UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Study of measurement of soil moisture by the plaster of Paris block method Day, John Howarth 1949

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1949_A4 D2 S7.pdf [ 17.12MB ]
Metadata
JSON: 831-1.0106847.json
JSON-LD: 831-1.0106847-ld.json
RDF/XML (Pretty): 831-1.0106847-rdf.xml
RDF/JSON: 831-1.0106847-rdf.json
Turtle: 831-1.0106847-turtle.txt
N-Triples: 831-1.0106847-rdf-ntriples.txt
Original Record: 831-1.0106847-source.json
Full Text
831-1.0106847-fulltext.txt
Citation
831-1.0106847.ris

Full Text

A STUDY OF MEASUREMENT OF SOIL MOISTURE BY THE PLASTER OF PARIS BLOCK METHOD by JOHN HOWARTH DAY, B.S.A. A Thesis submitted i n P a r t i a l Fulfilment of the Requirements for the Degree THE UNIVERSITY OF BRITISH COLUMBIA Of MASTER OF SCIENCE IN AGRICULTURE in the Department of AGRONOMY A p r i l , 1949 . A STUDY OF THE DETERMINATION OF SOIL MOISTURE BY THE PLASTER OF PARIS BLOCK METHOD. The P l a s t e r of Paris Block Method f o r measuring s o i l moisture is apparently affected by three f a c t o r s , ( l ) . C a l i -bration of the blocks i n terms of block r e s i s t a n c e - s o i l moisture, (2) S o i l temperature, (3) Soluble s a l t s i n the s o i l s o l u t i o n . Experiments designed to evaluate the method and to study these factors have revealed that ( l ) the Plas t e r of Paris Block Method is suitable for measurement of s o i l moisture. It operates s a t i s f a c t o r i l y over the c r i t i c a l moisture range and shows i t s greatest s e n s i t i v i t y i n the v i c i n i t y of the permanent w i l t i n g percentage, (2) a major source of v a r i a t i o n i n the method i s due to the Plaster of Paris blocks. In se l e c t i n g blocks for use their performance should be checked at both low and high resistances. The most s a t i s f a c t o r y c a l i b r a t i o n and performance of the Plas t e r of Paris blocks i s dependent upon the presence of roots of transpiring plants i n the immediate v i c i n i t y of the blocks, (3) s o i l temperatures may be estimated i n s i t u with the same resistance bridge used with the P l a s t e r of Paris blocks. Resistance thermometers found s a t i s f a c t o r y for this purpose are Thermistors type 14A, manufactured by the Western E l e c t r i c Company, (4) E l e c t r o l y t e s i n the s o i l s o l u t i o n a f f e c t moisture estimates made with the Plaster of Paris Block Method. . The magnitude of the effect produced with Na2S04 over the c r i t i c a l moisture range i s 0.3 to 0.4 percent decrease i n moistare estimate with each 500 pip.m. increase i n s a l t . The magnitude of the effec i s apparently the same in s o i l s of d i f f e r i n g texture. TABLE OF CONTENTS Page 1. Introduction 1. 2. A review of methods of estimating s o i l moisture i n S i t u : A. Methods based on gravimetric measurement. 2 B. Methods depending on e l e c t r i c a l properties of s o i l . 4 C. Methods based on thermal properties of s o i l . 22 D. Methods based on equilibrium tensions of s o i l . 25 E. Methods based on the mechanical resistance of the s o i l to penetration. 28 3. Advantages of methods f o r measuring s o i l moisture i n S i t u and reasons f o r differences i n data obtained by these methods. 30. 4. A comparison of methods f o r measuring s o i l moisture i n s i t u : A. Gravimetric sorption plugs 32 B. Plaster of Paris Blocks 34 C. Electrothermal u n i t s 35 D. Tensiometers 36 5• Ezper imental: A. Selection of s o i l samples 38 1. Estimation of moisture equivalents and permanent w i l t i n g percentages 40 2. Discussion of permanent w i l t i n g percentage, moisture equivalent and c r i t i c a l moisture range i n samples. 42 B. E s t a b l i s h i n g the r e l a t i o n s h i p between block resistance and s o i l moisture-Discussion of the block resistance-S o i l moisture curves. 44 C. S o i l temperature measurement and resistance correction-Discussion of thermistor c a l i b r a t i o n . 67 TABLE OF CONTENTS contd 5. Experimental contd. Page D. E f f e c t of sodium sulfate on the estimation of s o i l moisture by the Plaster of P a r i s Block method. Discussion of the e f f e c t of soluble s a l t on the block resistance. S o i l moisture curves. 77 6.. Conclusions. 91 7. Acknowledgements. 8. Bibliography. 1. INTRODUCTION A knowledge of s o i l moisture conditions i s of importance not only f o r crop production purposes but also as a guide to the proper ap p l i c a t i o n of i r r i g a t i o n water. Such information i s e ssential too, i n connection with dam and roadway construct-ion. It i s obvious then that an accurate and quick method fo r quantitative measurement of s o i l moisture under f i e l d conditions i s a major need f o r both a g r i c u l t u r i s t s and engineers. Despite this need, and the fac t that much time and ef f o r t have been devoted to the study, no e n t i r e l y s a t i s f a c t o r y method has yet been developed* Many of the investigations reported i n the l i t e r a t u r e have been directed to the establishment of a method providing (1) a quantitative determination over the entire moisture range, (2) an i n d i c a t i o n of s o i l moisture available to plant growth, (3) s i m p l i c i t y of operation, (4) suitability f o r use under f i e l d conditions. Methods that have been develcped meet one or other but not a l l these requirements. For convenience, these methods may be c l a s s i f i e d as follows, 1. Methods based on gravimetric measurements. Determination of moisture i n porous units i n equilibrium with the s o i l . 2. -Methods which depend upon the e l e c t r i c a l properties of s o i l or porous units i n equilibrium with the s o i l . a) conductance or resistance methods. b) capacitance or d i e l e c t r i c methods. 3. Methods based upon the thermal properties of s o i l or porous units i n equilibrium with the s o i l . 2. 4. Methods which depend upon the measurement of the equilibrium tension on the water i n a porous pot i n contact with the s o i l . 5. Methods which depend upon the resistance of the s o i l to penetration. In recent years, important advances have been made i n perfecting some of these methods p a r t i c u l a r l y those included, i n group 2a above. It i s to some factors influencing the methods of t h i s group that the present study i s devoted* A: RUSVIiaW <Jil METHODS 01? ESTIMATING SOIL MOISTUBE IN SITU Methods Based on Gravimetric Measurement Gravimetric Sorption Blocks Gravimetric sorption block methods, l i k e many other methods recently developed, approach the problem of moisture measurement through use of a porous medium permanently imbedded i n the s o i l . The use of thi s foreign absorbent material has been found to be a necessity to ensure a stable and standardized zone within which to secure the measurement of s o i l moisture* The gravimetric sorption plug method developed by Davis and Slater (13) "consists of a porous chamber or casing i n s t a l l e d i n the s o i l at the depth moisture readings are desired. The casing contains a c l o s e l y f i t t i n g tapered porous cone, or plug, which may be removed through a tube connecting the upper rim of the casing with the s o i l surface by means of a suitable hook." By then weighing the plug, a moisture c a l c u l a t i o n , based on the oven dry weight of the plug 3.--may be made. Since the plugs are of standard construction, energy equivalents to the s o i l moisture contents may be obtained by a laboratory c a l i b r a t i o n . From the work carried out, Davis and Slater observed promising r e s u l t s through the moisture range thus f a r experienced. Richards and Weaver (30) about the same time independ-ently developed a sorption block s o i l moisture meter which they think has promise. This sorption block s o i l moisture meter employs a ceramic disk which i s held i n contact with the s o i l , at the lower end of the mounting tube. This block has a d i f f e r e n t design than that of Davis and Slater and may be somewhat more d i f f i c u l t to manipulate. The authors have made provi s i o n for weighing the ceramic disk while suspended i n the s o i l , thus avoiding exposure to evaporation, f o r errors from this source would be s i g n i f i c a n t due to the low mass of the disk, 300-400 mgm. Davis and Slater have con-sidered effects due to evaporation during weighing to be i n s i g n i f i c a n t since the net weight of t h e i r block i s f i v e grams. When properly b u i l t the units should require l i t t l e s e r vicing or attention and accurate results should be immedia-t l y obtainable even with long periods of elapsed time between readings. The units are not susceptible to f r o s t i n j u r y since they are made of ceramic porcelain. Richards and Weaver (30) prefer ceramic sorption blocks over gypsum sorption blocks f o r t h e i r greater mechanical strength and lower s o l u b i l i t y . The gravimetric sorjjfcion blocks are influenced neither by 4. s a l t e f f e c t s nor by f l u c t u a t i o n i n s o i l temperature, since sorption block and s o i l are i n equilibrium i n respect to . moisture as well as to temperature. While gravimetric sorption block methods have c e r t a i n advantages, there also are disadvantages. The units are best suited to surface s o i l studies by reason of the short length of the connecting tube. While t h i s tube may conceiva-bly be of any length, i t i s f e l t that the method would not be e n t i r e l y suitable to deep s o i l studies* Another disadvantage of t h i s method i s the necessity f o r either carrying a balance i n the f i e l d for. weighing the blocks, or f o r removing the blocks i n an a i r t i g h t container to some central l o c a t i o n . Methods Based on E l e c t r i c a l Properties of S o i l Conductance or Resistance Methods The Two-Elect rode Method The f i r s t attempts to r e l a t e s o i l moisture to conduc-t i v i t y or resistance was made by Whitney, (36) Gardener (17) and Briggs, (11) who carried out rather elaborate i n v e s t i -gations. They used only two electrodes placed d i r e c t l y i n the s o i l and a l t e r n a t i n g current to avoid p o l a r i z a t i o n . They reported carbon electrodes as being most s a t i s f a c t o r y and concluded that t h i s method of moisture measurement had promise. However, others who later worked on the two-electrode method found that:-1. Variations i n s a l t content were too great to allow the use 5. of the resistance method so devised f o r measuring variations i n moisture content. 2. That the contact resistance between the electrodes and the s o i l may be e r r a t i c , for any expansion or contraction of the s o i l around the electrodes w i l l lower or raise the contact resistance. The two-electrode method measures the sum of the s o i l resistance plus the contact resistance, and sinee the l a t t e r i s very e r r a t i c and unrepro-duoible, i t s elimination i s highly desirable. Deighton (14), i n his work on the two-electrode method, found two main d i f f i c u l t i e s ; 1. Degree of compaction of the s o i l . 2. The v a r i a b i l i t y of the s o i l electrode resistance which he attributed to "actual differences i n moisture or other factors." Deighton attempted to standardize both the electrodes and the degree of packing of the s o i l by remixing and repack-ing the s o i l i n the experimental box at each determination. While i n f a c t he was r e a l l y changing three variables at each experiment, v i z . moist'ure content, state of packing, and i n t e r f a c i a l contact between s o i l and electrode, he hoped to reproduce the same conditions f o r the l a s t two. His exper-imental values showed marked f l u c t u a t i o n . Haines (18) t r i e d to overcome these d i f f i c u l t i e s by using a f l u i d (mercury) contact with the s o i l , and making measure-ments of resistance as the s o i l sample dried out, without disturbing i t i n any way. He concluded that "The results f o r s o i l s generally indicate a s u f f i c i e n t l y simple connection 6. between conductivity and moisture for t h i s property to be made the basis of s o i l moisture determinations." The Four-Elect rode Method McCorkle (26) recognized the great source of esror inherent i n the two-electrode method when used i n measuring the resistance of s o i l s , and hence he made a b r i e f investiga-t i o n using the four (or multiple) electrode. The four-electrode method was found to be successful since i t eliminated contact errors from the measurement of s o i l resistance and at the same time did away with the necessity of considering the resistance of lead wires, and thus increased the accuracy of the resistance measurements. From his experiments McCorkle concluded that "the method to be used to determine r e l a t i v e moisture contents, and as long as the apparatus i s c a l i b r a t e d frequently enough to overcome changes i n s a l t content, i t may possibly be used to measure the percentage of moisture i n s o i l s at various locations." Edlefsen and Anderson (15) continued work on the four* electrode method with the aim of determining: 1. Whether the variations i n sa l t content are too great to permit the use of the four-electrode method as an indicator of moisture content i n the s o i l , 2. The nature of the va r i a -t i o n i n the electrical resistance of the s o i l i n the neighbour-hood of the permanent w i l t i n g percentage. Edlefsen and Anderson were f i r s t to use tinned iron electrodes i n place of carbon electrodes, and found no measure-7. able difference i n t h e i r e l e c t r i c a l c h a r a c t e r i s t i c s . The i r o n electrodes were cheaper, sturdier and easier to i n s t a l l i n f i e l d work. Edlefsen and Anderson concluded that: 1. The e l e c t r i c a l resistance between any two electrodes varied inconsistently with variations i n moisture content due to the variable cont-act resistance, whereas by using four electrodes, the v a r i a -t i o n i n contact resistance between electrodes and s o i l was eliminated. 2. Very small changes i n moisture content taking place i n the v i c i n i t y of the permanent w i l t i n g point caused comparatively large changes i n e l e c t r i c a l resistance. When resistance was plotted as a function of moisture content a curve was obtained which showed a rapid increase i n resistance With decreasing moisture content i n the neighbourhood of. the permanent w i l t i n g point. 3. The four-electrode method of i n d i c a t i n g s o i l moisture has promise, and from the close r e l a t i o n between resistance and moisture content, the variations i n resistance due to s a l t content, or f a c t o r s other than moisture content, did not appear important f o r the conditions of the experiment. •".i x) The Two-Electrode Plaster-of-Paris Blook Method U n t i l i nvestigation of a rapid method of measuring s o i l moisture was undertaken by Bouyoucos and Mick (7), conducti-v i t y measurements of s o i l s had never been s a t i s f a c t o r i l y related to moisture content, due to the marked v a r i a b i l i t y of compaction, texture, s a l t content, and temperature of the s o i l . 8. Accordingly, the investi g a t i o n by Bouyoucos and Mick was undertaken with the following objectives; 1. To f i n d some means of minimizing the effects of s a l t content, compaction, texture and temperature on s o i l resistance measurements* 2. To study the relationship between resistances and s o i l moisture. 3. To devise an accurate, rapid method of making a continuous measurement of s o i l moisture under f i e l d conditions* P r i n c i p l e s employed i n the method. Tq avoid errors caused by compaction, texture, and s a l t concentration, i t was found necessary to control the area immediately around the electrodes• This was accomplished by embedding the electrodes i n a material which would remain more or less constant, with the exception of moisture content, under a i l f i e l d conditions. Two p r i n c i p l e s are here employed* F i r s t , a porous absorption block i s placed i n the s o i l , some of the s o i l water entering the block. Eventually equilibrium i s established between the s o i l moisture and the moisture i n the block. The block tends to esta b l i s h i t s own environment of equilibrium, since the plaster of p a r i s , which composes the matrix about the electrodes, i s s l i g h t l y soluble. I f the s o i l i s near saturation the block absorbs much moisture; while i f the s o i l i s dry, the block absorbs but l i t t l e moisture. I f a moisture gradient develops between s o i l and block, the block w i l l gain or lose moisture u n t i l equilibrium i s re-attained. Second, the moisture content of t h i s absorption block determines i t s e l e c t r i c a l resistance. The density, texture, s a l t eontent^nrfcompaction of the block remain f a i r l y constant. Hence a change i n resistance indicates a change i n the moisture content of the block, and thus a change i n s o i l moisture. Apparatus used i n the method. The apparatus used by Bouyoucos and Mick consisted of absorption blocks and of a sp e c i a l adaptation of the Wheatstone Bridge with which to measure resistances. After much experimenting, they found that blocks made of pla s t e r of par i s were most s a t i s f a c t o r y , due to i t s physical structure, s o l u b i l i t y c h a r a c t e r i s t i c s , i t s cheapness and ease of handling. Bouyoucos and Mick (8) state that p r i o r to t h e i r inves-t i g a t i o n of absorbent materials i t was believed that p l a s t e r of paris exhibited c e r t a i n disadvantageous properties, which possibly would not be encountered with other absorbent mater-i a l s . Chief of these was a tendency to soften when saturated. Also, the s o l u b i l i t y of CaS0 4 was at f i r s t considered a d i s -advantage because i t was thought that a completely insoluble absorbent might lend i t s e l f to Direct Current resistance measurements and thus avoid the high cost of Alternating Current bridges and many technical problems involved i n t h e i r u t i l i z a t i o n . To investigate this p o s s i b i l i t y , absorbent ceramic blocks of white porcelain clay with electrodes of hard lead were constructed. A f a i r l y s a t i s f a c t o r y saturation resistance reading was obtained with D.O. when the e l e c t r o l y t e content -10-was low, as i n d i s t i l l e d water, bat on being placed i n the s o i l , the s o i l s o l u t i o n c i r c u l a t i n g through the block increased the e l e c t r o l y t e content to such an extant that a constant measure-ment could not be obtained with simple D.C. instruments. For these reasons D.O. was abandoned i n favor of A.O. i n a l l sub-sequent investigations. It was also concluded that, instead of being a disadvantage, the s o l u b i l i t y of plaster of paris was of considerable useful-ness. This s o l u b i l i t y of plaster of paris creates a r e l a t i v e l y high e l e c t r o l y t e concentration* of most s o i l solutions, hence the s o l u b i l i t y , of plaster of p a r i s i s considered to e f f e c t a bu f f e r i n g action i n the r e l a t i o n between block resistance and changes i n the s a l t concentration of the s o i l water, thus minimizing conductance variations due to changing concentration of t h i s s o i l s o l u t i o n . Other substances were considered by Bouyoucos and Mick, i n addition to plaster of p a r i s , including cement, concrete, marble dusts, dental casting compounds, and lime plasters'. Laboratory examination of these materials, however, showed that compared to p l a s t e r of p a r i s , their rate of moisture absorption, and therefore, the attainment of an equilibrium between block moisture and s o i l moisture, was r e l a t i v e l y slow. From the evidence obtained by Bouyoucos and Mick they conclude that the desirable qualiti e s i i f o r an absorbent 1 - S o l u b i l i t y of Plaster of Paris 0.241 gm. per 100 ml.atO°C. - I I material are, 1. a hard, porous, physical structure capable of rapid absorption and rapid drying with regard to water, 2. a large absorption capacity to ensure a useful order of s e n s i t i v i t y , and 3, a moderate range of s o l u b i l i t y to mini-mize the e f f e c t s of changes i n the concentration of the s o i l s o l u t i o n . In conclusion then, i t appears that because of i t s physical structure, i t s s o l u b i l i t y c h a r a c t e r i s t i c s , and i t s cheapness and ease of handling, plaster of p a r i s i s at present the most sa t i s f a c t o r y absorbent material f o r the resistance method of measuring s o i l moisture. In a l a t e r investigation, Bouyoucos and Mick (10) point out that the pore size d i s t r i b u t i o n of plaster of paris i s such that i t measures s o i l moisture from f i e l d capacity down to the w i l t i n g percentage, through the range of " a v a i l a b l e water". Two c h a r a c t e r i s t i c s , however, have limited the usefulness of t h i s type of absorption unit. One l i m i t i n g c h a r a c t e r i s t i c i s that plaster of paris does not afford a wide range of s e n s i t i v i t y , that portion of the moisture curve from the w i l t i n g point to a i r dryness, i n addition to that from saturation to the w i l t i n g point, being of interest to engineers and hydrologists. S e n s i t i v i t y of the p l a s t e r blocks i s greatly diminished i n that portion of the moisture scale from the permanent w i l t i n g percentage to a i r dryness. A second l i m i t a t i o n i s encountered i n continuously wet locations, where plaster of paris i n s t a l l a t i o n s begin to disintegrate a f t e r functioning f o r several months i n -12-satura'ted environments. There i s a great need f o r units that w i l l function despite long periods of exposure to c i r c u l a t i n g ground water* With these l i m i t a t i o n s i n mind, Bouyoucos and Mick (id) carried out intensive investigations of a number of absorbent materials and electrode materials. Among the materials t e s t -ed f o r absorption were dental stone, f i r e d clay, concrete, c e l l u l o s e sponge, f i b r e glass and nylon. The materials tested for electrodes were n i c k e l , zinc coated screens, perforated zinc p l a t e s , and s t a i n l e s s s t e e l . Units having external electrodes with absorptive centers were tested, giving f i e l d r e s u l t s that had no very close r e l a t i o n s h i p to the laboratory s o i l moisture c a l i b r a t i o n . This discrepancy the authors attributed to external s o i l f a c t o r s . The f i n a l f a b r i c unit developed embodies the p r i n c i p l e of the p l a s t e r of paris block, consisting of two perforated, t h i n n i c k e l plat e s , or two pieces of f i n e monel screen a c t i n g as electrodes, to which are silver-soldered wire leads. The electrodes are separated by wrappings of nylon or f i b r e glass f a b r i c . The whole assemblage i s then placed i n a perforated n i c k e l case, pressed under high pressure and the edges of the case are united. None of these materials tested showed the same buffer capacity as p l a s t e r of p a r i s , although £ibre glass and nylon gave a greater range of s e n s i t i v i t y , from saturation to air-dryness. The f i n a l f a b r i c unit developed i s not intended to replace the p l a s t e r of paris block, but to provide additional mis-information to engineers and hydrologists, who are interested i n the entire moisture range. The f a b r i c unit may prove to be very useful i n locations of constantly poor drainage. For the resistance measurements, an adaptation of the Wheatstone Bridge was necessary. In the laboratory, a skeleton bridge, with a variable a i r condenser, a 1000-cycle reed-type a u d i o - o s c i l l a t o r , and earphones was used to obtain resistances. A vacuum tube o s c i l l a t o r , powered by dry c e l l b atteries, supplies alternating current at high frequency, eliminating p o l a r i z a t i o n and e l e c t r o l y s i s e r r o r s . Variable resistance arms increase the s e n s i t i v i t y to - 2$ between 50 and 90,000 ohms. A logarithmic potentiometric rheostat, with a 6-inch d i a l , i s the graduated arm. The c i r c u i t i s tuned to the " n u l l " point by means of earphones. Measurements can be obtained i n EO or 30 seconds by simply adjusting the rheostat and the condenser. Bridges with "magic eye" assembly replacing the earphones are obtainable, but opinion varies as to the ease of observ-ing t h i s "magic eye" i n bright sunlight. Bouyoucos and Mick also investigated the p o s s i b i l i t y of using direct current i n measuring conductivity changes i n the absorption blocks, but a s a t i s f a c t o r y c o r r e l a t i o n between block resistance and s o i l moisture could not be obtained fo r several reasons. Chief of these were the hydrolysis of the materials composing the blocks and d i s s o c i a t i o n of the absorbed s o i l s olution, both causing considerable e l e c t r o l y s i s and p o l a r i z a t i o n . It was hoped that conductivity might be -14-conveniently measured by differences i n e l e c t r i c a l current flow at a constant voltage with an ammeter. Neither t h i s method nor the employment of a D.C. Wheatstone Bridge, which makes possible a resistance measurement without appreciable current flow, proved successful.. Rapid and e r r a t i c d r i f t s pro-hib i t e d accurate and r e l i a b l e readings. Bouyoucos and Mick (9) have recently brought out a new improved bridge. Shis s p e c i a l bridge has several advantages; i t i s self-contained and portable, the power unit i s rugged enough to withstand shocks encountered i n f i e l d work, and a sharp n u l l point i s obtainable as a re s u l t of including the variable condenser i n one of the bridge arms. Compared, with the spe c i a l bridge o r i g i n a l l y designed for this work, the newer modification has a much wider range. 5,000,000 ohms as against 100,000 ohms f o r the old model. The o s c i l l a t o r has also been improved. Ca l i b r a t i o n of apparatus. The procedure used by Bouyoucos and Mick (7) for c a l i b -r a t ing the apparatus, involved placing a saturated block i n a shallow pan of s o i l , and determining the s o i l moisture content at equilibrium, as indicated by a constant e l e c t r i c a l resistance. By t h i s procedure, a ce r t a i n s o i l moisture content was found to be associated with a c e r t a i n block resistance value. Other pairs of values were s i m i l a r l y obtained over a range of s o i l moisture by allowing drying to proceed to vary-ing stages. These values were then plotted to form a moisture resistance curve f o r the p a r t i c u l a r s o i l . -15-Other investigators, Anderson and Edlefsen (4f), attempted to duplicate the c a l i b r a t i o n procedure of Bouyoucos and Mick, and found a tremendous lag i n response of the blocks to the moisture content of the surrounding s o i l when t h i s was near the permanent wilting percentage, l o r the s o i l used, and at constant moisture content, the curves for the two-electrode plaster of p a r i s blocks showed that even a f t e r one month the plaster block did not a t t a i n equilibrium i f surrounded by s o i l with moisture content i n the lower quarter of the range of available water* The d r i e r the s o i l , the more pronounced was the lag. The resistance was s t i l l r i s i n g r a p i d l y a f t e r 40 days, for s o i l s approximately 0.5 percent above the perman-ent w i l t i n g percentage. It i s evident, therefore, that considerable error may be made i n l o c a t i n g the c a l i b r a t i o n curve at the lower soil-moisture contents, by a method such as the foregoing. Anderson and Edlefsen (4) investigated lag i n response of the blocks to changes of soil-moisture content when the roots of a c t i v e l y t r a n s p i r i n g plants surrounded the blocks. Sunflowers were grown i n buckets, and the surface of \the s o i l was covered by overlapping sheets of l e a d f o i l , permitting i r r i g a t i o n of the s o i l without an appreciable loss of moisture by evaporation from the s o i l surface. The buckets of s o i l were heavily i r r i g a t e d and then allowed to become dry by the t r a n s p i r a t i o n of the s o i l water by the plants. This process was carried out repeatedly, readings being taken u n t i l the plants were very much wilted* -16-I f the blocks show a l a g i n response to changes of s o i l -moist ore content, the carve f o r any given cycle should f a l l lower i n the graph, the more rapid the rate of t r a n s p i r a t i o n during that c y c l e / Anderson and Edlefsen found a surprising absence of lag i n response to moisture changes f o r the two-electrode plaster blocks used, when the blocks were calibrated by the method described above. The reason f o r t h i s absence of lag i s probably that i n the s o i l where plants are growing, a very steep soil-moisture con-tent gradient i s developed and maintained i n the boundary layer surrounding the block, whereas i n the s o i l where no plants are growing, only a very small moisture content gradient i s developed. Since the rate of movement of moisture at any point i s proportional to the gradient of the moisture content, the movement out of the block i n the f i r s t case should be rapid, i n second case, very slow. Anderson and Edlefsen (4) conclude that: the block may be r e l i a b l y calibrated by determining the r e l a t i o n between the resistance and the s o i l -moisture content, the l a t t e r being changed by thor-oughly permeating the roots of a c t i v e l y . t r a n s p i r i n g plants i n the s o i l surrounding the block. The c a l i -bration should never begin u n t i l the roots of the plants have had time to permeate the s o i l surrounding the block, also' i t should be carried out i n a constant temperature tank since block resistance depends on temperature. K e l l y (23) found the method of Bouyoucos and Mick (7) unsatisfactory, and i s of the opinion that the plant method of Anderson and Edlefsen requires considerable time, space and labor. K e l l y used wire mesh baskets i n which were placed -17-the moist s o i l and the electrode block. The basket of s o i l and the electrode were placed i n a humidity chamber (93 to 98 percent r e l a t i v e humidity) for 19 hours i n order that s o i l moisture movements would reach equilibrium. At the end of that period the basket was weighed, and the resistance determined. It was then l e f t on a wire stand f o r 5 hours so that unrestricted evaporation from a l l sides could take place. The basket was then returned to the humidity chamber. This procedure was continued u n t i l the s o i l no longer l o s t moisture when exposed to the a i r * K e l l y found that there was a small v a r i a t i o n i n the moisture content between the outside layers and the center of the block and that t h i s v a r i a t i o n had l i t t l e e f f ect upon the shape or lo c a t i o n of the c a l i b r a t i o n curves. He concludes that when the described procedure i s used, the average moisture content of the whole s o i l block very c l o s e l y approximates that of the s o i l immediately surrounding the unit being calibrated* E f f e c t of changes i n s a l t content on the method* Minor variations i n block resistance s o i l moisture values may be due i n part to differences i n the salt content of the s o i l solution* Bouyoucos and Mick (7) studied t h i s effect i n the laboratory by applying the equivalent of up to 1000 pounds per acre of 4-16-8 commercial f e r t i l i z e r . They found no s i g n i f i -cant e f f e c t on the resistance readings on the blocks, and so concluded that "changes i n the s a l t content of ordinary s o i l s w i l l not influence the relat i o n s h i p between the resistance of -18. the block and s o i l moistare." The authors state that the effect of a l k a l i n e or s a l i n e s o i l s on the e l e c t r i c a l readings was not determined, hat may he large enough to render the method impractical. . y In a l k a l i or s a l i n e s o i l s , s a l t s i n the s o i l s o l u t i o n may or may not have an effect on the conductivity of the block moisture, depending on the s o l u b i l i t y of these s a l t s . It i s expected that s a l t s such as CaC03, CaS04 w i l l have l i t t l e e f f ect on lowering the e l e c t r i c a l resistance since t h e i r s o l u b i l i t i e s are less than that of p l a s t e r of p a r i s . Salts such as Wa2C0g and NagSO^ probably w i l l lower the block resistance since t h e i r s o l u b i l i t i e s are greater than that of p l a s t e r of p a r i s . E f f e c t of temperature on the method. Temperature changes cause variations i n the resistance of the absorption blocks at a constant moisture content. Bouyoucos and Mick (7) have shown the r e l a t i o n between . temperature, s o i l moisture and block resistance, and recommend that corrections f o r temperature be made f o r accurate s o i l moisture determination. Various types of thermometers have been used f o r measur-, ing s o i l temperature, including mercury, d i a l and b i m e t a l l i c thermometers." None of these are e n t i r e l y s a t i s f a c t o r y since they are f r a g i l e , unreliable, unsuitable at great depths, or not s u f f i c i e n t l y s e n s i t i v e . M e t a l l i c e l e c t r i c a l resistance thermometers overcome most of these d i f f i c u l t i e s but such -19-thermometers Involve the use of a potentiometer,, which consti-tutes an., extra piece of equipment. Bouyoucos (6) has developed a l i q u i d e l e c t r i c a l resistance thermometer which can he used on the same Wheatstone Bridge as the plaster of paris blocks. The thermometer i s very sensitive to temperature changes and gives a high value of resistance f o r every 1°JP. change of temperature. Thus f a r these thermometers have been made only manually. Bouyoucos and Mick (7) have published.a graph showing the r e l a t i o n between temperature, s o i l moisture and block r e s i s t -ance. Temperature corrections can be made from this graph with considerable accuracy. Slater and Bryant (34) received by private communication from T.C. Peele a mathematical temperature correction. Log R(t-70) = log Rfc 1 + 0.002(t-70) where R i s the resistance i n ohms and T i s the temperature i n degrees Fahrenheit. The equation i s an es s e n t i a l agreement " w i t h the temperature correction curves of Bouyoucos and Mick (7). Capacitance and D i e l e c t r i c Methods for Measuring  S o i l Moisture. The p r a c t i c a b i l i t y of the e l e c t r i c a l resistance of the two-electrode plaster of paris block as an indicator of the moisture content of s o i l i n which plants are growing has been shown by the work of Bouyoucos and Mick (7), and of Anderson and Edlefsen (4). Another e l e c t r i c a l property of the same two-electrode p l a s t e r of par i s block i s i t s e l e c t r i c a l capacity, which i s the quantity of e l e c t r i c i t y either of the electrodes w i l l hold when there i s unit potential difference between the two electrodes. The e l e c t r i c a l capacity of the pla s t e r of par i s block w i l l depend greatly on the nature of the medium between the electrodes. Since capacity depends upon the d i e l e c t r i o const-ant of the medium surrounding the electrodes then from the equation, 0 5 D 0 Q where Cs capacity, D - the d i e l e c t r i c constant of the medium, 0 Q z the capacity of electrodes when surrounded by a i r , i t may be seen that i f the d i e l e c t r i c constant i s low the capacity w i l l be low and viceversa. The e l e c t r i c a l capacity of the condenser composed of the electrodes embedded i n plas t e r of par i s w i l l be low when the medium i s dry, high when the medium i s wet. The capacity should therefore serve as an indicator of the moisture content of the medium surrounding the electrodes. Anderson and Edlefsen (5) concluded from t h e i r work on capacity methods that 1. the e l e c t r i c a l capacity of a p l a s t e r of paris block i s not changed appreciably by a change of 15 per cent i n spacing of the electrodes i n the midplane of the block, 2. i n comparison to the e l e c t r i c a l resistance, the e l e c t r i c a l capacity of the plaster of par i s blocks should be r e l a t i v e l y unaffected by changes i n the concentration of the s o i l s o l u t i o n . 3. With decrease i n s o i l moisture content, the e l e c t r i c a l capacity of the blocks begins to drop from a rather high value .(approximately 0,070 microfarad) at about the moisture equivalent, f i n a l l y approaching a r e l a t i v e l y constant value (around 0.0003 microfarad) a l i t t l e above the permanent w i l t i n g percentage of the s o i l , 4. the e l e c t r i c a l capacity of the plaster of paris block, over the entire range of moisture content r e a d i l y a v a i l a b l e to plants, w i l l serve as a p r a c t i c a l indicator of the soil-moisture content i n a body of s o i l where the blocks can be buried and where the changes i i n soil-moisture content are caused by the removal of the moisture by the roots of a c t i v e l y t r a n s p i r i n g plants. Fletcher (16) reported the res u t l s of an inv e s t i g a t i o n made on the d i e l e c t r i c method for determining s o i l moisture. He used copper plates separated by a thin membrane of baked clay which absorbed moisture from the s o i l and came into equilibrium with the s o i l moisture. He found that determinat-ions by thi s method are not greatly affected by the content of soluble s a l t s i n the s o i l , although the amount of c o l l o i d n may have,an e f f e c t . This method has the same disadvantage as the method of measuring resist'ance of plaster of paris blocks i n contact with the s o i l , an appreciable time lag i s found i n following changing moisture conditions i n the f i e l d . Thorne and Russell (35) found many inherent d i f f i c u l t i e s i n determination of s o i l moisture by d i e l e c t r i c methods, some of which may be caused by a high degree of ori e n t a t i o n of dipolar water molecules, by changes i n the el e c t r o l y t e concentration of the s o i l solution, or fey v a r i a t i o n i n cl a y -22-content of the s o i l * Methods Based on Thermal Properties of S o i l s . The accuracy of e l e c t r i c a l conductivity methods of measuring s o i l moisture i s doubtful because oi' conductivity of the s o i l at a given moisture content varies greatly with changes i n the s a l t concentration of the s o i l solution. A successful method of measuring changes i n s o i l moisture w i l l necessarily be one that uses some property of the s o i l and the s o i l s o l u t i o n that i s not influenced by changes i n the s a l t content. Heat conductivity should be a property of such a system-which would not be materially affected by the presence of eions i n solution, since rather large changes i n the concentration of a d i l u t e s a l t s olution have very l i t t l e influence on the thermal conductivity. The heat conductivity of a dry porous medium, such as s o i l , must of necessity be low, since the s o l i d materials make only point contacts. The area f o r continuous heat flow through s o l i d materials i s very small? a n e g l i g i b l e amount of the heat i s conducted by the a i r i n the pores, since a i r i s a much poorer conductor-that the s o i l s o l i d s . As water i s added to the s o i l , the area through which heat can flow w i l l , increase tremendously since the water w i l l form wedges around the points of contact. Water i s not as good a conductor of heat as the s o l i d s o i l material, but i t i s a f a r better conductor that a i r . Thus i t i s to be expected that the heat conductivity of a s o i l w i l l increase with i t s moisture content. -23-Shaw and Baver (32) undertook inves t i g a t i o n of the heat conductivity of s o i l . The instrument devised was a modified Wheatstone bridge, two of the arms being equiresistant c o i l s of No. 40 enamelled copper wire wound on glass.tubing. One of these c o i l s i s placed i n oven dry s o i l , while the other c o i l i s placed i n moist s o i l . It can be seen that i f the s o i l s surrounding the two c o i l s have d i f f e r e n t heat c o n d u c t i v i t i e s , the f i n a l equilibrium temperatures of the c o i l s , a f t e r passage of a c e r t a i n amount of c u r r e n t , . w i l l be d i f f e r e n t . The c o i l i n the s o i l of lowest heat conductivity w i l l have the highest temperature and also the highest resistance. By rebalancing the bridge the differenqe i n resistance between the two c o i l s may be r noted arid related to moisture content of the s o i l . Shaw and Baver (32) .conclude from their study of thermal conductivity i n r e l a t i o n to s o i l texture, moisture content, and s a l t content that 1. It has been possible to devise an apparatus f o r measuring changes i n the heat conductivity of a s o i l at various moisture contents, 2. It has been established that heat conductivity gives a r e l i a b l e index of the moisture content of the s o i l , 3. It has been shown that changes i n sa l t concentration of the s o i l s olution do not materially a f f e c t the heat conductivity of the s o i l . In a l a t e r work Shaw and Baver (33) showed the r e l a t i o n -ship between instrument readings and moisture content f o r various s o i l s . They observed a unique reading for each moisture content. The curves obtained showed that t h i s method -24-of moisture measurement covers the entire range of s o i l moisture with i t s greatest s e n s i t i v i t y "below the moisture equivalent. Shaw and Baver are of the opinion that the readings are influenced neither by external s o i l temperatures nor by changes i n the s a l t concentration of the s o i l s o l u t i o n . Work with Bouyoucos' pl a s t e r e l e c t r i c a l conductors suggested to Johnston (22) that i f the heater element i n the s o i l were given a covering of porous material that would have i t s own moisture-holding c h a r a c t e r i s t i c s , this jacket would absorb the heat emanating from the heater c o i l , making the re s u l t i n g readings a function of the moisture content of the jacket and eliminating the contact d i f f i c u l t i e s between s o i l and elements that ar i s e when nonjacketed heaters are used. Accordingly, jacketed heater elements were made, and were buried i n pots of s o i l , one contained Yolo f i n e sandy loam, and the other, f i n e sand. Sunflowers were grown i n the pots. Johnston observed comparable readings at the moisture l i m i t s -f i e l d capacity and permanent wi l t i n g percentage - even though these l i m i t s were di f f e r e n t f o r the two s o i l s . The p l a s t e r of p a r i s jacket appeared capable of rapid adjustment to com-paratively rapidly changing moisture content i n the s o i l i n the pots. Thermal conductivity methods using the jacketed heater elements of Johnston (22) appear to be quite promising, provided that the readings are influenced neither by changes i n the s a l t concentration of the s o i l s olution nor by external s o i l temperature. -25-Methods Bases on Equilibrium Tensions or Qapillary P a l l of the S o i l . It has long been known that the s.oil exerts a c e r t a i n " c a p i l l a r y p a l l " caused by the surface energy of the f i l m of water surrounding i t s p a r t i c l e s and that t h i s p u l l varies with the moisture content of the s o i l . Kornev (25) described i n a report on the "Absorbing Power of S o i l s and the P r i n c i p l e of Automatic S e l f - I r r i g a t i o n of S o i l s " h i s use of a porous pot connected to a mercury mano-meter to measure the " p u l l " of the s o i l . The " p u l l " of the s o i l on the water i n the porous pot caused the mercury to r i s e u n t i l equilibrium was reached. He observed that the "absorbing power" of the s o i l varies with the structure, degree of compactness, size of mechanical p a r t i c l e s of the s o i l , and degree of s o i l moisture. The d r i e r the s o i l , the greater the p u l l shown on the manometer. Haines (19) discussed the theoret i c a l side of the r e l a t i o n -ship between s o i l and pressure deficiency and threw much new l i g h t on this problem. Using a porous pot f i l l e d with water and connected to a mercury manometer he obtained moisture; pressure-deficiency curves f o r a good textural range. These showed a sharp r i s e i n pressure deficiency as the percentage of saturation f e l l from 100 percent of the pore space to about 80 percent, then a gradual r i s e as the pore spaces emptied from 80 to 20 percent of saturation. He mentioned, however, that --86-the main features of the pressure-defioiency curve, namely a 'distinct bend at each end with a f l a t intervening portion, became smoothed out as the p a r t i c l e size becomes less uniform, for there i s c. then a smooth and wide graduation i n the pore s i z e s . This i s borne out by the work of Kornev, Heath and Rogers.. Haines (20) raised the important point that the pressure-deficiency curve f o r r i s i n g moisture does not exactly coin-cide with that f o r f a l l i n g moisture, but makes a hysteresis loop. He found that the region of s t r i c t l y r eversible changes was confined to the two ends of the curve. The work of Hogers (31) indicates that this hysteresis effect does cause a cert a i n error i n measurement of actual percentage of s o i l moisture obtained with a manometer, but i n an ordinary s o i l consisting of p a r t i c l e s of mixed sizes t h i s error i s comparatively small, not greater than three percent. Further, i t may well be that the c a p i l l a r y p u l l of the s o i l , as shown by the manometer, i s a more accurate index of the resistance of the s o i l to the obtaining of water -by the plant than i s any figure f o r actual moisture percentage. Richards and Gardner (27) describe in d i c a t i n g , recording and d i f f e r e n t i a l instruments f o r measuring c a p i l l a r y p o t e n t i a l . The expression " c a p i l l a r y tension" i s used as a name f o r the negative pressure e x i s t i n g i n the water i n unsaturated s o i l , and porous cell-vacuum gauge instruments used i n i t s measure-ment are c a l l e d tensiometers by the writers. Richards and Lamb (29) conducted f i e l d measurements of c a p i l l a r y tension. They found r e l a t i v e values of c a p i l l a r y -27-tension at varioas layers of the s o i l p r o f i l e to be consistent. Changes occurred where water was being lost or added before corresponding changes reached the other layers. The changes followed closely the p r e c i p i t a t i o n data. Experimental curves r e l a t i n g moisture percentage and c a p i l l a r y tensions f o r samples of the Lordstown surface s o i l were found to d i f f e r , depending on whether the s o i l was wetting or drying; the hysteresis curve not exceeding three percent moisture content at constant tension. The authors prefer the expression of s o i l moisture conditions i n terms of tension, rather than i n percentage, for d i f f i c u l t i e s due to hysteresis are avoided. Tensions cannot be measured with porous clay apparatus when they exceed one atmosphere, but within the range of one atmosphere they are readily obtainable. Richards (28) has outlined a method whereby the quantity of water held i n a column of s o i l i s calculated from c a p i l l a r y tension records. The calculations require a knowledge of the functional r e l a t i o n between tension and the mofeture content f o r the p a r t i c u l a r s o i l i n question. Experimental curves r e l a t i n g moisture content and tension showed a measurable hysteresis loop. E f f e c t s of the compaction of the s o i l on this r e l a t i o n were observed. A pressure c e l l employing a porous ceramic wall i s described f o r obtaining curves r e l a t i n g tension and moisture content over a tension range greater than one atmosphere. The tensiometer provides data on the wet end of the -28-energy-moisture carve. The maximum tension cannot exceed one ' atmosphere; when the poten t i a l of the s o i l water exceeds t h i s value, a i r enters the cup and the instrument i s no longer operative. Nevertheless, t h i s technique o f f e r s many p o s s i b i l -i t i e s for studying soil-moisture changes from, values about L the moisture equivalent to c a p i l l a r y saturation. Methods Based on the Mechanical Resistance of the S o i l to Penetration. It i s generally observed that the higher the moisture content of s o i l , the more p l a s t i c and less stable i t becomes when subjected to a deforming f o r c e . As the moisture content decreases, the force necessary to cause a predetermined deformation i n s o i l of a given compaction increases. This phenomenon i s very marked i n s o i l s of high clay content. A l l y n and Work (2) have developed the "Availameter", which i s e s s e n t i a l l y an instrument for measuring the p l a s t i -c i t y , s t a b i l i t y or degree of hardness of a s o i l sample. The primary form of t h i s instrument required andundisturbed s o i l core of an approximately uniform |?-inch diameter r e a d i l y obtained with the King s o i l tube. In essence the "Availameter" consists of two plungers or needles of standard form and s i z e . Resistance to penetration of the s o i l core by these needles i s read d i r e c t l y from a pressure gauge graduated i n pounds. If the moisture c o n t e n t - s t a b i l i t y r e l a t i o n s h i p is known fo r any p a r t i c u l a r s o i l , the s t a b i l i t y determinations may be -29-converted into, or readings may be made d i r e c t l y i n terms of, moisture content or available moisture content. The authors observed a close r e l a t i o n s h i p between availameter measurements and the corresponding moisture content. The availameter was not t r i e d on l i g h t s o i l s , but i t , i s not considered by the authors to be adaptable to s o i l s which f a i l to give cohesive s o i l tube cores; therefore i t s use may be limited to medium or heavy s o i l types. In a l a t e r investigation A l l y n and World3) studied the «. c a l i b r a t i o n and use of the availameter i n quantitative studies of i r r i g a t i o n problems. They found close agreement i n compar-isons of s o i l moisture as determined by oven-drying and by the availameter. On heavy clay s o i l the difference was less than two percent of the availa b l e s o i l moisture, and less than 0.5 percent of the t o t a l s o i l moisture. A l l y n ( l ) describes a new device, known as the "Stabilime-ter" f o r quick f i e l d determination of s o i l moisture conditions. The apparatus consists, e s s e n t i a l l y , of a diamond-shaped blade point mounted on a shaft, by means of which the blade point can be driven to the desired s o i l depths. The measurement of the resistance of the s o i l to r o t a t i o n of t h i s point i s termed "the s o i l s t a b i l i t y " and i s evaluated i n terms of torque i n inch-pounds by the use of an es p e c i a l l y designed handle which may be attached quickly to the head of the shaft. This s t a b i l i t y measurement, when correlated with the corresponding s o i l moisture content, showed a consistant r e l a t i o n by means of which s o i l moisture content may be -30-estimated, usually within 0.5 percent, i n heavy s o i l s such as were covered i n the in v e s t i g a t i o n . The range of s o i l types on which th i s method would, s a t i s f a c t o r i l y operate was not determined by the author, since the investigation was confined to heavy s o i l s . A l l y n believes, however, that s a t i s f a c t o r y operation w i l l be found on s o i l s as l i g h t as clay loams and, possibly, on much l i g h t e r s o i l s . ADVANTAGES OF METHODS FOR MEASURING SOIL MOISTURE IN SITU AND REASONS FOR DIFF-ERENCES IN DATA OBTAINED BY THESE METHODS Methods and instruments which enable the following of s o i l moisture changes i n s i t u present very d e f i n i t e theoret-i c a l and p r a c t i c a l advantages over other methods of s o i l moisture determination. The p r i n c i p a l advantages of the instruments which have been discussed are these: 1. The values obtained i n moisture determinations by these instrument methods r e f l e c t , either d i r e c t l y or i n d i r e c t l y , the security with which the water i s held i n the s o i l , meas-ures of the a v a i l a b i l i t y to plants. S o i l texture and structure are automatically considered, a feature of paramount importance i n heterogeneous s o i l s . 2. Information of the moisture condition of the s o i l i s immediately available when the instrument reading i s taken, with no delay for weighings, ca l c u l a t i o n s , or other procedures. 3. After i n s t a l l a t i o n of the instruments, the s o i l and plants are not disturbed by successive samplings. In many experimental plots i t would be impractical to take adequate s o i l samples -31-frequently, due to the small area a v a i l a b l e . Instrument methods allow moisture measurements to he made as frequently as desired. 4. The cost of moisture studies i s much reduced due to savings i n manpower, equipment and laboratory space. In any method used f o r following s o i l moisture changes, there i s nearly always v a r i a b i l i t y i n the data obtained from r e p l i c a t e plots of the same moisture treatment. These differences may be due to any one or a combination of the following causes: 1. S o i l heterogeneity. Marked variations i n moisture-holding capacity often accompany variati o n s i n s o i l texture and structure. Under similar conditions s o i l s of high moisture-holding capacity are not dried to the permanent w i l t i n g percen-tage by plants as rapidly as those s o i l s of low moisture-holding capacity. S o i l heterogeneity may resu l t also i n large differences i n water percolation during i r r i g a t i o n . 2. Lack of uniform permeation of the s o i l by roots. This i s a primary cause of nonuniform drying of the s o i l . An a c c u r a t e ^ measure of s o i l moisture conditions cannot be obtained f o r a s o i l not having uniform root permeation, since over small areas there are steep cajillary p o t e n t i a l gradients, or large differences i n moisture content, and the value obtained i n any single determination depends upon the p o s i t i o n of the sampling location with respect to plant roots. 3. Uneven growth of plants. Sometimes plants on experimental areas of si m i l a r treatment grow unevenly, due to differences -32-i n f e r t i l i t y , physical condition of the s o i l , or other f a c t o r s . Large plants usually transpire water from the s o i l more rapidly and i n greater quantity than do small plants. 4. Inequalities i n s o i l surface. These may resu l t i n uneven wetting of the s o i l by i r r i g a t i o n or r a i n f a l l . t'A COMPARISON OF METHODS FOR MEASURING: MOISTURE CONTENT OF THE SOIL IN SITU* o Several investigators have attempted to obtain comparable data on the accuracy and general s u i t a b i l i t y of four d i f f e r e n t methods of measurement. These methods are: 1. Gravimetric sorption plugs 2. Bouyoucos Plaster of Paris blocks 3. Electrothermal units 4. Tensiometers The opinions and conclusions of the aforementioned investigators are now presented. Gravimetric Sorption Plugs. Slater and Bryant (34) state that the gravimetric plugs used i n t h e i r investigation were subject to d i s i n t e g r a t i o n . The.effects of s o l u b i l i t y were confined l a r g e l y to the casing which protects the plug. The change i n dry weight of the plug did not exceed + 0.278 percent. The units are e a s i l y i n s t a l l e d and may be produced at low cost. Readings can be made quickly, although not so quickly as on resistance blocks, i t i s necessary to either weigh the plug i n the f i e l d or to remove i t i n a small closed container to some ce n t r a l l o c a t i o n . The gravimetric units are not materially affected by temperature of el e c t r o l y t e concentrations, and no corrections -33-of the basic data are necessary. The instrument responds to temperature only as temperature potentials cause a trans-? l o c a t i o n of s o i l moisture. Gravimetric sorption plugs measure 9.7 percent of the moisture percentage scale, or 58.4 percent of the availa b l e moisture scale, on a s i l t loam s o i l . The gravimetric plug i n i t s range of operation occupies a pos i t i o n intermediate between the Bouyoucos block and the tensiometer. On a s i l t loam s o i l the plugs were better than the blocks with respect to errors of estimate, e f f i c i e n c i e s of d i f f e r e n -t i a t i o n , and location of th e i r s e n s i t i v e ranges on an available moisture scale. On a very sandy s o i l . t h e plugs appeared to be i n f e r i o r to the blocks i n a l l respects, although the performance of neither instrument was s a t i s f a c t o r y throughout the available moisture range. It i s the opinion of Slater and Bryant that under a majority of s o i l conditions, gravimetric sorption plugs are best adapted to accurate measurement. K e l l y et a l . (24) obtained only limited information with these units since only 13 units'were a v a i l a b l e . In view of the rela t i o n s h i p between moisture content and moisture tension i n gypsum (21), these units were expected, by K e l l y et a l , to r e g i s t e r maximum changes below four atmospheres tension, with considerable changes taking place below one atmosphere. F i e l d data did not bear this out and K e l l y et al- at the time of investigation, were not able to make a direc t c a l i b r a t i o n -34-of these units versus moisture tension i n the laboratory. The authors f e e l that an i n s u f f i c i e n t number of units were used to give this method a f a i r t r i a l , but on the basis of the data obtained they were not as s a t i s f a c t o r y as the Bouyoucos blocks. The data suggests a considerable lag i n approaching equilibrium. Their main advantage i s i n being not affected by s a l t s . Bouyoucos Plaster of Paris Blocks. Slater arid Bryant (34) state that the Bouyoucos p l a s t e r of paris blocks appear to be adapted to f i e l d use in a number of respects; they are not d i f f i c u l t to i n s t a l l and measurements can be made quickly, the cost of the.individual unit is low, making considerable r e p l i c a t i o n f e a s i b l e . Slater and Bryant indicate several disadvantages of the Bouyoucos blocks; t h e i r i n s e n s i t i v i t y at the higher moisture l e v e l s , the p o s s i b i l i t y of e f f e c t of e l e c t r o l y t i c concentra-t i o n , the effect of temperature, the d i s i n t e g r a t i o n of the blocks over a period to time. The Bouyoucos blocks measure 11.8 percent of the moisture percentage scale or 57.2 percent of the available moisture scale i n a s i l t loam s o i l . The upper l i m i t of block operation coincides approximately with the lower l i m i t of tensiometer operation. On a s i l t loam s o i l the Bouyoucos blocks were i n f e r i o r to the gravimetric plugs with respect to errors of estimate, e f f i c i e n c i e s of d i f f e r e n t i a t i o n , and l o c a t i o n of t h e i r -35-sensitive ranges on an available moisture scale. On a very sandy s o i l the blocks appeared to be superior to the gravimetric plugs, i n . a l l respects, although the performance of neither instrument was s a t i s f a c t o r y through-out the available moisture range. It i s the opinion of Slater and Bryant that Bouyoucos* plaster of paris blocks are best adapted to semi-quantitative measurements of the variable f i e l d moisture of large areas under conditions that do not require the improved accuracy, that can be obtained by temperature corrections. The r e l a t i o n of the Bouyoucos block resistance to moisture tension has been determined by Haise and K e l l y (21). K e l l y et al, (24) states that Bouyoucos blocks w i l l measure s o i l moisture held at tensions of from 1 to 15 atmospheres or more. It i s important that the pore s i z e of the gypsum be similar f o r a l l blocks, since water drains from large pores at lower tensions than from small pores. It i s the opinion of K e l l y et a l . that Bouyoucos 1 blocks are the most p r a c t i c a l instruments a v a i l a b l e at the present time f o r measuring moisture changes at tensions above one atmosphere, and that there are, however, c e r t a i n l i m i t a t i o n s to the accuracy obtainable by t h e i r use. Electrothermal Units. Oummings and Chandler (12) conclude that electrothermal data showed extreme v a r i a b i l i t y between units and did not give a s a t i s f a c t o r y measure of s o i l moisture conditions i n -36-th e f i e l d . Laboratory studies, on the other hand, gave good r e s u l t s . ' The d i f f i c u l t y of obtaining and maintaining adequate contact between s o i l and unit .under f i e l d conditions seemed to be the p r i n c i p a l source of d i f f i c u l t y . E e l ley et a 1.(24) state that the thermal units used i n t h e i r investigation were capable of following changes i n s o i l moisture tension from s l i g h t l y less than one atmosphere up to about four atmospheres (21). Thus they cover a considerably wider tension range than the tensiometers, but w i l l not measure moisture at tensions below one atmosphere as accurately as w i l l the tensiometers. The thermal method has the advantage over the Bouyoucos block method that readings are apparently not affected by the presence of s a l t s . K e l l y et a l . (24) indicate c e r t a i n disadvantages to t h i s method, which include physical properties and dimensional losses due to the s o l u b i l i t y of the gypsum. At i t s present stage of development, the thermal unit i s not as useful f o r following s o i l moisture changes as either the tensiometer or Bouyoucos block. ' Tensiometers. Sla t e r and Bryant (34) are of the opinion that the tensiometers gave t h e i r best performance i n a range of higher moisture eontente, and that tensiometers are decidedly limited i n usefulness, since their operation i s r e s t r i c t e d to less than h a l f of the available moisture range. K e l l y et al.(24) consider tensiometers to be the most accurate and p r a c t i c a l instruments for measuring s o i l moisture tension within the range over which they function. Their usefulness i s l i m i t e d , however, by the f a c t that at 850cm. tension some s o i l s may have l o s t less than h a l f the water available to pla n t s . The maintenance and operation of tensiometers presents l i t t l e d i f f i c u l t y . •38"» Experimental The Plaster of Paris Block method for measuring s o i l moisture i n S i t u as proposed by Bouyoucos and Mick (7) was selected f o r further study since i t appears to be one of the more promising methods. The review, of l i t e r a t u r e dealing with the method revealed that results obtained are affected by several factors, notably: (1) The method of c a l i b r a t i n g blqcks i n the s o i l , (2) The temperature of the s o i l , (3) The soluble s a l t content of the s o i l . With these i n mind a series of experiments were conducted to obtain information with respect to the method and the influence that these factors have on i t s p r a c t i c a b i l i t y . ~ . Selection of S o i l Samples. It was considered desirable to study the a p p l i c a t i o n of the method i n s o i l s d i f f e r i n g i n moisture holding character* For t h i s reason s o i l samples were selected that d i f f e r e d greatly i n t e x t u r a l , s t r u c t u r a l and chemical properties. The s o i l s used were from the Prince George d i s t r i c t of B r i t i s h Columbia and were from the surface and subsoils o f three s o i l s e r i e s , Eega sandy loam, Bednest* s i l t loam and Pineview clay.''' The Eena sandy loam i s a Grey wooded s o i l derived from g l a c i a l f l u v i a l material. A description of a representative p r o f i l e i s as follows: 1. B r i t i s h Columbia S o i l Survey Report No. 2, Zelowna, B.C. March 1946. -39-Sample Depth. Description. 0 - 6 " Whitish brown ash-like and p l a t y sandy loam. Finger l i k e pofikets extend downward for several inches around roots. pH 5.8. 6 - 18" Light brown medium to coarse loamy sand, with scattered small g r a v e l . Firm and structureless BH 6.5. 18 - 30" Greyish brown structureless loamy sand, small amounts of s i l t due to t h i n s i l t y bands i n the parent material 6.4. 30 - 42" Grey s t r a t i f i e d sand with brownish colour inclusions 6.8. The Bednest* s i l t loam is a Grey Wooded s o i l derived from g l a c i a l - f l u v i a l material. A description of a representative p r o f i l e i s as follows: Sample Depth. Description. 0 - 6 " Ash grey to brownish s i l t loam, fine granular structure i n p l a t y arrange-ment, F r i a b l e and porous. )»'H 4.65. 6 - 14" Yellow brown to grey brown s i l t y clay loam with soft fragmentary structure breaking e a s i l y into granules. Compact but f a i r l y porous. ^H^5.82. 14 - 26" Brown s i l t y clay loam with large fragmentary structure, less e a s i l y broken and heavier than horizon above compact but well drained. pE 6.00 26 - 34" Buff coloured s t r a i i f i e d s i l t i n compact layers about one eighth inch thick. No stones, g r i t or gravel. BH 6.65 The Pineview clay i s a Grey Wooded s o i l derived from lacustrine material. A description of a representative profile, i s as follows: -40-Sample Depth. Description. 0 - 6" Upper part of horizon f r i a b l e mineral s o i l , with small granular structure. Lower portion i s l i g h t grey clay with granular structure becoming larger with depth. pH 5.1. 6 - 14" Brown heavy clay with s l i g h t l y reddish tinge. Large fragmentary structure and heaviest texture of any horizon. Cracks v e r t i c a l l y on drying 5.2. 14 - 20" Heavy clay but not as heavy as horizon above. Light grey broken laminations alternating with dark ones. No evidence of accumulated lime. "pH 6.3. 20 - 26" Heavy varved clay, with t h i n horizontal cleavage. Alternation of l i g h t and dark layers with range of colour from l i g h t grey to greyish brown. ^H 7.56. Estimation of Moisture Equivalent and Permanent Wilt i n g Percentage. In evaluating the Plaster of Paris Block method f o r estimating s o i l moisture one of the important features i s i t s range of s e n s i t i v i t y and the rela t i o n s h i p of this range to s o i l moisture avai l a b l e to pl a n t s . The range of s o i l moisture c r i t i c a l f o r the growth of plants usually i s taken as that between the permanent w i l t i n g percentage and the moisture equivalent. In.evaluating the method these two points there-fore were determined for the s o i l s under study. In determining the permanent w i l t i n g percentage the samples were a i r - d r i e d and passed through a screen having 2 mm. openings. An inch of s o i l was then placed i n the "bottom of a p i n t - s i z e ice cream container and packed l i g h t l y . The plaster block^was then placed on end on this s o i l and more s o i l was placed around the block with l i g h t packing. S o i l was added with l i g h t packing u n t i l the block was covered and the s o i l surface within one-half inch of the top of the carton. Four sunflower seeds were then planted and the pots were kept moist with either top water or nutrient so l u t i o n . When the plants were about 12 inches t a l l and with four or f i v e sets of leaves the permanent w i l t i n g percentage and the accompanying block resistance^were determined. They were then moved to a humidity chamber f o r 16 to 24 hours. If the plants regained turgor, they were removed from the humid-i t y chamber and allowed to transpire more moisture from the s o i l . When the plants f a i l e d to regain turgor they were considered to be permanently wi l t e d . The percentage moisture content of the s o i l was then determined by weighing the pot and i t s contents. In the determination of the moisture equivalent the sample's were a i r - d r i e d , the lumps crushed, and the s o i l passed through a screen having 2mm. openings. Determination of the moisture equivalent by the centrifuge method ' of Briggs and McLane was made on duplicate samples. 1. The Plaster blocks and conductivity bridge were purchased from In d u s t r i a l Instruments Inc. Z. Briggs, L.J. and McLane, J.W., "The moisture equivalent of s o i l s . " U.S. Dept. Agr. Bur.-Soils Bui.45, 1907. 42-Permanent Wilting Percentage, Moisture Equivalent. J•_?(*(? us.?.ion ^r. and G r i t i o a l Moisture Range i n Samples. The permanent Wilting percentage, moisture equivalent and c r i t i c a l moisture range for the s o i l samples studied are presented i n Table 1. TABLE 1. MOISTURE CONSTANTS AND THE CRITICAL MOISTURE RANGE FOR EENA SANDY LOAM, BEDNESTI SILT LOAM, AND PINEVIEW CLAY. S o i l Series Sample Depth Ins. Texture Perm. Moisture Equiv.% C r i t i c a l Mois. Range fo ( a v a i l , moisture 0-6 • sandy loam 5.0 19.1 14.1 0-6 sandy loan 5.5 22.5 17.0 Eena 6-18 loamy sand 4.0 12.5 8.5 18-30 loamy sand 4.0 15.1 11.1 30-42 sand 4.5 16.3 11.8 0-6 s i l t loam 7.0 30.5 23.5 0-6 s i l t loam 12.0 37.3 22.3 Bednesti 6-14 s i l t clay loam 3.8 31.8 28.0 14-26 i t i i t i 6.7 31.9 25.2 26-34 s i l t - 6.5 32.4 25,9 0-6 clay 21.5 37.4 15.9 0-6 clay 21.0 36.2 15.2 Pineview 6-14 heavy clay 27.0 36.8 9.8 14-20 heavy clay 23.5 37.9 14.4 20-26 clay 26.7 39.5 12.8 Some of the major points i n connection with t h i s table should be noted. The samples showed a range i n permanent w i l t i n g percentage from 3.8 percent to 27.0 percent and the moisture equivalent from 12.5 percent to 39.5 percent. In the case of the c r i t i c a l moisture the range was from 8.5 percent to 28.0 percent. From these values i t i s evident that the samples -43-selected showed a wide variation i n moisture holding Ch a r a c t e r i s t i c s . The Eena sandy loam samples have the l i g h t e s t texture, the lowest permanent w i l t i n g percentages and moisture equivalents of the three s o i l s e r i e s ; the subsurface horizons are esren l i g h t e r i n texture than the surface horizons. The Bednesti samples are of medium texture and are quite uniform, changing but l i t t l e with depth. The Pineview clay samples are the heaviest i n texture and have the highest moisture equivalent. The subsurface samples are s l i g h t l y heavier, i n texture and have higher moisture equivalents than the surface samples. <Dhe permanent w i l t i n g percentages and c r i t i c a l moisture ranges f o r the samples show very inte r e s t i n g r e l a t i o n s h i p s . The Eena samples have very low w i l t i n g percentages, the Bednesti intermediate, and the Pineview samples have very high w i l t i n g percentages. These s t r i k i n g differences have a pronounced ef f e c t upon the c r i t i c a l moisture ranges and i t w i l l be noted that the l i g h t textured Eena samples have c r i t i c a l moisture ranges equal to the very heavy Pineview clay samples. In p a r t i c u l a r , the 6-18 inch sample of Pineview clay shows a c r i t i c a l moisture range of only 9.8 percent, which i s considerably lower than the surface and subsoil samples of Eena sandy loam. Although the Bednesti samples are l i g h t e r i n texture and have lower moisture equivalents than the Pineview clay samples, the c r i t i c a l moisture range f o r the Bednesti samples &.is wider. The explanation for these 44-relationships l i e s i n the marked differences i n structure of the samples. The Bednesti series has good structure while the Pineview series has i n f e r i o r structure . In view of these facts i t i s evident that the samples chosen were very s a t i s f a c t o r y f o r use i n t e s t i n g a method designed to estimate s o i l moisture and to r e l a t e i t to the c r i t i c a l moisture range f o r plants. Establishing the Relationship Between Block Resistance and S o i l Moisture. Of the several methods f o r determining the r e l a t i o n -ship between s o i l moisture content and block resistance mentioned i n the l i t e r a t u r e , one of the more s a t i s f a c t o r y appears to be that suggested by Anderson and Edlefsen (4). This method avoids the main disadvantage of that o r i g i n a l l y proposed by Bouyoucos and Mick (7) and i s r e l a t i v e l y simple. With only s l i g h t modifications, i t was used i n the present study. The s o i l used was a i r - d r i e d , the lumps crushed and the s o i l passed through a screen with 2mm. openings. About one inch of s o i l was placed i n the bottom of a p i n t - s i z e ice cream carton and packed l i g h t l y . The p l a s t e r block was then placed on end on t h i s s o i l and more s o i l was l i g h t l y packed around i t . S o i l was added u n t i l the block was covered by s o i l and u n t i l the surface was about one-half inch from the top of the carton. Four sunflower seeds were then planted and the pots were i r r i g a t e d with either tap water or nutrient s o l u t i o n . When the plants had reached a height of -45-about 12 inches the s o i l surface was covered with overlapping sheets of lead f o i l which served to reduce evaporation while allowing easy watering. S o i l moisture and block resistance determination were not commenced u n t i l the sunflower roots apparently had permeated the entire s o i l mass about the plaster block. At that time, watering was stopped and the plants were allowed to remove the s o i l moisture by t r a n s p i r a t i o n . At in t e r v a l s resistance readings of the buried blocks were taken and at the same time the s o i l moisture content of the s o i l was estimated by weighing the entire pot and i t s contents. When the plants were temporarily wilted the s o i l moisture was brought to a high l e v e l and the process repeated. In a l l , t h i s cycle was repeated f i v e to eight times f o r each pot, moisture and resistance readings being obtained i n each case at intervals as the moisture was removed by t r a n s p i r a t i o n . To f a c i l i t a t e c a l c u l a t i o n of s o i l moisture contents at the end of .the c a l i b r a t i o n t r i a l , the weights of the carton, oven dry s o i l , plug lead, stake and lead f o i l were recorded. At the end of the t r i a l , the weight of the plants on each pot was recorded. These weights were subtracted from the t o t a l weight of the carton of moist s o i l i n order to obtain the percentage moisture content of the s o i l . On the f i n a l cycle, the plants were allowed to reach permanent w i l t i n g . They were then moved to a humidity chamber for 16-24 hours. I f the plants regained turgor, they were removed from the humidity chamber and allowed to -46-transpire more mois tare from the s o i l . When the y f a i l e d to regain turgor they were considered to be permanently w i l t e d . The s o i l moisture content was then determined. To permit the correction of the block resistance readings f o r temperature v a r i a t i o n , s o i l temperatures were recorded f o r each resistance reading. In correcting the resistance readings the method suggested by T.C. Peele to Slater and Bryant (34) was used. A l l readings were corrected ^to 70°F. -47-Pisoussion of the Block Kesistance-Soll Moist are Carves» Block r e s i s t a n c e - s o i l moisture curves f o r the s o i l series Eena sandy loam, Bednesti s i l t loam and Pineview clay are presented i n F i g s . 1-15. It w i l l he noted from these carves that as the s o i l moisture content decreases the block resistance increases and that t h i s rate of increase as shown by the slope of the carves i s greater f o r s o i l s with narrow c r i t i c a l mo is tore ranges. For example, Fi g s . 6,7,8,9 and 10 represent s o i l s with wide c r i t i c a l moisture ranges and the corresponding curves have gentle slopes. F i g s . 1,2,3,4,5,11,12,13,14 and 15 represent s o i l s with narrow c r i t i c a l moisture ranges and the resistance-moisture curves are steep. The figures also show that the resistance-moisture curves are very steep i n the v i c i n i t y of the w i l t i n g -percentage,' the rate of change being i n the order of 20,000 to 40,000 ohms f o r each percent change in s o i l moisture. As the moisture equivalent i s approached the curves become f l a t t e r and the rate of change i s only i n the order of a hundred ohms for each percent change i n s o i l moisture. At moisture contents above the moisture equivalent the ecurves are almost f l a t . These curve c h a r a c t e r i s t i c s are i n agreement with the observations made by Bouyoucos and Mick (7). In fac t i t i s the great change i n block resistance that accompanies small changes in s o i l moisture i n the lower portion of the c r i t i c a l moisture range that makes the method of i n t e r e s t . -48-After studying s o i l moisture-resistance curves f o r a number of s o i l s Bouyoucos and Mick concluded that for a l l p r a c t i c a l purposes the w i l t i n g point may be considered as occuring between 60,000 to 75,000 ohms, and that saturated s o i l s w i l l give readings of 400 to 600 ohms. It i s of inter e s t to compare the values obtained f o r the s o i l s under study as indicated by F i g s . 1 to 15. Examination of these figures reveals that of the 15 samples tested, 10 of the curves obtained are i n reasonable agreement with the l i m i t s suggested by Bouyoucos and Mick, these compared to F i g s . 1,2,3,5,9,11,12, 13,14 and 15. For example Fi g s . 1 and 2 f o r Eena sandy loam show permanent w i l t i n g percentages of about f i v e percent and a block resistance at: t h i s moisture of about 70,000 ohms, while F i g s . 11 and 12 f o r Pineview clay with a permanent w i l t i n g percentage of 22 percent shows a resistance of 80,000 ohms at t h i s moisture content. The block r e s i s t a n c e - s o i l moisture curves that do not agree reasonably well with the l i m i t s suggested by Bouyoucos and Mick are those shown i n F i g s . 4,6,7,8 and 10. The curves i n F i g s . 4,6,8 and 10 indicate resistance readings of considerably less than 60,000 ohms at the w i l t i n g percentage while the curve i n F i g . 7 indicates a resistance of over 4,000 ohms at the moisture equivalent. With respect to a possible explanation f o r these discrepancies i t i s of interest to note that examination of the cartons i n which low resistance readings were obtained revealed rather poor root development. In f a c t , i n some cases, the roots were confined to the upper -50 portion,of the pot and consequently did not have good d i s t r i -bution through the s o i l adjacent to the p l a s t e r paris block. In view of t h i s f a c t the low resistance readings may have been due to a lag i n response of the blocks to change i n s o i l moisture. This e f f e c t has been described by various investigators and i n p a r t i c u l a r Anderson and Edlefsen (4) have concluded that s a t i s f a c t o r y performance of the blocks depends upon the presence of roots. The present results would tend to confirm t h i s conclusion. A further observation which supports the suggestion that the low resistance readings were due to a lag i n response i s that the resistance readings gradually increased when the pots were held at constant moisture i n a humidity chamber. An examination of the figures also reveals that i n some cases, the resistance readings f a l l close together along a smooth curve, e.g. F i g s . 1,3,4,5,8,9,10 and 11. However i n other cases, marked scattering of points i s evident, as i n F i g s . 2,6,7,12,13,14 and 15. Some explanation of this scattering e f f e c t may be offered by r e f e r r i n g to the colouring of the points from which the curves have been drawn. Thus i n Figures 12,13,14 and 15 representing Pineview clay, some of the points l y i n g to the lower l e f t of the curves are c i r c l e d i n black. It appeared that these readings resulted from the fact that when the moisture was reduced to the v i c i n i t y of the w i l t i n g percentage, severe cracking resulted. Upon i r r i g a t i o n , the water penetrated these cracks and produced lo c a l f l o o d i n g . U n t i l t h i s moisture d i s t r i b u t e d i t s e l f -51-the resistance readings remained low. In F i g s . 2,6 and 7 red and green c i r c l e s have been used to i l l u s t r a t e an i n t e r e s t i n g effect that was observed with repeated drying. The red c i r c l e s represent readings obtain-ed during the f i r s t drying cycle and the green c i r c l e the second drying c y c l e . It was also noted that the readings are displaced downward with the second and subsequent cy c l e s . No e n t i r e l y s a t i s f a c t o r y explanation may be offered f o r these r e s u l t s . It should be noted however that the greatest displacement appears i n Figs. 6 and 7 which are duplicates of the same s o i l . This would suggest that the ef f e c t may be associated with a s o i l c h a r a c t e r i s t i c . Some displacement i s evident i n other figures as well though not as marked. These results suggest that i n preparing curves of this type more than one drying cycle should be-studied. Of 15 s o i l samples studied, the curves f o r 14 of these s o i l s suggest that the Plas t e r of Paris Block method serves well for measurementoof s o i l moisture. For 10 of the s o i l s the block resistance at the permanent w i l t i n g percentage and saturation agree reasonably well with the l i m i t s suggested by Bouyoucos and Mick (7), while the block resistances at the w i l t i n g percentages of four s o i l s lack close agreement. The lack of agreement i n these four s o i l s i s thought to be due to lag i n response caused by poor root development. However, the s u i t a b i l i t y of the method cannot be discredited for t h i s reason by these four s o i l s . One other s o i l lacked agreement with the suggested l i m i t s but the reason for this i s unknown. -63-FI6.2 BLOCK RESISTANCE-SOIL MOISTURE CURVES FOR EENA SANDY LOAM 0-6 INCHES -54--55--57-FIG.6 BLOCK RESISTANCE—SOIL MOISTURE CURVES FOR BEDNESTI SILfzLOAM 0—6 INCHES -58-& 7 & f ff£sysmvc£-smtEAfo/swM CURVE FOR BEDNESTI SILT LOAM -66--67. S o i l Temperature Measurement and Resistance Correction As indicated i n the previous sections i t usually i s considered desireable to correet the Plaster of P a r i s Block resistance readings i n accordance with changes i n s o i l temperature. However, unless increased accuracy i s desired, correction of resistance measurements fo r temperature f l u c t u a t i o n i s unnecessary (7). Seventy degrees Fahrenheit i s usually taken as a standard reference temperature. T.C. Peele i n communication with S l a t e r and Bryant (34) suggests an equation that provides the correction of resistance readings to t h i s temperature. This equation is Log R (t=70) = Log Rfc [ l + .002(t-70)J where R-= observed resistance, R(t - 70) the resistance corrected to 70°F. and t = the temperature i n degrees Fahren-h e i t . A short series of tests-were conducted to appraise the s u i t a b i l i t y of t h i s correction. In making these tests the calculated correction was ehecked i n s o i l s with very low soluble s a l t content and with the same s o i l to which varying amounts of soluble s a l t had been added. The results of these tests are given i n Table 2. The r e s u l t s presented i n Table 2 indicate that the method of correction i s s a t i s f a c t o r y f o r the purpose intended. 68-TABLE 2 . THE SUITABILITY OF A TEMPERATURE CORRECTION FACTOR FOR USE IN SOILS CONTAINING VARIABLE AMOUNTS OF SOLUBLE SALT. Resistance i n hundred ohms. Salt. Calc. Difference i n Cart on Content R.@70°F R.@80°F R.@70°F resistance betwee p .p .m ohms ohms ohms observed and calc Na 2S0 4 values 101 500 1800 1400 1820 20 10S 1500 960 770 978 18 103 2500 490 405 500 10 104 4000 780 625 776 4 105 6000 690 57 5 724 34 106 10000 1250 1020 1290 40 202 1500 1720 1400 1780 60 204 4000 1070 8700 1150 800 205 6000 8100 6300 8320 :.220 206 10000 9250 7200 9340 90 301 500 1020 800 1000 20 305 6000 665 540 675 10 When attempting to determine s o i l temperature i n s i t u much the same problems are encountered as i n measuring s o i l moisture i n s i t u . To be e n t i r e l y s a t i s f a c t o r y f o r t h i s purpose the method should combine these c h a r a c t e r i s t i c s ; (1) accuracy of estimate, accuracy to 1°F. i s suitable for t h i s purpose, (2) s u i t a b i l i t y f or use at any s o i l depth and s.n under any moisture condition, (3) speed and s i m p l i c i t y of op-eration. In attempting to f i n d a suitable method i t was hoped that a method could be found which would u t i l i z e the same conductivity bridge required f o r the P l a s t e r of Paris Block method. For t h i s reason resistance thermometers were given -69-S p e c i a l consideration. A f t e r some investi g a t i o n a resistance thermometer was found that seemed to possess the necessary c h a r a c t e r i s t i c s . This resistance thermometer i s manufactured by the Western E l e c t r i c Company and i s catalogued as Thermistor Type 14A. T h i r t y - s i x of these Thermistors were purchased f o r t r i a l . The cost of each unit was $1.85. The thermistors as purchased were not equiped with leads and consequently leads of the same type as were used on the Plaster of Paris Blocks were ^soldered to the terminals. The soldered connection and a l l but about one-quarter inch at the end of the thermistor were then coated heavily with r De Khotinsky cement made by mixing dry orange shellac and pine tar i n the r a t i o of 3:1 by weight. This cement has good i n s u l a t i n g powers and considerable mechanical strength. In applying the cement i t was found necessary to carry i t well over the rubber insul a t i o n on the leads i n order to obtain a water-tight s e a l . When the thermistors were prepared they were immersed i n a constant temperature water bath and t h e i r resistances determined over a. temperature range from 32°]?. to 120°F. In taking the readings the water temperature of the bath was held to within * 0.5°3?. of the desired temperature. The readings were taken both ascending and descending i n order to check the thermistors f o r constancy. A l l readings were taken with the resistance bridge used with the pla s t e r of paris blocks. -70-TABLE 3. THERMISTOR RESISTANCES AT VARIOUS TEMPERATURES Resistance i n hundred ohms.v 4. Temperature °F z 32 40 50 60 70 80 90 100 110 120 1 3725 2825 2275 1710 1230 965 742 581 462 365 2 3900 3300 2400 1760 1340 1055 815 645 510 410 5 3400 2800 2150 1620 1230 970 752 590 461 370 6 3175 2550 2150 1560 1120 882 680 532 423 335 7 3325. 2650 2150 163? 1205 955 740 585 461 363 8 3275 2600 1932 1515 1162 921 722 565 456 360 10 3725 2800 2150 1545 1170 940 725 580 456 370 12 3275 2600 2100 1560 1205 9 57 740 585 465 373 13 2900 2250 1790 1330 1040 830 642 506 404 321 14 3600 2870 2225 1725 1330 1042 810 627 496 385 16 3300' 2600 2025 1595 1227 9 70 757 592 471 375 17 3350 2700 2100 • 1540 1180 935 730 568 454 355 19 3575 2725 2100 1540 1175 885 770 557 420 340 21 3775 3000 2365 1730 1325 1050 . 812 635 501 400 22 3475 2725 2125 1625 1235 975 757 590 471 360 23 3350> 2650 2100 1630 1240 985 775 607 483 385 25 3075 2425 1975 1457 1120 890 710 556 450 350 26 2900 2425 1875 1435 1135 900 707 551 436 348 27 2750 2300 1850 1410 1097 890 691 546 430 345 28 3200 2670 2200 1610 1275 1002 792 620 490 390 29 3300 2425 1925 1445 1110 892 710 557 454 356 32 2975 2475 2050 1470 1175 942 727 576 452 355 34 2850 2250 1900 1440 1137 917 722 565 450 357 36 3075 2800 2000 1500 1195 960 757 647 472 375 average min 2750 2250 1790 1330 1040 830 643 507 404 321 absolute min 2600 2200 1780 1310 1040 820 640 505 402 average max 3900 3300 2400 1760 1340 1055 815 648 511 410 absolute max 3900 3350 2430 1770 1350 1060 820 695 513 average r e s i s t a n c e 3300 2670 2080 1558 1190 946 741 559 460 365 •F AT o hmS 7780 5920 5218 3680 2436 2050 1825 990 9 50 -71-i'able 3 gives the r e s u l t s of the thermistor c a l i b r a t i o n t r i a l . Results for only 24 thermistors are included as the res u l t s f o r the other 12 are highly v a r i a b l e . This v a r i a t i o n seems to be due to f a u l t y application of the i n s u l a t i n g cement. The values included i n Table 3 are the average of two resistance readings, one found with ascending and the other with descending temperature. Though not shown i n d e t a i l i n the table the agreement i n individual readings was found to depend upon the" temperature. Thus at 110°F the individual resistance readings were found to agree almost exactly, the difference corresponding to a temperature v a r i a t i o n of less than 0.5°F. However at 32°F. the range was wider and duplicate readings showed variation? i n resistance correspond-ing to temperature differences of 3°to 4°F. The reason f o r the v a r i a t i o n at the lower temperature i s thought to be associated with errors involved i n reading the thermistor resistances.- It was found very d i f f i c u l t to obtain a clear n u l l point. A modification i n the' bridge would be necessary to overcome t h i s d i f f i c u l t y . The data included i n Table 3 i s shown graphically i n Figs. 16-19. In these figures each curve represents the c a l i b r a t i o n of one thermistor. From the curves i t i s evident that a l l the thermistors show c h a r a c t e r i s t i c changes i n resistance with temperature. It i s also evident that while some thermistors show close agreement with each other, others show considerable curve displacement, the displacement being -72-greater at lower temperatures. This v a r i a t i o n would suggest that before use, each thermistor would have to be calib r a t e d separately. However, the curves are so uniformly changeable that c a l i b r a t i o n at three temperatures should be s u f f i c i e n t for each unit. C a l i b r a t i o n of each unit in a manner similar to that used i n t h i s experiment should make possible s o i l temperature readings to within * 0.250ff. at 100°F. and about * 1.5°2F. at 320F. Increased accuracy at the lower temperatures would probably depend upon obtaining an improved bridge. However, fo r the purpose intended, the present bridge would be s a t i s f a c t o r y provided the average of several resistance readings were taken. -73-FI6.16 CALIBRATION CURVES FOR THERMISTORS 60 80 100 TEMPERATURE °F. 120 -7b-F/G. 18 CALIBRATION CURVES FOR THERMISTORS TEMPERATURE °F. -77* EFFECT OF SODIUM SULFATE ON THE ESTIMATION OF SOIL MOISTURE BY THE PLASTER OF PARIS BLOCK METHOD. It has been indicated that the presence of e l e c t r o l y t e s i n the s o i l solution i s a f a c t o r to be considered i n the evaluation of the Plast e r of P a r i s Block Method. The observation of others that normal s o i l s of humid regions do not carry enough e l e c t r o l y t e i n solution to render the method impractical, has been substantiated by the studies just reported. However, s o i l s of the a r i d and semi-arid regions frequently carry appreciable amounts of soluble s a l t s . Since s i g n i f i c a n t areas of such s o i l s occur i t was considered desirable to investigate the e f f e c t that such.salts might have on the estimation of s o i l moisture by the Pl a s t e r of P a r i s Block Method. Sodium Sulfate i s one of the soluble s a l t s often found i n s o i l s of a r i d and semi-arid regions and i t was therefore selected f o r use i n the proposed study. In order to tes t the eff e c t of various concentrations of Na 2S0 4 on the moisture estimation, surface s o i l samples from the three s o i l series previously described were prepared i n a manner similar to that used i n the resistance block t r i a l s described i n a previous section. The experiment was set up i n such a way that the ch a r a c t e r i s t i c s of three groups of Pl a s t e r of Paris Blocks of di f f e r e n t manufactore could be compared. Group A and B blocks, purchased from I n d u s t r i a l Instruments Inc., were selected by the manufacturer to give maximum uniformity within each group. -78-Group C blocks were manufactured i n the laboratory by the procedure of Anderson and Edlefsen (4) and selected f o r uniformity on the basis ofsaturation resistance as suggested by Bouyoucos and Mick (7). The experiment also included s i x concentrations of Ka 2S0 4, 500, 1500, 2500, 4000, 6000 and 10,000 p.p.m. Five hundred gram samples of a i r dry s o i l were weighed into heavily waxed i c e cream cartons and subsamples taken f o r determination of the oven dry weight of s o i l used. Varying amounts of Glauber's Salt (NagSO^lOHgO) were added to the s o i l samples i n the cartons. The s o i l sample and s a l t were then emptied into a mortar and tap water was added to bring the moisture content to the moisture equivalent. The. mixture was s t i r r e d well to ensure good d i s t r i b u t i o n of the s a l t and water. About one h a l f - i n c h of the moist s o i l was packed l i g h t l y into the carton. A saturated plaster block was then placed on end on the s o i l , and more s o i l was l i g h t l y packed around the block. S o i l was added u n t i l the block was covered and the s o i l surface within one h a l f - i n c h of the top of the carton. The lead of the block was put through a hole i n the l i d of the carton and the l i d was slipped into place. The carton and l i d , as well as the hole through which the lead passed, were sealed with molten p a r a f f i n . Sealing with p a r a f f i n was intended to create a constant humidity within the carton i n order that moisture equilibrium would be established i n the minimum period of time. -79-At i n t e r v a l s resistance of the blocks was measured. A considerable l a g i n attainment of moisture equilibrium was observed, the time required ranging from 4 to 10 days. When the block resistance became constant, the cartons were opened, temperature measurement was made to provide f o r temperature correction, and moisture samples were taken of the s o i l i n contact with the block. The remainder of the s o i l i n the car* ton was spread t h i n l y on a piece of heavily waxed brown paper and allowed to dry f o r from one to f i v e hours. The s o i l was then mixed well and with the block was repacked into the carton and resealed with p a r a f f i n . This procedure was r e -peated many times u n t i l resistance measurements and moisture samples had been taken f o r the entire range of s e n s i t i v i t y of the pl a s t e r blocks. -80-FIG.20 BLOCK RESISTANCE-SOIL MOISTURE CURVES AT VARIOUS SALT CONCENTRATIONS EEEz GROUP A BLOCKS EEEl -82-FIG. 22 BLOCK RESISTANCE-SOIL MOISTURE CURVES AT VARIOUS SALT CONCENTRATIONS GROUP B BLOCKS -83-FIG.23 BLOCK RESISTANCE-SOIL MOISTURE CURVES AT VARIOUS SALT CONCENTRATIONS GROUP B BLOCKS SOIL MOISTURE -64-FIG.24 BLOCK RESISTANCE-SOIL MOISTURE CURVES AT VARIOUS SALT CONCENTRATIONS GROUP C BLOCKS -86-Dlscussion of Block Resistance - S a l t Content - S o i l Moisture  Curves. The r e s u l t s obtained from t h i s experiment are shown graphically i n F i g s . 20-25. In these f i g u r e s the block resistance - s o i l moisture curves are arranged by P l a s t e r of P a r i s Block group and by s o i l type. Examination of the curves i n F i g s . 20-25 with respect to block uniformity shows some good and some poor agreement. Group A blocks have the greatest uniformity and only three blocks appear to give r e s u l t s that vary s i g n i f i c a n t l y from the group as a whole. Group B blocks are not as s a t i s f a c t o r y , a high proportion of these showing inconsistencies. The group C blocks of laboratory manufacture gave the poorest performance of the three groups. The f a c t that a l l the blocks used gave readings at saturation which agreed within l e s s than 100 ohms, suggests that t h i s i s not a s u f f i c i e n t l y accurate measure of uniformity. This suggestion i s supported by the f a c t that some of the blocks gave moisture-resistance curves of d i f f e r e n t general slope character. From t h i s r e s u l t i t appears that to be assured of the consistent be-havior of the blocks, they should be checked at 500, 10,000 and 70,000 ohms. I t i s evident from the curves i n F i g s . 20-25 that added Na gS0 4 has affected the resistance readings, as would be expected, the curves i n general are displaced downward. In some cases the curves are displaced upward with further s a l t addition; t h i s displacement i s attributed to block v a r i a t i o n . -87-Also, i t appears from the curves that the ef f e c t varies i n di f f e r e n t s o i l s and at d i f f e r e n t moisture l e v e l s . In appraising the Plaster of Paris Block Method, the important point i s not so much the e f f e c t that s a l t content has on the actual block resistance, but i t i s rather the ef f e c t that s a l t content has on the moisture values i n t e r -preted from the resistance readings. To a s s i s t i n the appraisal of the method on t h i s basis Table 4 has been pre-pared from F i g s . 20-25. The s o i l moisture percentages r e -corded i n t h i s table were obtained by readings from the curves f o r the various s o i l s and s a l t additions the moisture percentages corresponding to 1000 and 50,000 ohms. It i s evident from Table 4 that additions of sodium sulfate lower the moisture estimate. The uniformity with which the moisture estimate i s lowered i s determined by the uniformity of the blocks. Thus the group B blocks, gave good uniformity of estimate i n the Bednesti s o i l as i s shown by the r e s u l t s i n Table 4 and also from an examination of Fig.21. In the case of blocks showing poor uniformity the moisture values are also v a r i a b l e . In f a c t i n some cases the e f f e c t of the added s a l t i s e n t i r e l y o f f s e t by block v a r i a b i l i t y * This e f f e c t may be e a s i l y observed i n the values f o r the group C blocks. The v a r i a b i l i t y i n moisture estimate introducted by the blocks has made evaluation of the e f f e c t of the s a l t added rather d i f f i c u l t . However, the e f f e c t may be appraised to some extent from Table 5. This table has been prepared to -88-show the extent of the lowering of the moisture estimate r e -s u l t i n g from each 500 p.p.m. addition of NagSO^.. In c a l c u l a t i n g the values f o r i n c l u s i o n i n t h i s table, only data obtained from' blocks showing good uniformity were used. The values i n Table 5 indicate that the added s a l t had about the same e f f e c t i n a l l three s o i l s , the magnitude of the e f f e c t being about the same i n the clay as i n the sandy loam. The table also shows that the moisture estimate was lowered by 0.44% on the average f o r each 500 p.p.m. increase at 1000 ohms-resistance, and that at 50,000 ohms resistance the moisture estimate was lowered by 0.31%. From these r e s u l t s i t i s evident that the v a r i a b i l i t y i n moisture estimate, introduced by a change of a few hundred parts per m i l l i o n of e l e c t r o l y t e i n the s o i l solution, i s small i n comparison to the error introduced by block v a r i a b i l i t y . For t h i s reason the successful operation of the method i n s o i l s of humid regions depends c h i e f l y upon s a t i s f a c t o r y block manufacture. -89-TABLE 4 EFFECT OF Na gS0 4 ADDITIONS ON SOIL MOISTURE DETERMINATIONS USING PLASTER OF PARIS BLOCKS ( S o i l moisture i n fo) S o i l Block Resistance s a l t added(p.p.m. dry we ight of s o i l Series Group ohms 500 1500 2500 4000 6000 10000 Eena A 1000 18.3 < 14.5 16.5 14.5 12.5 9.0 Sandy B 17.5 21.0 13. 12.5 13. . 12. Loam C 20.5 15.5 16.5 14.5 12.3 24.5 Bednesti A 26. 25.5 24.5 22. 19.5 11. S i l t B 25. 21. 23,.' 6 20. 20. 18. Loam C 31. 29.5 27. 25. 26.2 22.5 Pineview A 41.2 40.5 40. 42.5 37.3 36. Clay B 39.5 43.5 41.5 40. 38.5 36. G 45.5 43. 38.3 41.4 41.3 37.2 Eena A 50,000 9. 7.9 8.2 6.5 6.0 4.7 Sandy B 8.5 10. 6.5 5.5 6.0 5.5 Loam C 13. 11. 11.5 11. 8.7 10. Bedne s t i A 13. 12. 11. 9.5 9. 8. S i l t B 17. 14. 15.5 13. 14. 12. Loam C 17.5 12.2 11.5 13.5 10.5 14.7 Pineview A 31. 30.2 29.2 27.3 25.5 24.7 Clay B 31. 30. 29.5 28. 26.5 24.5 C 32.5 33. 29.5 29.3 30.5 31. TABLE 5 CHANGE IN MOISTURE ESTIMATION WITH EACH 500 p.p.m. ADDITION OF NapSO, S o i l Block Moisture Estimate Decrease (&) per 500 p.p.m. Series Group addition of Na £S0 4 LOW MEDIUM HIGH OVERALL BLOCK 500-2500 2500-6000 6000-10000 CHANGE RESIST-p.p.m. p.p.m. p.p.m. AN0E Eena A 6.45 1.55 0.44 0.49 1000 Sandy- B 1.1 0.17 0.71 ohms Loam C 1.0 0.6 0.75 Bednesti A 0.25 0.6 1.1 0.32 S i l t B 2.0 0.1 0.25 0.37 Loam C 0,75 0.9 0.2 0.45 Pineview A 0.3 0.4 0.2 0.22 Clay B 0,75 0.4 0.3 0.18 C 0.7 0.03 0.5 0.44 Ave $rage moie ture estimate decrease r 0.44 Eena A 0.2 0.6 0.15 0.16 50,000 Sandy B 0.5 0.17 0.04 0.13 ohms Loam C 0.25 0.6 0.39 Bednesti A 0.5 0.3 0.13 0.26 S i l t B 0.8 0.17 0.26 Loam C 1.5 0.14 0.64 Pineview A 0.45 0.5 0.1 0.33 Clay B C 0.37 0.45 0.25 0.34 Av« jrage moie i ture estime ite decrease s 0.31 -91 CONCLUSIONS The P l a s t e r of Paris Block Method is p r a c t i c a l f o r following s o i l moisture changes in s i t u . It operates s a t i s -f a c t o r i l y over the c r i t i c a l moisture range and shows i t s greatest s e n s i t i v i t y i n the v i c i n i t y of the permanent w i l t i n g percentage. A major source of v a r i a t i o n i n the method i s due to the Pla s t e r of Paris blocks. In selecting blocks f o r use t h e i r performance should be checked at both low and high resistances. The most s a t i s f a c t o r y c a l i b r a t i o n and performance of the P l a s t e r of Paris blocks i s dependent upon the presence of roots of tra n s p i r i n g plants i n the immediate v i c i n i t y of the blocks. S o i l temperatures may be estimated i n s i t u with the same resistance bridge used with the p l a s t e r of p a r i s blocks. Resistance thermometers found s a t i s f a c t o r y for t h i s purpose are Thermistors type 14A, manufactured by the Western E l e c t r i c Company. El e c t r o l y t e s i n the s o i l solution a f f e c t moisture estimates made with the P l a s t e r of P a r i s Block Method. The magnitude of the eff e c t produced with Na2S04 over the c r i t i c a l moisture range is 0,3 to 0.4 percent decrease i n moisture estimate with each 500p.p.m. increase i n s a l t . The magnitude of the eff e c t i s apparently the same i n s o i l s of d i f f e r i n g texture. ACKNOWLEDGEMENTS To Dr. C. A. Howies, Associate Pro-fessor i n the Department of Agronomy, under whose guidance the work was done; to Dr. D. G. Laird, Professor of S o i l s , f o r his helpf u l c r i t i c i s m ; and to the University Com mittee on Research f o r f i n a n c i a l assistance, the author wishes to express his sincere appreciation. BIBLIOGRAPHY 1. A l l y n , R.B., "A calibrated s o i l probe f o r measuring f i e l d s o i l moisture." S o i l Sc. 53:273-285 1942. 2. A l l y n , R.B i t and Work, R.A., "The availameter and i t s use i n s o i l moisture control:-1. The instrument and i t s use." S o i l Sc. 51:307-321 1941. 3. A l l y n , R.B., and Work, R.A., "The availameter and i t s use i n s o i l moisture c o n t r o l : - I I . C a l i b r a t i o n methods." S o i l Sc. 51:391-406 1941. 4. Anderson, A.B.C., and Edlefsen, N.E., "Laboratory study of the response of 2- and 4-electrode p l a s t e r of paris blocks as soil-moisture content indicators." S o i l Sc. 53:413-428 1942. 5. Anderson, A.B.C., and Edlefsen, N.E., "The e l e c t r i c a l capacity of the 2-eleetrode p i a s t e r of-paris blocks as an indicator of s o i l moisture content." S o i l Sc. 54:35-46 1942. 6. Bouyoucos, G.J., "Anew e l e c t r i c a l thermometer f o r s o i l s . " S o i l Sc. 63:291-298 1947. 7. Bouyoucos, G.J., and Mick, A.H., "An e l e c t r i c a l method for the continuous measurement of s o i l moisture under f i e l d conditions." Mich.Agr.Exp.Sta.Tech.Bui. 172:1-38 1940. 8. Bouyoucos, G.J., and Mick, A.H., "Comparison of absorbent materials employed i n the e l e c t r i c a l resistance method of making a continuous measurement of s o i l moisture under f i e l d conditions." S o i l Sc. Soc.Proc. 5:77-79 1940. 9. Bouyoucos, G.J., and Mick, A.H., "Improvements, i n the plaster of paris absorption b l o c k - e l e c t r i c a l resistance method f o r measuring s o i l moisture under f i e l d condi-tions." S o i l Sc. 63:455-465 1947. 10. Bouyoucos, G.J., and Mick, A.H., "A f a b r i c absorption unit f o r continuous measurement o-f s o i l moisture i n the f i e l d . " S o i l Sc. 66:217-232 1948. 11. Briggs, L.J., " E l e c t r i c a l instruments for determining the moisture, temperature and soluble s a l t content of s o i l s . " U.S. Dept. A g r . Bui. 15 1899. ff 12. Cummings, R.W., and Ohandler, R.F., J r . , "A f i e l d comparison of the electro-thermal and gypsum block, e l e c t r i c a l resistance methods with the tensiometer method f o r estimating s o i l moisture i n s i t u . " S o i l Sc. Soc. Amer. Proc. 5:80-85 1941. 13. Davis, W.E., and Slater, G.S., "A di r e c t weighing method for sequent measurements of s o i l moisture under f i e l d conditions." Jour. Amer. Soc. Agron. 34:285-287 1942. 14. Deighton, T., "Some investigations of the e l e c t r i c a l method of soil-moisture determination." Jour. Agr. Sc. 12:207 1922. 15. Edlefsen, N.E., and Anderson, A.B.C., "The four-electrode resistance method fo r measuring soil-moisture content under f i e l d conditions." S o i l Sc. 51:367-376 1941. 16. Fletcher,J.E,, "A d i e l e c t r i c method for determining s o i l moisture."- S o i l Sc. Soc. Amer. Proc. 4:84-88 1939. 17. Gardner, F.D., "The e l e c t r i c a l method of moisture determination i n s o i l s : results and modifications i n 1897." U.S. Dept. Agric. Bui. 12 1897. 18. Haines, W.B., "Studies i n the physical properties of s o i l s . I I I . Observations on the e l e c t r i c a l conductivity of s o i l s . " Jour. Agr. Sc. 15:536 1925. 19. Haines, W.B., "Studies i n the physical properties of s o i l s . IV. A further contribution to the theory of c a p i l l a r y phenomena i n s o i l s . " Jour. Agr. Sc. 17: 264-290 1927. - . 20. Haines, W.B., "Studies i n the physical properties of s o i l s . V. The Hysteresis effect i n c a p i l l a r y properties, and the modes of moisture d i s t r i b u t i o n associated therewith." Jour. Agr. Sc. 20:97-116 1930. 21. Haise, H.R. and Kelley, O.J., "The r e l a t i o n of e l e c t r i -c a l and heat conductivity to moisture tension i n pl a s t e r of paris blocks." S o i l Sc. 61:411-422 1946. Johnston, G.N.t "Water-permeable jacketed thermal radiators as indicators of f i e l d capacity and per-manent w i l t i n g percentage i n s o i l s . " S o i l Sc. 54:123 1942. Kelley, Omar J., "A rapid method of c a l i b r a t i n g various instruments f o r measuring s o i l moisture i n s i t u . " S o i l Sc. 58:433-440 1944. 22. 23. 24. Kelley, O.J., Hunter, A.S., Raise, H.R., and Hobbs, G.H., "A comparison of methods of measuring s o i l moisture under f i e l d conditions." Jour. Amer. Soc. Agron. 28:759 1946. 25. Xornev, V.G., "The suction force of s o i l s . " Zhurnal Opitnaii Agron-(Russian Jour. Exp. Agron.) 1924 22:105-111 1921-23. Abstract i n S o i l Sc. 17:428 1924. 26. McOorkle, W.H., "Determination of s o i l moisture by the method of multiple electrodes." Texas Agr. Exp. Sta. Bui. 426 1931. 27. Richards, L.A., and Gardner, W., "Tensiometers f o r measuring the c a p i l l a r y tension of s o i l water." Jour. Amer. Soc. Agron. 28:352-358 1936. 28. Richards, J.S., " S o i l moisture content calculations from c a p i l l a r y tension records." S o i l Sc. Soc. Amer. Proc. 3: 1938. 29. Richards, S.J., and Lamb, John J r . , " F i e l d measurements of c a p i l l a r y tension." Jour. Amer. Soc. Agron. 29:772-780 1937. -30. Richards, L.A., and Weaver, L.R., "The sorption-block s o i l moisture meter and hys t e r e s i s - e f f e c t s r,elated to i t s operation." Jour. Amer. Soc. Agron. 35:1002-1011 1943. 31. Rogers, W.A., "A s o i l moisture meter." Jour. Agr. Sc. 25:326-343-1935. 32. Shaw, Byron, and Baver, L.D.,."Heat conductivity as an index of s o i l moisture." Jour. Amer. Soc. Agron. 31:886 1939. 33. :' Shaw, Byron and Baver, L.D., "An electrothermal method fo r following moisture changes of the s o i l i n s i t u . " S o i l Sc. 'Soc. Amer. Proc. 4:78-83 1939. 34. Slater, C.S., and Bryant,.J.0., "Comparison of'four methods of s o i l moisture measurement." S o i l Sc. 61:131-156 1946. 35. Thome, M.D., and Russell, M.B., " D i e l e c t r i c properties of s o i l moisture and the i r measurement." S o i l Sc. Soc. Amer. Proc. 12:66 1947. -36. Whitney, M., Briggs, L.J., Gardner, F.D., "An e l e c t r i c a l method of determining the moisture content of arable s o i l s . " U.S. Dept. Agric. Bui.6 1897 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.831.1-0106847/manifest

Comment

Related Items