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Surface charge properties of selected soils Hendershot, William Hamilton 1978

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SURFACE CHARGE PROPERTIES OF SELECTED SOILS by WILLIAM HAMILTON HENDERSHOT B . S c , University of Toronto, 1972 M . S c , McGill University, 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Soil Science) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA June 1978 William Hamilton Hendershot, 1978 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s an advanced degree at the U n i v e r s i t y o f B r i t i s h Co lumb ia , I ag ree t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s tudy . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d tha t c o p y i n g o r p u b l i c a t i o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i thout my w r i t t e n p e r m i s s i o n . Department o f Soil Science The U n i v e r s i t y o f B r i t i s h Co lumbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 June 8, 1978 - i -ABSTRACT The aim of this thesis was to examine the use of surface charge characteristics as a measure of pedogenic development in selected soi ls of the humid, temperate environment of southwestern Brit ish Columbia. A secondary objective, using a wider range of soi l types, was to gain further knowledge of the factors which influence the values of ZPC (the zero point of charge) and (its displacement from the zero point of t i t ra t ion , ZPT), so that these measurements might be better understood. Measurement techniques, using potentiometric t i t r a t i o n , have a s i g -nif icant effect on the values of ZPC and obtained in the laboratory. The fast-adsorption procedure measures the fast reaction taking place at the part ic le surface, while the slow-adsorption procedure measures the additional slow reaction resulting from the incorporation of H + and OH" ions into the structure of oxide coatings. Therefore the values obtained using the slow-adsorption procedure result in higher ZPC values and lower values than the fast-adsorption procedure. NaCl-saturation of the exchange complex pr ior to t i trat ion results in the exchange of strongly bonded ions such as A l 3 + and P0h3~ and thus alters the proper-ties of the surface. In the case of untreated samples with ZPC values below pH 5.0 and Oj values less than -0.2 meq/lOOg there was a shi f t to lower ZPC and values; i f the values for the untreated samples were above th is , then the shift was in the opposite direct ion. The fast-adsorption method using untreated samples is therefore considered to be the most accurate means of measuring the surface charge properties as they exist in the f i e l d . - i i -Indirect evidence suggested that the presence of organic matter caused a decrease in the ZPC and values. This evidence is in the form of simple correlation coefficients between organic C and a... The results of part ial correlation analysis removing the effect of organic matter indicate strong relationships between ZPC and so i l age and also between cr.j and so i l age. Evidence of this relationship is also supported by direct measurement. The removal of a portion of the organic matter using a pretreatment of NaOCl resulted in a significant increase in both the ZPC and values. Potentiometric t i trat ion is a useful measure of surface charge cha-racterist ics and of so i l genesis. In soi ls with large amounts of clay minerals, as the sesquioxide content increased the ZPC became better defined and moved to a higher pH and a- approached zero. In sandy s o i l s , the values of ZPC and are only s ignif icantly related to so i l age when the effect of so i l organic matter content has been eliminated. On the other hand, the ApH value, defined as the difference between ZPC and pH(KCl), compensates for differences in organic matter and provides a good indicator of the extent of soi l development. i i i -TABLE OF CONTENTS 1. INTRODUCTION 1 Li terature Ci ted 5 2. THE USE OF ZPC TO ASSESS PEDOGENIC DEVELOPMENT 6 Abstract 6 Introduction 7 Materials and Methods 10 Results and Discussion 11 Conclusions 21 Literature Cited 22 3. MEASUREMENT TECHNIQUE EFFECTS ON THE VALUE OF ZERO POINT OF CHARGE AND ITS DISPLACEMENT FROM ZERO POINT OF TITRATION 24 Abstract 24 Introduction 25 Materials and Methods 26 Results and Discussion 26 Conclusions 31 Literature Cited 32 4. VARIATION IN SURFACE CHARGE CHARACTERISTICS IN A SOIL CHRONOSEQUENCE 33 Abstract 33 Introduction 34 Materials and Methods 35 Results and Discussion 36 Conclusions 46 Literature Cited 47 - iv -5. THE EFFECT OF NaCl-SATURATION AND ORGANIC MATTER REMOVAL ON THE VALUE OF ZERO POINT OF CHARGE 49 Abstract 49 Introduction 50 Materials and Methods 51 Results and Discussion 54 Conclusions 58 Literature Cited 59 6. SUMMARY AND CONCLUSIONS 61 APPENDIX 65 - V -LIST OF TABLES 2.1 Morphological properties of three soi l profiles ranging from Dystric Eutrochrept to Spodic Ferrudalf. 12 2.2 Organic carbon, particle size analysis, and chemical extraction data for the three s o i l s . 14 2.3 Mineralogy of the <2um fraction. 16 2.4 pH and ZPC values for the three so i l s . 19 3.1 Surface charge characteristics of the three soi ls measured by three different techniques. 27 4.1 Some chemical and surface charge characteristics of the seven soils studied. 38 4.2 Extractable sesquioxide data for the seven s o i l s . 39 4.3 Selected columns of the correlation matrix. 40 5.1 Soil c lass i f icat ion and sampling location for the ten s o i l s . 52 5.2 Soil texture and extractable sesquioxide for the ten s o i l s . 55 5.3 pH, surface charge characterist ics , and total carbon for the ten so i l s . 56 - vi -LIST OF FIGURES 2.1 Potentiometric t i trat ion curves showing the relation-ship between ZPC, ZPT, IEP(s) and a 1 . 9 2.2 Potentiometric t i trat ion curves of three horizons indicating the three classes of t i t ra t ion curves. 18 3.1 Graph showing the relationship of ZPC and a i for the three s o i l s . 28 4.1 Showing the relationship between the ZPC and age for the seven s o i l s . 42 4.2 Showing the relationship between and age for the seven soiIs . 43 4.3 Showing the relationship between ApH and age for the seven soiIs . 45 ACKNOWLEDGEMENTS Numerous people in the Department of Soil Science greatly aided my research during the course of my program. Speci f ical ly , I would l ike to thank: Bev Herman for drafting and help with the laboratory analysis; Jul ie Armanini, Terry Walters and Margaret Holm for laboratory analysis; Bernie von Spindler for photographic reproduction of the figures; Mark Sondheim for help with the s tat i s t i ca l analysis; and Donna Farley and Valerie Marshall for typing earl ier drafts of some of the chapters. I would also l ike to thank Les Lavkulich for his supervision and friendship during the past three years. Glen Singleton has assisted me a great deal by his useful cr i t ic i sm of my manuscripts and by allowing me to include in my research samples from his study of Cox Bay. Very special thanks are due to S .E. Stewart, who spent a great deal of effort editing my material and typing the final draft of my thesis. This research would not have been possible without the financial assistance of a Leonard S. Klinck Fellowship from the Faculty of Graduate Studies and the National Research Council (Grant Number A4463). - 1 -Chapter One INTRODUCTION The processes of soi l genesis operate at the boundary between the soi l particles and the soil solution. If we visualize the development from infancy of a spodosol (podzol), for example, then we can trace the alteration of the particle surfaces of the parent material. In i t ia l ly the soil solution may be in direct contact with the crystal l ine surface of the soi l mineral grains; with time, the most easily soluble minerals weather through processes such as solution, oxidation, and hydrolysis. Eventually, weathering rinds composed of the least soluble constituents of the underlying minerals develop on the surfaces of the soil p a r t i -cles, subsequently reducing the rate of mineral weathering. On minerals such as quartz, coatings of amorphous material accumulate as a result of precipitation; iron and aluminum hydrous oxides have been shown to be common agents in the formation of such amorphous coatings. The result is that the surface in contact with the soil solution changes with time from one composed of crystal l ine minerals to one composed of the alteration products of soi l weathering. Pedologists studying soi l genesis have used different approaches to study the chemistry of non-crystalline material making up the sur-faces of the soi l particles . Since 1922, selective chemical dissolution techniques have been employed with considerable success (Mitchell et a l . , 1964). Despite the length of time that these various techniques - 2 -have been in use, the exact form of the material extracted is s t i l l not entirely identif ied. Although in the past this approach has provided a considerable amount of information on soi l genesis and is used today as an aid to the c lass i f icat ion of soi ls in Canada and the United States (Canada Soil Survey Committee, 1978; Soil Survey Staff, 1975), the author believes that relat ively small gains in knowledge are to be made by fur-ther research along these l ines . One of the possible alternative approaches to the study of amorphous coatings on soi l particles is to study the exchange reactions which take place at the interface between the soi l particles and the soil solution. This can be done using the potentiometric t i t rat ion technique, which ut i l i zes the adsorption and desorption of H + and OH" ions to measure the surface charge characterist ics . Spec i f ica l ly , the t i t rat ion curves at different ionic strengths cross at a common point, thus defining the zero point of charge (ZPC, the pH at which the net e lectr ic charge is zero), and providing a measure of the permanent or pH-independent exchange capacity (a-,-, the displacement of the crossover from the zero point of t i t r a t i o n ) . This technique has been applied to a limited extent to soils of tropical regions (Gallez e_t al_., 1976; Morais et^  al_., 1976; van Raij and Peech, 1972), and to areas of soi ls developed on volcanic ash (Espinoza et al_., 1975), but only very recently has attention been given to soi ls of temperate regions (Laverdiere and Weaver, 1977; Laverdiere et al_., 1977). These researchers have focussed their attention on the type of exchange reactions taking place in these so i l s . The present research attempts to expand the scope of investigation to concepts of soi l genesis. - 3 -The theory that measurement of ZPC and an- could be used as measures of so i l genesis was suggested by the work of Parks and de Bruyn (1962), who state that the ZPC is the pH of minimum solubi l i ty of pure oxide minerals. This statement was largely based on empirical evidence, although theoretical considerations dictate that the amount of hydro-lysis taking place at the surfaces of particles wi l l be at a minimum at the ZPC. In a system containing many different compounds, the ZPC wi l l be a function of a l l of the surfaces, and the ZPC wil l indicate the point of minimum so lubi l i ty for the whole system. Using this concept with soils leads to the hypothesis that as the ZPC of the sol id phase approaches the pH of the soi l solution, the so lubi l i ty of the so i l wi l l approach a minimum and the soi l particles wi l l approach equilibrium with the soil solution. Two experiments were performed to test the hypothesis that with increasing pedogenic development the ZPC approaches the pH of the s o i l . In the f i r s t experiment (Chapter Two), morphological and chemical evi-dence of sesquioxide accumulation was used as the measure of pedogenic development. In the second experiment (Chapter Four), the additional evidence of the ages of soi ls in a dated chronosequence was included. Two additional experiments were performed to evaluate some of the fac-tors which control the values of ZPC and o-j obtained in the laboratory. In Chapter Three the effects of fast-adsorption and slow-adsorption potentiometric t i trat ion techniques were discussed, and the effect of NaCl-saturation of the exchange complex prior to t i t ra t ion was evaluated. Chapter Five also evaluated the effect of NaCl-saturation; in addition, direct evidence is presented regarding the effect of organic matter. - 4 -Throughout the thesis, so i l c lass i f icat ion according to the Soil Taxonomy manual (Soil Survey Staff, 1975) has been used. The purpose was two-fold: f i r s t , the chapters presented in this thesis were written in a style suitable for publication in the Soil Science Society of America Journal (American Society of Agronomy, 1976), and second, the terminology of this system provides more descriptive information than is provided by the terminology of the Canadian System of Soil C l a s s i f i -cation (Canada Soi l Survey Committee, 1978). For the convenience of the reader, the c lass i f icat ion according to the Canadian system is inc lu -ded in the f i r s t table of each chapter. - 5 -LITERATURE CITED 1. American Society of Agronomy. 1976. Handbook and Style Manual. Am. Soc. Agron., Madison, Wisconsin. 2. Canada Soil Survey Committee. 1978. The Canadian System of Soil Class i f icat ion . Agriculture Canada, Ottawa, Ont. (In press) 3. Espinoza, W., R.G. Gast, and R.S. Adams, J r . Charge characteris-tics and nitrate retention by two Andepts from south-central Chile . Soil S c i . Soc. Am. Proc. 39: 842-846. 4. Gallez, A . , A.S.R. Juo, and A . J . Herbillon. 1976. Surface and charge characteristics of selected soi ls in the tropics. Soil S c i . Soc. Am. J . 40: 601-608. 5. Laverdiere, M.R., and R.M. Weaver. 1977. Charge characteristics of spodic horizons. Soil S c i . Soc. Am. J . 41: 505-510. 6. Laverdiere, M.R., R.M. Weaver, and A. D'Avignon. 1977. Characte-r i s t ics of the mineral constituents of some albic and spodic horizons as related to their charge properties. Can. J . Soil Sci . 57: 349-359. 7. Mitchel l , B . D . , V .C. Farmer, and W.J. McHardy. 1964. Amorphous inorganic materials in so i l s . Adv. in Agron. 16: 327-384. 8. Morais, F . I . , A .C. Page, and C.S. Lund. 1976. The effect of pH, sa l t concentration and nature of electrolytes on the charge characteristics of Brazi l ian tropical s o i l s . Soil S c i . Soc. Am-; J . ; . 40: 521-527. 9. Parks, G . A . , and P .L . de Bruyn. 1962. The zero point of charge of oxides. J . Phys. Chem. 66: 967-973. 10. Soil Survey Staff. 1975. Soil Taxonomy. Agriculture Handbook No. 436, Washington, D.C. 754 p. 11. Van R a i j , B . , and M. Peech. 1972. Electrochemical properties of some Oxisols and Alf isols in the tropics. Soi l Sc i . Soc. Am. Proc. 36: 587-593. - 6 -Chapter Two THE USE OF ZPC TO ASSESS PEDOGENIC DEVELOPMENT ABSTRACT The pH at the zero point of charge (ZPC) has been used to charac-terize the electrochemical properties of s o i l s . To date, most studies have focussed on soi ls high in oxides with low phyl los i l icate content. The study reported here attempts to use ZPC determinations to charac-terize the surface charge characteristics of soi ls from a temperate region. Three soi ls were examined; these ranged in pedogenic develop-ment, as expressed by so i l morphology, from a small accumulation of sesquioxides to a relatively high one. The soi ls were a Dystric Eutro-chrept, a Typic Ferrudalf, and a Spodic Ferrudalf. With increased pedogenic development the ZPC became more clearly defined and approached the natural pH of the s o i l , and there was a decrease in the relative significance of the pH-independent charge. The ZPC may be used as a measure of pedogenic development. - 7 -INTRODUCTION Previous researchers (Es pi noza et a]_., 1975; Gallez et al_., 1976; Laverdiere and Weaver, 1977; Morais e_t al_., 1976; van Raij and Peech, 1972) have examined the surface charge characteristics of a number of soils of varying pedogenic age from different parts of the world. The evidence presented by these authors suggests that with increased soi l development, the surface charge characteristics of soi ls in oxidizing, free leaching conditions become dominantly amphoteric. In other words, with increased pedogenic age the pH-dependent charge generated by oxide surfaces tends to become dominant over the pH-independent charge gene-rated by isomorphous substitution in clay minerals. This is thought to result from a combination of factors: (i) blocking of pH-independent sites by amorphous oxides; ( i i ) dissolution of clay minerals; and ( i i i ) the increasing predominance of pH-dependent charge surfaces of amorphous or crystal l ine oxides and hydrous oxides, primarily of iron and alumi-num. In discussing surface charge characteristics i t is important to c lar i fy the definitions of the terms used. In the recent l i terature these terms have been used differently by different authors. The ZPC is the pH at which the net total charge on the sol id phase is zero, whether the charge arises from pH-independent charge associated with isomorphous substitution or from pH-dependent charge associated with hydroxy*!ated oxide or organic matter surfaces. The IEP(s) ( isoelectric point of the solid) is the pH at which the net charge on the hydroxy-lated surfaces (pH-dependent charge) is zero. The ZPT (zero point of - 8 -t i trat ion) is the pH or range of pH values resulting from the reaction of the sol id species with the indifferent electrolyte of varying con-centrations in the absence of added acid or base (Figure 2.1) (Parks, 1967). In a pure oxide system the ZPC, IEP(s), and ZPT are coincident. In a system containing a source of pH-independent charge, such as clay minerals, the measured ZPC wi l l be different from the IEP(s) of the Hydroxylated surface and the cross-over point wi l l be displaced above or below the ZPT by an amount (a^) equal to the net charge of the per-manent (pH-independent) exchange s i tes . Parks (1967) states that materials with a large pH-independent negative charge w i l l not have a unique ZPC; potentiometric t i t rat ion wi l l not y i e ld a unique cross-over point until a l l of the net negative charge has been satisf ied by added H* ions. This depresses the ZPC below the ZPT, and often makes the ZPC unattainable within a range of pH ( i . e . , pH above 3) that does not result in an excessive amount of the oxides being so lubi l ized. An accumulation of iron and aluminum oxides and hydroxides causes the ZPC to approach the ZPT; ultimately, i f the soi l behaves as a pure oxide system, the ZPC wil l occur at the ZPT. The ZPC is the pH at which hydrolysis of the particulate surfaces by H + or OH" ions is at a minimum; as a result , the so lubi l i ty of the material (in a pure oxide system at least) is also at a minimum (Parks and de Bruyn, 1962). Using this concept with soi ls leads to the hypo-thesis that as the ZPC of the sol id phase approaches the pH of the soil solution, the so lubi l i ty of the soi l w i l l also approach a minimum and the soi l particles wil l approach equilibrium with the soil solution. FIGURE 2 .1: Potentiometric t i trat ion curves showing the relationship between ZPC, ZPT, IEP(s) and a-j of: a) a pure oxide system, and b) a soi l system with a f ini te amount of net negative charge. - 10 -The objective of this study was to test the hypothesis that with increased pedological development (increasing amounts of sesquioxides) the ZPC approaches the pH of the soil . In contrast to the soils used in the studies referred to above, which had a low phyl los i l icate-c lay content, the soi ls in this case contain high amountsoof clay minerals (phyllosil icates) and varying amounts of sesquioxides. They are deve-loped from glacio-marine deposits and support a 20- to 30-year-old, second-growth Douglas f i r forest. MATERIALS AND METHODS Field Study The three soi ls were from Vancouver Island, Bri t i sh Columbia, Canada. The soi ls examined were: the Saanichton Series (Dystric Eutro-chrept); the Alberni Series (Typic Ferrudalf); the Memekay Series (Spo-dic Ferrudalf) (Day et al_., 1959; Soil Survey Staff , T975). Soils were described and sampled in the f i e ld and the samples were returned to the laboratory for study. Laboratory Procedures Following a ir-drying, the samples were passed through a 2 mm sieve and this material was used for a l l subsequent determinations. Soil pH was measured in water (1:1, Peech, p. 922, 1965), 0.01M CaCl 2 (1:2, Peech, p. 923, 1965), and 1N_ KCl (1:2.5, Hesse, p. 30, 1971). Three different extractions of amorphous material were made using: (i) sodium pyrophosphate (p), one overnight extraction at pH 10 (McKeague, 1976); ( i i ) acid ammonium oxalate (o), one four-hour extraction at pH 3.0 (McKeague and Day, 1966); and ( i i i ) sodium-citrate, -bicarbonate, ^dithionite (c) , two fifteen-minute extractions (Mehra and Jackson, 1960). 1 ( - 11 -The extracted Fe and Al were determined by atomic absorption spectro-scopy, and pyrophosphate-extractable carbon was determined by the Walkely-Black wet oxidization technique (Al l i son, pp. 1374-1376, 1965). Total so i l organic carbon was determined by Leco Analyser. The samples used for part ic le s ize analysis and X-ray di f fract ion analysis were pre-treated with sodium hypochlorite to remove organic matter (Lavkulich and Wiens, 1970) and acid ammonium oxalate (McKeague and Day, 1966) to remove amorphous material. Particle size analysis was accomplished by sieving and pipette analysis (McKeague, 1976). X-ray diffraction was performed on the <2 ym fraction of selected horizons (B and C) in unsaturated random powder mounts and also as oriented aggregate mounts in the Mg-saturated, a i r -dr ied , and ethylene-glycol-solvated states, and K-saturated, a i r -dr i ed , heated to 300°C and 550°C states (Whittig, 1965). Potentiometric t itrations for determination of the ZPC were done on NaCl-saturated samples using the method of van Raij and Peech (1972). RESULTS AND DISCUSSION The description of the soi ls is presented in Table 2.1. The three soi ls represent a transition in morphological expression varying from a Dystric Eutrochrept to a Spodic Ferrudalf. In the former, the colours in the B22 are relat ively yellow (10YR) and the accumulation of extractable sesquioxides (Table 2.2) was the least of the three s o i l s . The Typic Ferrudalf indicates further pedogenic development by the presence of abundant hard concretions, in the B21cn, redder hues and an increased amount of extractable sesquioxides. The Spodic Ferrudalf TABLE 2.1: Morphological properties of three soi l profiles ranging from Dystric Eutrochrept to Spodic Ferrudalf. HORIZON DEPTH COLOUR MOIST DRY , (crushed) TEXTURE STRUCTURE DYSTRIC EUTROCHREPT (ORTHIC DYSTRIC BRUNISOL) - SAANICHTON SERIES 0 2-0 10YR 2/2 10YR 3/3 med. granular Al 0-2 7.5YR.-3/2 10YR 4/3 SiL wk. B21 2-33 7.5YR 4/4 1QYR 6/4 SiL wk. med. subangular blocky B22 33-62 10YR 5/4 1QYR 7/3 SiL St. med-coarse subangular blocky CI 62-95 10YR 4/3 10YR 5/3 SiCL St. coarse platy C2g 9 5-, 105+ 2.5Y 4/2 2.5Y 6/2 SiCL St. coarse platy TYPIC.FERRUDALF (ORTHIC DYSTRIC BRUNISOL) - ALBERNI SERIES 0 2-0 10YR 2/2 10YR 4/3 -. med. granular B21Cn 0-5 5YR 3/2 5YR 4/3 SiC mod B22Cn 5-25 5YR 4/4 7.5YR 5.5/4 C: mod . fine subangular blocky B23 t 25-55 7.5YR 5/4 10YR 6/4 HC mod . med. subangular blocky B3 55-73 7. ,5YR 5/6 - 2.5Y 5/2 2.5Y 7/4 C. St. med. angular blocky C 73-105+ 7. ,5YR 3/2 - 2.5Y 5/2 2.5Y 6/3 C St. coarse platy SPODIC FERRUDALF (PODZOLIC GRAY LUVISOL) - MEMEKAY SERIES 0 9-0 5YR 2,5/2 5YR 3/3 -A2 0-1 5YR 3/3 5YR 5/6 SiL wk. fine granular B 21 i r 1-12 2.5YR 3/4 5YR 5/6 SiL mod . med. granular B22ir 12-40 5YR 4/4 7.5YR 5/6 SiL mod . coarse subangular blocky B23 t '40-50 5YR 3/4 7.5YR 5/6 SiL mod . coarse subangular blocky B3 50-70 2.5YR 4/3 - 5YR 4.5/2 2.5Y 6/3 SiL mod . s t . coarse subangular blocky C 70-100 5Y 4/2 5Y 6/2 SiL St. coarse platy TABLE 2.1 (cont'd): HORIZON CONSISTENCE CONCRETION CUTANS MOIST DRY DYSTRIC EUTROCHREPT (ORTHIC DYSTRIC BRUNISOL) - SAANICHTON SERIES 0 Al v. fr iable soft B21 v. friable soft few small weak B22 firm hard - few thin clayey CI v. firm hard to v. hard C2g v. firm hard to v. hard - dark brown Mn TYPIC FERRUDALF (ORTHIC DYSTRIC BRUNISOL) - ALBERNI SERIES 0 B21cn fr iable s i . hard ab. fine to med. hard B22cn fr iable s i . hard few med. hard B23t fr iable s i . hard few med. hard * com. thin clayey B3 v. firm hard * f e w thin clayey C v. firm hard to v. hard - dark brown Mn SPODIC FERRUDALF (PODZOLIC GRAY LUVISOL) - MEMEKAY SERIES 0 - - - -A2 v. friable soft -B21ir friable s i . hard p l . fine hard -B22ir fr iable s i . hard few med. hard -B23t friable si . hard few med. hard * com. thin clayey B3 fr iable to firm si . hard to hard - * few thin clayey C v. firm hard to v. hard - -* Personal communications, A.C. Research Station, Vancouver, B.C. TABLE 2.2: Organic carbon, particle size analysis, and chemical extraction data for the three so i l s . HORIZON ORGANIC C DYSTRIC EUTROCHREPT 0 Al B21 B22 CI C2g 18.6 9.4 1.7 1.7 • .3 ' .3 TYPIC FERRUDALF 0 B21cn B22cn B23t B3 C 22. 7. 2, 1. SPODIC FERRUDALF 0 A2 B21ir B22ir B23t B3 C PARTICLE SIZE ANALYSIS Sand S i l t - Clay F. Clay 2-.5mm .5-.002mm <.002mm <.0002mm PYROPHOSPHATE Fe n Al„ C„ Fe, OXALATE A L Fe, CITRATE Al , - - - .21 .25 .002 .-5.3 :36 .86 .39 7.6 62.9 23.5 6.4 .37 .44 .017 .95 .58 1.28 .45 7.4 67.6 25.0 6.5 .40 .44 .006 .99 .78 1.38 .45 5.5 76.6 17.9 4.0 .25 .20 .003 .82 .24 .93 .14 1 .9 62.1 36.0 7.7 .07 < .12 .001 .79 .25 1 .26 .10 .8 60.2 39.0 9.8 •.06 .08 .002 .60 .21 1.34 .12 16.8 - - - .35 .51 .035 .66 .64 .95 .76 39.9 43.3 8.8 .51 .82 .015 1 .21 1 .07 2.41 1 .03 11.9 34,0 54.1 11.3 .53 .85 .007 .78 1.06 2.54 1.04 11.2 28.7 60.1 12.9 .32 .64 .005 .98 .80 2.01 .58 18.1 27.8 54.1 5.5 .06 .27 .001 .66 .63 1.50 .26 8.2 32.2 59.6 9.0 .06 .10 .001 .74 .51 1.32 .16 35.7 - • - - - .21 .34 .045 4.1 11.9 72 (0 16.1 2.0 .59 .39 .013 2.6 11 .7 66.1 22.2 3.2 .83 .88 .010 2.0 22.7 ' 55.1 22.2 .7 .28 >.86 .008 1 .5 21 .3 56.8 21.9 1 .3 .16 .69 .006 • .9 7.8 77.1 21.1 2.9 .15 .51 .004 .2 .5 78.5 21.0 2,8 ' .07 .28 .001 .29 1 .43 1.64 1 .40 1 .12 .79 .57 .34 .48 1.16 2.34 2.02 1 .41 .72 .62 .51 .60 .62 .25 .16 .90 .48 .52 .94 1 .33 1.33 .66 .26 - 15 -represents a degraded Al f i so l with accumulation of amorphous iron and aluminum, the presence of hard concretions, the red hues, and the large amounts pf extractable sesquioxides in the B21ir and B22ir horizons, a l l of which tend to mask the a l f i c nature of the prof i l e . Pyrophosphate, oxalate, and citrate extraction data indicate that the amount of sesquioxide iron and aluminum increased substantially from the Dystrtc Eutrochrept to the Typic Ferrudalf and then again to the Spodic Ferrudalf (Table 2.2). The pyrophosphate-extractable iron and aluminum (Fep and Alp) have somewhat different distributions in the three s o i l s . This is interpreted as indicating a difference in the distribution of active organic-complexing agents. In the Dystric Eutro-chrept and Spodic Ferrudalf, the maximum accumulations are reeordedfin the B21 horizons, while the Typic Ferrudalf has a relatively even dis-tribution in the B21 and B22 horizons. Oxalate extraction data i n d i -cate that the maximum amount of amorphous iron compounds is found in the B21 horizons of a l l three so i l s ; however, with increased pedogenic development, as inferred from total extractable sesquioxides, the amor-phous aluminum content maxima are displaced downwards in the prof i l e . In the present case the Alo maximum is in the B21 horizon of the Dystric Eutrochrept, evenly distributed between B21 and B22 of the Typic Ferru-dalf , and in the B22 of the Spodic Ferrudalf. These results suggest ahatncreased degree of spodic character with increased prof i le develop-ment. Similar results were obtained from the citrate-extractable iron and aluminum data. The mineralogy of representative horizons of the three soi ls is presented in Table 2.3. X-ray dif fract ion has not indicated major -- 16 -TABLE 2.3: Mineralogy of the <2ym fract ion. HORIZON Chlorite Vermiculite m i t e Quartz Feldspar Zircon DYSTRIC EUTROCHREPT B21 2 3 3 1 2 2 C2g 3 2 2 1 2 2 TYPIC FERRUDALF B22cn . 2 - - 2 3 2 C 3 2 3 2 2 2 SPODIC FERRUDALF B21ir 2 - 2 2 2 C 2 - 3 2 2 2 1: Dominant 2: Common 3: Trace ^: None - i n -differences in the mineralogy, although some variations do occur. Spe-c i f i c a l l y , the amounts of chlorite in the Dystric Eutrochrept and Typic Ferrudalf decrease with increasing depth from the B2 to the C horizons, with a corresponding increase in the i l l i t e and vermiculite content. This trend is not evident in the Spodic Ferrudalf. The relative amount of quartz in the Dystric Eutrochrept is s l ight ly greater than in the other two s o i l s . The B22 horizon of the Typic Ferrudalf contains only a trace of feldspar, whereas this mineral is found in greater quantities in the other s o i l s . There is a trend in each of the profiles to have a lower amount of feldspar in the B horizon as compared to the C horizon. Potentiometric t itrations Three types of potentiometric t i trat ion curves were encountered in this study. Type I, represented by Figure 2.2a, shows no cross-over point of the four t i trat ion curves at different concentrations of the indifferent electrolyte; therefore the ZPC is undefined. Type II (Figure 2.2b) has a dis t inct cross-over point for the three t i trat ion curves at concentrations of 0.1N, 0.01J\[, and O.OOlN^NaCl. The fai lure of the l.ON^ NaCl t i trat ion curve to achieve cross-over at the same pH value is an indication that the surface has a lower than expected net negative charge at this concentration; the mechanism for this is not known, but is probably related to the ordering of Na + and H + ions in the double layer. In this case the ZPC was defined as being the cross-over point of the three lower concentration t i trat ion curves. Type III possessed a clearly defined ZPC; a l l four of the t i trat ion curves crossed over at very nearly the same point (Figure 2.2c). The ZPC data for the three samples are presented in Table 2.4 and identif ied according to type. The Dystric Eutrochrept B21 horizon and FIGURE 2.2: Potentiometric t i trat ion curves of three horizons indicating the three classes of t i t ra t ion curves: a) Type I - no cross-over point; b) Type II - three curves only cross over; and c) Type III - a l l four t i trat ion curves cross over. - 19 -TABLE 2.4: pH and ZPC values for the three s o i l s . HORIZON H20 .01M CaCl 2 N. KCL ZPC a i meq/lOOg pH(KCL)-ZPC DYSTRIC EUTROCHREPT 0 5.57 5.24 5.01 - -Al 6.11 5.58 5.26 u -B21 5.97 5.19 4.68 4.0 * -10.0 0.68 B22 5.54 4.56 4.00 u - -CI 5.55 4.87 3.83 u - -C2g 6,23 5.48 4.04 u - -TYPIC FERRUDALF 0 5,57 5,11 4.85 - - -B21cn 5.62 5.07 4.61 3.9 * -20.0 0.46 B22cn 5,74 5.00 4.49 4.2 * - 9.5 0.29 B23t 5.10 4.43 3.93 3.65* -20.0 0.28 B3 5.01 4.49 3.70 u - -C 5.54 5.20 3.84 u - -SPODIC 1 FERRUDALF 0 4.32 3.55 3.27 - -A2 4.62 3,83 3.63 - - -B21ir 5.40 4.52 4.34 4.25 - 5.0 0.09 B22ir 5.99 5.26 5.11 5.0 - 2.0 0.11 B23t 6.05 5.32 5.11 4.85 - 2.0 0.26 B3 6.05 5.09 4.74 4.5 - 2.5 0.24 C 6.09 5.02 4.39 4.2 * - 5.0 0.19 u: Undefined - : Not determined *: Defined by three curves only - 20 -the Typic Ferrudalf B21, B22 and B23 horizons have ZPC values defined as Type II . The a-j values indicate the presence of 9.5 to 20 meq/lOOg of pH-independent charge in these horizons. The Spodic Ferrudalf demonstrates the dominant effect of the pH-dependent charge generated by the sesquioxide surfaces; the ZPC of the B21ir, B22ir, B23t, and B3 horizons are a l l Type III with relatively small a-,- values (-2.0 to -5 .0) . The +pH(KCl) - ZPC values indicate that the ZPC is approaching the soi l pH with increased soi l development. The B21 horizon of the Dystric Eutrochrept has a ZPC within 0.68 pH units of the pH(KCl), while the horizons above and below do not have a defined ZPC. A l l of the B2 horizons of the Typic Ferrudalf have a ZPC defined by only three t i t rat ion curves, and the difference between the ZPC and the pH(KCl) is between 0.46 and 0.28 pH units; the ZPC of the B3 and C horizons are undefined. The Spodic Ferrudalf, the soi l with the largest accu-mulation of extractable iron and aluminum, has a defined ZPC for a l l of the subsurface horizons, and except for the C horizon they are defined by a l l four t i trat ion curves. In this so i l the differences between the ZPC and the pH(KCl) decrease to within 0.09 and 0.26 pH units. Comparison of the ZPC, a-j, and pH(KCl) - ZPC values with the total organic C, pyrophosphate-rextractable C, and particle size ana-lysis data do not reveal any obvious correlations. The data do ind i -cate, however, that with increasing sesquioxide content the ZPC approaches the ZPT (decreasing a-j) and the ZPC also approaches the pH (KCl) . - 21 -Although the pH values obtained in KCl are not necessarily the pH of the soi l solution, they are an indication of the pH of the solu-tion in close proximity to the particle surfaces, since most of the exchangeable acidity w i l l be brought into solution. For this reason they were chosen for the comparison with the ZPC. Similar results , with larger difference values, are obtained by comparing the ZPC with the pH(H20) and pH(CaCl 2 ). CONCLUSIONS The experiment reveals that the three soi ls studied range in the degree of their pedological development from a soi l with morphological and chemical characteristics indicative of only a small accumurliation of sesquioxides to a soi l in which sesquioxide accumulation is exten-s ive . The surface charge properties indicate that the increase in morphological development hassbeen accompanied by a decrease in the relative significance of pH-independent charge, as compared to the pH-dependent charge. As a result , the ZPC becomes more clearly defined and approaches the natural pH of the s o i l . - 2 2 -LITERATURE CITED 1 . A l l i s o n , L . E . 1 9 6 5 . Organic carbon. J J I C.A. Black (ed.) . Methods of Soil Analysis. Part 2 . Agronomy 9 : 1 3 6 7 - 1 3 7 8 . Amer. Soc. Agron., Madison, Wisconsin. 2 . Day, J . H . , L . Farstad, and D.G. La ird . 1 9 5 9 . Soil Survey of Southeast Vancouver Island and Gulf Islands, Bri t i sh Columbia. Report No. 6 , B . C . Soil Survey. 3 . Espinoza, W., R.G. Gast, and R.S. Adams, J r . 1 9 7 5 . Charge charac-ter is t ics and nitrate retention by two Andepts from south-central Chi le . Soil S c i . Soc. Am. Proc. 3 9 : 8 4 2 - 8 4 6 . 4 . Gallez, A . , A.S.R. Juo, and A . J . Herbi l lon. 1 9 7 6 . Surface and charge characteristics of selected soi ls in the tropics . Soil S c i . Soc. Am. J . 4 0 : 6 0 1 - 6 0 8 . 5 . Hesse, P.R. i l>971 . A Textbook of Soil Chemical Analysis. John Murray Publishers, London. 5 2 0 p. 6 . Laverdiere, M.R. , and R.M. Weaver. 1 9 7 7 . Charge characteristics of spodic horizons. Soil S c i . Soc. Am. J . 4 1 : 5 0 5 - 5 1 0 . 7 . Lavkulich, L . M . , and J . H . Wiens'. 1 9 7 0 . Comparison of organic matter destruction by hydrogen peroxide and sodium hypochlo-r i t e and its effect on selected mineral constituents. Soil S c i . Soc. Am. Proc. 3 4 : 7 5 5 - 7 5 8 . 8 . McKeague, J . A . (ed.) . 1 9 7 6 . Manual on Soil Sampling and Methods of Analysis. S . R . I . , Agriculture Canada, Ottawa. 9 . McKeague, J . A . , and J . H . Day. 1 9 6 6 . Dithionite and oxalate extractable Fe and Al as aids in differentiating various classes of s o i l s . Can. J . Soil S c i . 4 6 : 1 3 - 2 2 . 1 0 . Mehra, O.P . , and M.L. Jackson. 1 9 6 0 . Iron oxide removal from soils and clays by a dithionite-c i trate system buffered with sodium bicarbonate. Clays and Clay Minerals 7 : 3 1 7 - 3 2 7 . 1 1 . Morais, F ; I . , A?6.APage,?aand QnS. Lund. 1 9 7 6 . The effect of pH, sa l t concentration and nature of electrolytes on the charge characteristics of Brazi l ian tropical s o i l s . Soil S c i . Soc. Am. J . 4 0 : 5 2 1 - 5 2 7 . 1 2 . Parks, G.A. 1 9 6 7 . Aqueous surface chemistry of oxides and complex oxide minerals. Adv. Chem. 6 7 : 1 2 1 - 1 6 0 . 1 3 . Parks, G.A. , and P .L . de Bruyn. 1 9 6 2 . The zero point of charge of oxides. J . Phys. Chem. 6 6 : 9 6 7 - 9 7 3 . - 23 -14. Peech, M. 1965. Hydrogen-ion ac t iv i ty . Jji C A . Black (ed.) . Methods of Soil Analysis. Part 2. Agronomy 9: 914-926. Am. Soc. Agron., Madison, Wisconsin. 15. Soil Survey Staff . 1975. Soil Taxonomy. Agricultural Handbook No. 436, Washington, D.C. 16. Van R a i j , B . , and M. Peech. 1972. Electrochemical properties of some oxisols and a l f i so ls in the tropics. Soil S c i . Soc. Am. Proc. 36: 587-593. 17. Whittig, L . D . 1965. X-ray techniques for mineral identif ication and mineralogical composition. IJX C A . Black (ed.). Methods of Soil Analysis. Part 1. Agronomy 9: 671-698. Am. Soc. Agron., Madison, Wisconsin. - 24 -Chapter Three MEASUREMENT TECHNIQUE EFFECTS ON THE VALUE OF ZERO POINT OF CHARGE AND ITS DISPLACEMENT FROM ZERO POINT OF TITRATION ABSTRACT Three procedures for measuring ZPC (the zero point of charge) and a-j (its displacement from the zero point of t i trat ion) were investigated. The slow-adsorption potentiometric t i trat ion technique was found to pro-duce higher ZPC values and lower values than the fast-adsorption tech-nique. NaCl-saturation of the exchange complex prior to t i t ra t ion resulted in a shi f t of ZPC and a^ values compared to untreated samples. For ZPC and an- values below pH 5.0 and -0.25 meq/lOOg respectively, as measured on untreated samples, the NaCl-saturation resulted in lower values; for values above th i s , the change was to higher values. The fast-adsorption procedure performed on untreated samples is considered to be the most suitable for soils because the values obtained wi l l most closely characterize the surface charge properties in the f i e l d . c. - 25 -INTRODUCTION In recent years there has been an increasing interest in the use of potentiometric t i trat ion to study the surface charge characteristics of s o i l s . This interest stems from the recognition of the importance of the pH-dependent exchange capacity of many soi ls high in oxides and hydroxides of iron and aluminum. It has been recognized that these soils respond differently to management practices than soi ls in which the pH-independent exchange capacity dominates. In order that the results of different authors in different parts of the world be compa-rable, i t is important to know whether the methods used by the different authors produce the same or similar results for the surface charge cha-racter i s t i c s . Several different techniques have been used in the past for the determination of the zero point of charge of soi ls (ZPC, the pH at which the net charge is zero). Van Raij and Peech (1972) and Espinoza et a l . (1975) employed a technique using an equilibration period of thRee days during the potentiometric t i t r a t i o n . Laverdiere and Weaver (1977) state that identical information could be obtained using a much faster technique which employed an equilibration period of only two minutes; and that NaCl-saturation of the samples prior to t i trat ion did not affect the value obtained for the ZPC, although i t did cause an increase in the displacement of the cross-over point (defined here as a^) below the zero point of t i t rat ion (ZPT). - 26 -MATERIALS AND METHODS The samples used in this study are from three soils on Vancouver Island, Br i t i sh Columbia, developed on similar glacio-marine parent material. In contrast to the soi ls used by previous researchers, the soi ls used in this case contained high amounts of clay minerals (phyllo-s i l icates) and therefore have the potential of developing high permanent cation exchange capacities. These soi ls are discussed fu l ly in Chapter •Two. The NaCl-saturated samples were prepared with four washings of lf[ NaCl and then washed free of excess salt with methanol /water and acetone/water solutions. The samples were subsequently oven-dried at 4 0 ° C. The three t i trat ion methods investigated in this study were: (i) the slow-adsorption method using NaCl-saturated samples and an equilibration period of three days (van Raij and Peech, 1972); ( i i ) the fast-adsorption method using NaCl-saturated samples and equilibration period of two minutes (Laverdiere and Weaver, 1977); and ( i i i ) the fast-adsorption method using untreated samples (Laverdiere and Weaver, 1977). In a l l cases the so i l to electrolyte ratio was 1:20 (gm:ml) and the indifferent electrolyte was NaCl in concentrations of 0.001NU O.OIJi, 0.1N, and IN. RESULTS AND DISCUSSION Comparing the results for the NaCl-saturated samples by the slow-adsorption and fast-adsorption methods, Table 3.1 and Figure 3.1 show that with the exception of the Spodic Ferrudalf B22, B23t and B3 hor i -zons, the slow procedure resulted in higher ZPC values for the three soi ls studied. In a l l cases, the slow technique resulted in a greater displacement of the cross-over point below the ZPT, as indicated by TABLE 3.1: Surface charge characteristics of the three soils measured by three different techniques. Slow F a s t F a s t l i n n T 7 m i NaCl-Saturated NaCl-Saturated Untreated HORIZON Z P C meq/lOOg Z P C meq/lOOg Z P C meq/lOOg DYSTRIC EUTROCHREPT (ORTHIC DYSTRIC RRUNISOL) - SAANICHTQN SERIES B21 4.00* -10.0 3.17* - 9.0 3.72* - 4.3 B22 u - u u -CI u - u - u -TYPIC FERRUDALF (ORTHIC DYSTRIC BRUNISOL) - ALBERNI SERIES B21cn 3.90* -20.Q 3.10* -19.5 3.89* - 7.0 B22cn 4.20* - 9.5 3.76* - 6.9 3.89 - 4.6 B23t 3.65* -20.0 u - 3.56 - 5.3 C u - u - . u -SPODIC FERRUDALF (PODZOLIC GRAY LUVISOL) - MEMEKAY SERIES B21 i r 4.25 - 5.0 4.19 - 1 .9 4.51 - 0.4 B22ir •' 5.00 - 2.0 5.31 - 0.1 5.27 - 0.3 B23t 4.85 - 2.0 5.36 + 0.2 5.18 - O U B3 4.50 - 2.5 4.61 - 0.8 4.71 - 0.6 C 4.20* - 5.0 3.42* - 4.8 3.91* - 2.1 *: Defined by three of the four curves only u: Undefined - 28 -ZPC FIGURE.3.1: Graph showing the relationship of ZPC and a-j for the three so i l s . The three regression lines are: A-A, log (-1.9 - a-,-) = 8.76-1.97 ZPC, R 2 = 0.94; B-B, log (0.58 - a,-) = 3.05-0.63 ZPC, R2 = 0.95; C-C, log (-0.25- a ^ = 4.95-1.15 ZPC, R2 = 0.93. - 29 -larger negative values of a j . Otherwise, the samples, which did.not have a defined ZPC when measured by the slow-adsorption method, were also undefined by the fast-adsorption method. Of the samples studied in this experiment, the Spodic Ferrudalf had the highest amounts of extractable iron and aluminum in the B21 and B22 horizons, and the pH-dependent charge dominated over the pH-independent charge (Chapter Two). In these samples the ZPC was defined more clearly and at a higher pH for both methods. As a result , the difference between the ZPC values of these samples measured by the fast-adsorption and slow-adsorption methods was much less pronounced. This can be seen graphically on Figure 3.1, where the regression lines representing the NaCl-saturated, fas t - , and slow-adsorption ZPC values come closest together at high values of ZPCandda-j values close to zero. The differences in the results obtained with the fast-adsorption as compared to the slow-adsorption procedure are due to the differences in the adsorption reactions with the two methods. Atkinson et a l . (1967) and Blok and de Bruyn (1970) found that the i n i t i a l rapid adsorp-tion is a reaction between the part ic le surfaces and the potential determining ions (H + and OH"), whereas the slow-adsorption which follows is a reaction involving the incorporation of H + and OH" ions into the sol id phase. For this reason, the fast procedure, which measures the i n i t i a l fast-adsorption, is a better indicator of the charge characte-r i s t i c s of the soi l samples at the surface proper. Comparing the results for the fast-adsorption methods with NaCl-saturated and untreated samples, Table 3.1 shows that, with the excep-tion of the Typic Ferrudalf B23t horizon, the ZPC was undefined for the same samples. As with the comparison between the fast- and slow-- 30 -adsorption methods, the differences in the values measured by the two methods were at a minimum with the samples which had the highest ZPC values and a-j values closest to zero. In the samples in which the pH-independent charge is comparatively more important than the pH-dependent charge (as indicated by larger, negative values), there were re la -t ively large differences in the measured values of ZPC and a-j between the NaCl-saturated and untreated samples. Figure S.^showshcthat'for ZPC values below 5.0 (and a-j values lower than -0.25), there was a decrease in the measured values of both ZPC and for the NaCl-saturated samples compared to the untreated samples. At ZPC vailues above 5.0 (and values above -0.25), the reverse result was obtained and there was an increase in the values of both ZPC and a-j. As can be seen from Figure 3.1, the regression lines for the NaCl-saturated and untreated samples measured by the fast-adsorption method are s imilar , and the results measured by the different techniques are merely shifted in one direc-tion or the other along the curve (assindicated by the arrows connecting pairs of samples). The differences between the values of ZPC and a-,- for the NaCl-saturated and untreated samples, for the fast-adsorption method are attributed to the replacement of strongly-bonded cations and anions adsorbed on the exchange sites by Na + and CI~. In the case of the sam-ples which responded to the NaCl-saturation by yielding lower ZPC and values, the treatment caused more pH-independent cation exchange sites to be made accessible during the potentiometric t i t r a t i o n . This 3 + would result from the replacement of Al ions, which block permanent cation exchange s i tes , by the readily exchangeable Na + ions. In the case of the samples which responded to NaCl-saturation by yielding - 31 -higher ZPC and values, the treatment caused more anion exchange sites to be made accessible. This would result from the replacement of 3 • -strongly-bonded anions such as PCH by readily exchangeable CI". A l -though both of these processes wi l l occur in a l l of the samples, the dominance of one or the other wil l determine the net change which \ results. Due to the change in the measured values following NaCl-satu-rat ion, i t is concluded that untreated samples wi l l give a more accurate estimate of the surface charge characteristics as they occur in the f i e l d . CONCLUSIONS Laverdiere and Weaver (1977) stated that the ZPC values obtained for NaCl-saturated and untreated samples would be the same, and that NaCl-saturation would lower the value of a-j. The present study found that: (a) the ZPC values for the two techniques would be very close only when the cross-over occurs close to the ZPT, and (b) the change in the value of a-j wi l l depend on the nature of the part ic le surfaces under study. This study has shown that in some cases these methods of measuring ZPC and a-j do not y i e ld the same results . When the value of a-j is -en-close to zero, the differences obtained with various measurement tech-niques wi l l be at a minimum. When a-j is not close to zero, the values wi l l be quite different. Therefore, care must be taken when comparing the work of different researchers, i f identical laboratory procedures have not been employed. LITERATURE CITED Atkinson, R . J . , A.M. Posner, and J . P . Quirk. 1967. Adsorption of potential-determining ions at the f err i c oxide-aqueous elec-trolyte interface. J . Phys. Chem. 71: 550-558. Blok, L . , and P . L . de Brjayn. 1970. The ionic double layer at the ZnO/solution interface: I . The experimental point of zero charge. J . Col lo id . Interface S c i . 32: 518-525. Espinoza, W., R.G. Gast, and R.S. Adams, J r . 1975. Charge charac-terist ics and nitrate retention by two Aridepts from south-central Chi le . Soil S c i . Soc. Am. Proc. 39: 842-846. Laverdiere, M.R., and R.M. Weaver. 1977. Charge characteristics of spodic horizons. Soil S c i . Soc. Am. J . 41: 505-510. van R a i j , B . , and M. Peech. 1972. Electrochemical properties of some Oxisols and Alf isols in the tropics . Soil S c i . Soc. Am. Proc. 36: 587-593. - 33 -Chapter Four VARIATION IN SURFACE CHARGE CHARACTERISTICS IN A SOIL CHRONOSEQUENCE ABSTRACT The surface charge characteristics of the B horizons of seven soi ls in a chronosequence developed on sandy beach material were studied. The soi ls range in age from 127 to 550 years and exhibit morphological and chemical characteristics ranging from Typic Udipsamment to Aquic Haplorthod. The difference between the zero point of charge (ZPC) and the pH of the soi l decreased as the age of the soi ls increased, from a maximum value at the youngest s i te of 0.27 to a minimum value at the oldest s i te of 0.11. The decrease in this ApH value is interpreted as indicating that the soi ls are approaching a steady-state with time. The ApH value is presented as a measure of pedogenic development. - 34 -INTRODUCTION With time and increasing pedogenic development, the surface charge characteristics of soi l particles change. In the B horizon of soils developing under the climate and vegetation that result in the development of a spodosol, this change is a function of the increase in sesquioxide content. As a result of this increase, the part ic le sur-faces approach a condition in which these oxides and hydroxides gene-rate the majority of.exchange sites (Clark, McKeague, and Nichol, 1966), When this has occurred, the exchange capacity w i l l be largely pH-depen-dent and the surfaces can be said to be amphoteric (Chapter Two). Potentiometric t i trat ion can be used to measure certain para-meters which reveal the surface charge characteristics of amphoteric particles (Parks, 1965). The zero point of charge (ZPC) is the pH at which the net total charge on the particle surfaces is zero, i . e . , the number of positive and negative charge sites is equal. At pH values above the ZPC a net negative charge wi l l be generated at the oxide sur-face; and at pH values below the ZPC, a net positive charge wi l l be generated, as represented schematically for a ferr ic oxide surface below. For Fe 2 0 3 and A1 2 0 3 the ZPC has been measured at 8.5 and 6.9 respectively (Parks and de Bruyn, 1962). pH< ZPC \ OH. 3 H 20 + 0 OH ? \/ Fe ' ^ OH, 3H-.0 pH • ZPC \ ro + 0 OH \ l / Fe |\ OH / pH >ZPC Fc 3 OK - . 0 0 > X Fe / + 3 H 20 - 35 -According to Parks (1967), the ZPC is a measure of the charge pro-perties of the system as a whole, and the ZPC of a mixture wi l l be deter-mined by the weighted average of the charge on the different surfaces. In a soil:: system containing mixtures of crystall ine and amorphous inor- , ganic compounds and various organic compounds, the ZPC wi l l be a function of the proportion of the surface that is made up of different components. Therefore, as a soi l develops, measurable differences in surface charge characteristics should be observed. The objective of this study was to test the hypothesis that with increasing pedogenic development o f the B horizon, as indicated by increasing age and sesquioxide content: (i) the ZPC increases; ( i i ) the absolute value of decreases; and ( i i i ) the ApH value decreases, where ApH is defined as the difference between the ZPC and the pH(KCl). This hypothesis was tested on a chronosequence developed on sandy beach material in the humid temperate environment of west-coast Vancouver Island. MATERIALS AND METHODS Characterization of the Soil Samples The soils used in this study were collected along a chronosequence on the West Coast o f Vancouver Island. The detailed descriptions o f these sites are part of the Ph.D. program of G.A. Singleton (Department o f Soil Science, University of Bri t i sh Columbia, Vancouver, Canada). The soi ls range in age from 127 to 550 years old and are classif ied as a sequence from a Typic Udipsamment to an Aquic Haplorthod. For this study, samples of the f i r s t B horizon and the next lower horizon were used. The so i l profiles were described and sampled in the f i e l d ; on - 36 -return to the laboratory the samples were a ir -dr ied and passed through a 2 mm sieve. This material was used for a l l subsequent determinations. Total organic carbon content was measured with a Leco Analyser. Extrac-table iron and aluminum was determined on samples ground to pass a 100 mesh sieve using three methods: (i) sodium pyrophosphate (Bascomb, 1968); ( i i ) acid ammonium oxalate (McKeague and Day, 1966); and ( i i i ) sodium-citrate, -bicarbonate, -dithionite (Mehra and Jackson, 1960). pH measurements in Iff KCl were made according to the method of Hesse Cp. 30, 1971), and are interpreted as being a good indicator of the pH in close proximity to the particle surfaces. Potentiometric Titrat ion As was discussed more fu l ly in the previous chapter, the fast-adsorption method using untreated samples produces results which are most applicable to the exchange reactions that take place in the f i e l d . For this reason the fast-adsorption potentiometric t i t rat ion method of Blok and de Bruyn (1970) was applied to untreated so i l samples (Laverdiere and Weaver, 1977). NaCl was used as the indifferent elec-trolyte in concentrations of 0.001^, 0.01N, 0.05N, and 0.2N. Forty ml of the indifferent electrolyte and 4 g of untreated, <2 mm soi l were t i trated with 0.1 ml aliquots of O.lN^HCl or NaOH. The samples were s t irred continuously and, after a two-minute equil ibration period, the resulting pH was measured with a calomel reference-glass electrode pair on a Radiometer PHM 62. RESULTS AND DISCUSSION The data for the f i r s t two horizons below ei-ther an 0 or A horizon in the seven soi ls ranging from a Typic Udipsamment to an Aquic - 37 -Haplorthod (Soil Survey Staff, 1976) are presented in Tables 4.1 and 4.2. At a l l sites the two horizons examined were both B horizons, except at s i te 1, where there was only one B horizon ident i f ied. The seven soi ls form a transect in a continuous depositional beach chrono-sequence and the sites range in age, as dated by dendrochronology, from 127 years at s i te 1 to 550 years at s i te 7. As an aid to assessing the functional relationships between the variables measured, a Pearson product-moment coefficient of correlation analysis program was run on the data in Tables 4.1 and 4.2. Selected columns of the resulting correlation matrix are presented as Table 4.3. With increasing age, there is a s ta t i s t i ca l l y s ignif icant increase in the amounts of extractable iron and aluminum and organic matter (Table 4.3). The increase in these values with increasing age is not constant from site 1 to s i te 7; the variations are explained by minor variations in soi l s i te factors such as microtopography and particle size d i s t r i -bution of the parent material. With the exception of s i te 1, the f i r s t subsurface horizon had the highest content of these materials. The best correlation between extractable sesquioxides and age was obtained with the pyrophosphate and citrate-bicarbonate-dithiom*te extractable aluminum; the correlations with extractable iron are not as strong. The best correlation between iron plus aluminum and age was obtained with the pyrophosphate extraction values. This supports the conclusion of the Canadian Soil Survey Committee (1978) and the Soil Survey Staff (1976) that the pyrophosphate-extractabl e iron plus aluminum is the best indicator of Spodosol (Podzol) development. Due to the increasing amounts of extractable iron and aluminum - 38 -TABLE 4.1: Some chemical and surface charge characteristics of the seven soi ls studied. . Organic SITE HORIZON ZPC a i ApH Carbon pH(KCl) (meq/lOOg.) ^ y r S ; % TYPIC UDIPSAMMENT (ORTHIC DYSTRIC BRUNISOL) B2 3.45 -1.15 0.72 127 0.61 4.17 CI 3.70 -0.68 0.42 127 0.46 4.12 TYPIC UDIPSAMMENT (ORTHIC DYSTRIC BRUNISOL) B2 3.60 -0.40 0.30 170 0.30 3.90 B3 3.87 -0.25 0.25 170 0.23 4.12 TYPIC HAPLORTHOD (ORTHIC DYSTRIC BRUNISOL) B2 3.68 -0.60 0.48 265 0.76 4.16 B3 4.05 T0.23 0.43 265 0.08 4.48 TYPIC HAPLORTHOD (ORTHIC HUMO-FERRIC PODZOL) B21ir 3.55 -0.73 0.22 370 0.91 3.77 B22 3.82 -0.35 0.26 370 0.61 4.08 TYPIC HAPLORTHOD (ORTHIC HUMO-FERRIC PODZOL) B21ir 3.35 -1.35 0.26 446 1.67 3.61 B22ir 3.80 -0.28 0.23 446 1.03 4.03 TYPIC HAPLORTHOD (ORTHIC HUMO-FERRIC PODZOL) B21ir 3.80 -0.55 0.17 480 1.60 3.97 B22ir 3.98 -0.30 0.12 480 1.14 4.10 AQUIC HAPLORTHOD (ORTHIC HUMO-FERRIC PODZOL) B21ir 3.65 -0.45 0.19 550 1.67 3.84 B22ir 3.85 r0.25 0.11 550 1.14 3.96 TABLE 4.2: Extractable sesquioxide data for the seven soils (percent of total sample). SITE SOIL CLASSIFICATION HORIZON PYROPHOSPHATE Fe Al Fe+Al ACID AMMONIUM OXALATE CITRATE-BICARBONATE -DITHIONITE Fe Al Fe+Al Fe Al Fe+Al 1 TYPIC UDIPSAMMENT B2 0.13 0.09 0.22 0.27 0 .12 0.39 0.32 0.10 0.42 CI 0.11 0.12 0.23 0.28 Q .17 0.45 0.35 0.13 0.48 2 TYPIC UDIPSAMMENT B2 0.13 0.15 0.28 0.26 0 .15 0.41 0.30 0.12 0.42 B3 0.07 0.10 0.17 0.19 0 .13 0.32 0.22 0.09 0.31 3 TYPIC HAPLORTHOD B2 0.16 0.14 0.30 0.27 0 .15 0.42 0.32 0.12 0.44 B3 0.06 0.10 0.16 0.20 0 .14 0.34 0.21 0.08 0.29 4 TYPIC HAPLORTHOD; B21ir .0.28 0.14 0.42 0.36 0 .17 0.53 0.44 0.17 0.61 B22 0.14 0.18 0.32 0.25 0 .22 0.47 0.29 0.19 0.48 5 TYPIC HAPLORTHOD B21ir 0.49 0.19 0.68 0.54 0 .20 0.74 0.63 0.23 0.86 B22ir 0.21 0.22 0.43 0.28 0 .26 0.54 0.36 0.27 0.63 6 TYPIC HAPLORTHOD B21ir 0.50 0.26 0.76 0.60 0 .30 0.90 0.64 0.32 0.96 B22ir 0.31 0.32 0.63 0.39 0 .32 0.71 0.45 0.30 0.75 7 AQUIC HAPLORTHOD B21ir 0.34 0.29 0.63 0.48 0 .29 0.77 0.48 0.28 0.76 B22ir 0.19 0.25 0.44 0.27 0 .25 0.52 0.33 0.23 0.56 C O - 40 -TABLE 4.3: Selected columns of the correlation matrix (R-values). - • - ' '• — -IT —• '"" ZPC a i ApH Age ZPC 1.00 0.87** -0.34 0.16 a i 0.87** 1 .00 -0.45 0.17 ApH -0.34 -0.45 1 .00 -0.76** Age 0.16 0.17 -0.76** 1.00 Organic Carbon -0.33 -0.32 -0.50 0.81** pH(KCl) 0.67** 0.46 0.47 -0.45 PYROPHOSPHATE Fe 0.33 -0.41 -0.48 0.69** Al 0.22 0.25 -0.74** 0.88** Fe+Al -0.15 -0.20 -0.62** 0.82** ACID AMMONIUM OXALATE Fe -0.35 -0.43 .0.39 0.61* Al 0.29 0.28 -0.73** 0.86** Fe+Al -0.14 -0.20 -0.56* 0.76** CITRATE-BICARBONATE-DITHIONITE Fe -0.41 -0.50 -0.39 0.59* Al 0.09 0.08 .0.71** 0.88** Fe+Al \ -0.23 -0.30 ' -0.55* 0.75** • ~ — ~ ~ <r * Significant at the .05 level ** Significant at the .01 level - 41 -with increasing soi l age, the ZPC was expected to increase. Although the general trend in Figure 4.1 indicates an increase in ZPC with i n -creasing soi l age, there is an extremely large amount of scatter, and Table 4.3 indicates that this relationship is not s ta t i s t i ca l l y s igni -f icant. On Figure 4.1, the f i r s t and second B horizons appear to be separated, with the f i r s t having a lower ZPC than the second; this is in spite of the higher sesquioxide content of the f i r s t B horizon. The higher organic matter content of the upper horizons causes this decrease in the ZPC values; the large fluctuation in ZPC values between sites is also strongly influenced by organic matter, as has been found by other researchers (van Raij and Peech, 1972; Gallez et al_., 1976; Morais et a l . , 1976; Laverdiere and Weaver, 1977). This explanation is further supported by the fact that the partial correlation coefficient between ZPC and age, with effect of organic matter removed (0.77), is s tat i s -t i c a l l y significant at the 0.01 leve l , while the simple correlation coefficient between ZPC and age (0.16) is not. In a similar manner, the pH(KCl) tends to decrease with increasing age of the s o i l ; super-imposed on this is the effect of organic matter. High levels of organic matter content cause a decrease in the measured pH(KCl); there-fore there is a co-variation of ZPC and pH(KCl) (R = 0.67, s ignificant at the 0.01 level ) . The displacement of the crossover point below the ZPT, defined as o-j, was expected to approach zero with increasing so i l age. The results presented in Table 4.1 and on Figure 4.2 show that this was not proven by the data. Although the general trend indicates the predicted re la-tionship, the great amount of scatter prevents any definite conclusion. 4 . 4 0 . 4 . 2 0 . 4 . 0 0 . pH 3 . 8 0 . 3 . 6 0 . 3 . 4 0 J 3 . 2 0 . O Z P C O Z P C O F F I R S T B H O R I Z O N O F S E C O N D B H O R I Z O N _ O O -O <•> <3> <3> o o . o —5 o o o o 0 0 i 2 0 0 3 0 0 \ A G E ( Y E A R S ) 4 0 0 \ 5 0 0 6 0 0 S I T E I S I T E 2 S I T E 3 S I T E 4 S I T E 5 S I T E 6 S I T E 7 FIGURE 4.1: Showing the relationship between the ZPC and age for the seven so i l s . 0 FIGURE 4.2: Showing the relationship between a-j and age for the seven so i l s . - 44 -The poor correlation between a-j and age also appears to be due to the variations in organic matter content. This is suggested by the compa-rison of the simple correlation coefficient between a^  and age (0.17) with the partial correlation coefficient between and age with the effect of organic matter removed (0.78, s ignificant at the 0.01 l eve l ) . The differences between the ZPC and pH(KCl) are presented as ApH values in Table 4.1 and on Figure 4.3. In this figure, the scatter of points around the regression l ine is much reduced and s ta t i s t i ca l ana-lysis reveals that the regression l ine is s ta t i s t i ca l ly s ignif icant at the 0.01 level . The trend of decreasing ApH values with time indicates that, as hypothesized, the ZPC is approaching the pH of the soi l with increased pedogenic development. The ApH value is highly correlated with the aluminum extracted by pyrophosphate, oxalate, and c i trate -bicarbonate-dithionite, and with the amounts of iron plus aluminum removed by these extractants. The ApH value also indicates that with increasing age the "active" surfaces in the soi l are approaching equi-librium with the soil solution. This last statement is derived from the empirical relationship between the ZPC and the pH of minimum solu-b i l i t y of pure oxides; since the hydrolysis of the surfaces is at a minimum at the ZPC, the pH of minimum solubi l i ty wi l l occur at the ZPC (Parks and de Bruyn, 1962). Although the so i l is not a pure system, the decrease in ApH with time is interpreted as indicating that the soi l is approaching steady-state. Most of the components in the system wil l not actually be in thermodynamic equilibrium with the soil solution; nonetheless, the results indicate that the "active" surfaces, which form the interface 1.0 O ApH OF FIRST B HORIZON O <•> ApH OF SECOND B HORIZON 100 ! 1 200 ! 1 - : 300 400 ; ; 1 r \ 500 600 j ! AGE (YEARS) j i ! • i " : I i : SITE I SITE 2 SITE 3 SITE 4 SITE 5 SITE 6 SITE 7 FIGURE 4.3: Showing the relationship between ApH and age for the seven soils (significant at the 0.01 l eve l ) . - 46 -between the so l id phase and the solution, are approaching a steady-state when considered as a whole. CONCLUSIONS The study reveals that as spodic soils develop, the surface charge characteristics change. The ZPC value was expected to increase and the a-j value to approach zero from the youngest to oldest s i t e . Due to differences in organic matter content, these trends were not clearly defined. The results show that both the ZPC and pH(KCl) values varied together, as a function of organic matter content, such that the di f fe-rence between the ZPC and the soi l pH decreased with time from a maximum value of 0.72 at s i te 1 to a minimum value of 0.11 at s i te 7. This trend is related to the increase in sesquioxide content of the B hori-zons. The hypothesis presented is part ia l ly supported by the experi-mental results; the increase in the ZPC and the decrease in the absolute value of aj were not proven to be significant with the data available. However, the ZPC does approach the pH of the s o i l , indicating that the chemistry of the "active" surface is approaching a steady-state. The ApH value is presented as a measure of pedogenic development. - 47 -LITERATURE CITED 1. Bascomb, C L . 1968. Distribution of pyrophosphate extractable iron and organic carbon in soi ls of various groups. J . Soil S c i . 19: 251-267. 2. Blok, L . , and P . L . de Bruyn. 1970. The ionic double layer at the ZnO/solution interface. I. Composition model of the surface. J . Co l lo id . Interface S c i . 32: 527-532. 3. Canada Soil Survey Committee. 1978. The Canadian System of Soil Class i f icat ion. Agriculture Canada, Ottawa, Ont. (In Press) 4. Clark, J . S . , J .A . McKeague, and W.E. Nichol. 1966. The use of pH-dependent C . E . C . for characterizing the B horizons of brunisolic and podzolic so i l s . Can. J . Soil S c i . 46: 161-166. 5. Gallez, A . , A.S.R. Juo, and A . J . Herbil lon. 1976. Surface and charge characteristics of selected soi ls in the tropics. Soil S c i . Soc. Am. J . 40: 601-608. 6. Laverdiere, M.R. , and R.M. Weaver. 1977. Charge characteristics of spodic horizons, Soil S c i . Soc. Am. J . 41: 505-510. 7. Hesse, P.R. 1971. A textbook of Soil Chemical Analysis. John Murray Publishers ( L t d . ) , London. 520 pp. 8. McKeague, J . A . , and J . H . Day. 1966. Dithionite and oxalate extrac-table Fe and Al as aids in differentiating various classes of s o i l s . Can. J . Soi l S c i . 46: 13-22. 9. Mehra, O.P . , and M.L. Jackson. 1960. Iron oxide removal from soi ls and clays by a dithionite-c i trate system buffered with sodium bicarbonate. Clays and Clay Minerals 5: 317-327. 10. Morais, F . I . , A . C . Page, and C S . Lund. 1976. The effect of pH, sa l t concentration and nature of electrolytes on the charge characteristics of Brazi l ian tropical s o i l s . Soil S c i . Soc. Am. J . 40: 521-527. 11. Parks, G.A. 1967. Aqueous surface chemistry of oxides and complex oxide minerals. Adv. Chem. 67: 121-160. 12. Parks, G.A. 1965. The isoelectric points of sol id oxides, so l id hydroxides and aqueous hydroxo complex systems. Chem. Rev. 65: 177-198. 13. Parks, G .A . , and P . L . de Bruyn. 1962. The zero point of charge of oxides. J . Phys. Chem. 66: 967-973. - 48 -14. Soil Survey Staff . 1975. Soil Taxonomy. Agriculture Handbook No. 436, Washington, D.C. 754 pp. 15. Van R a i j , B . , and M, Peech. 1972. Electrochemical properties of some Oxisols and Al f i so l s in the tropics. Soil S c i . Soc. Am. Proc. 36: 587-593, - 49 -Chapter Five THE EFFECT OF NaCl-SATURATION AND ORGANIC MATTER REMOVAL ON THE VALUE OF ZERO POINT OF CHARGE ABSTRACT Ten soi ls with a wide range of surface charge properties and extrac-table sesquioxide contents were studied. The zero point of charge (ZPC) and its displacement from the zero point of t i t rat ion (o^ -) were measured on untreated samples, on samples which were NaCl-saturated, and on sam-ples from which a portion of the organic matter had been removed. NaCl-saturation was shown to cause a shift of the ZPC and . In addition, removal of organic matter resulted in an increase in both the ZPC and the a,. - 50 -INTRODUCTION The factors which affect the measured value of the zero point of charge (ZPC) are important to the interpretation of the surface charge characteristics of s o i l s . Recent work by Laverdiere and Weaver (1977) indicates that for soi ls with low contents of phyl los i l icate clay mine-ra l s , NaCl-saturated samples and untreated samples give v ir tual ly the same measured value of ZPC. In contrast, soi ls with higher clay content, and hence with a potentially high permanent negative charge capacity, do not y i e l d the same ZPC values when measured in the NaCl-saturated as compared to untreated states (Chapter Three). Further, on the basis of indirect evidence, several authors have concluded that organic matter has a depressing effect on the ZPC (van Raij and Peech, 1972; Gallez et al_., 1976; Morais et al_., 1976; Laver-diere and Weaver, 1977; and Chapter Four). The evidence presented by these authors is based on comparison of the surface charge characteris-tics of samples which contain different amounts of organic matter, but which are similar in other respects. In the present experiment, ten soils with a broad range of surface charge properties and extractable sesquioxide contents were used. The ZPC and an- values were measured on untreated samples, NaCl-saturated samples, and samples from which a portion of the organic matter had been extracted in the laboratory. The aim was to determine: (i) whether the results of Chapter Three regarding the effect of NaCl-saturation could be confirmed for a wider range of soi l types; and ( i i ) whether direct evidence of the effect of organic matter on the ZPC and could be obtained. - 51 -MATERIALS AND METHODS Soil Samples The samples used in this study were collected from southwestern Bri t i sh Columbia, Canada, with the exception of the Typic Acrorthox and the Typic Hapludult, which were provided by members of the Soil Conser-vation Service, United States Department of Agriculture. The soi l c lass i f icat ion of the ten soils (Soil Survey Staff, 1975; Canada Soi l Survey Committee, 1978) and the sampling location of each is given in Table 5.1 . Sample Pretreatment Following a ir -drying , the samples were passed through a 2 mm sieve. Subsamples were taken and prepared as follows: (i) The untreated sam-ples were analysed after grinding to pass a 0.50 mm sieve but without any further treatment, ( i i ) The NaCl-saturated samples were prepared with 6 washings of IN NaCl followed by repeated washings with methanol/ water and acetone/water until free of CI" (AgN03 test ) , then washed twice with acetone and oven-dried at 40° C for 48 hours; f ina l ly the samples were ground to pass a 0.50 mm sieve, ( i i i ) The organic matter  extracted samples were treated three times with NaOCl according to the procedure of Lavkulich and Wiens (1970), followed by the same NaCl-satu-ration procedure as above, except that during the final NaCl washing the pH of the samples was adjusted to that of the NaCl-saturated samples and maintained until equilibrium was obtained (about one week). This step was necessary to prevent the high pH of the NaOCl extraction from inter-fering with the measurement of the ZPC (Ferreiro and Helmy, 1976)*. TABLE 5.1: Soil c lassif ication and sampling location for the ten s o i l s . SAMPLE NUMBER U.S.D.A. CLASSIFICATION CANADIAN CLASSIFICATION SOIL SERIES SAMPLING LOCATION 1 TYPIC HAPLUMBREPT B21 Orthic Ferro-Humic Podzol Bhf Marblehi l l 1 Agassiz, B.C. 2 TYPIC HAPLUMBREPT B21 Sombric Humo-Ferric Podzol B f l Durieu2 Durieu, B.C. 3 TYPIC HAPLAQUEPT B2g Orthic Humic Gleysol Bg - Durieu, B.C. 4 SPODIC FERRUDALF B21ir Orthic Humo-Ferric Podzol B f l Memekay3 Campbell River, B.C. 5 TYPIC UDIPSAMMENT B21 Orthic Dystric Brum'sol Bmx Cox Bay #2" Tofino, B.C. 6 TYPIC HAPLORTHOD B21ir Orthic Humo-Ferric Podzol B f l Cox Bay #4" Tofi no , B .C. 7 TYPIC HAPLORTHOD B21ir Orthic Humo-Ferric Podzol B f l Cox Bay #6h Tofino, B.C. 8 AQUIC HAPLORTHOD B21ir Orthic Humo-Ferric Podzol Bf x Cox Bay #7k Tofino, B.C. 9 TYPIC ACRORTHOX B22 No Canadian Equivalent Nipe 5 Oeste SCD, Puerto Rico 10 TYPIC HAPLUDULT B22t No Canadian Equivalent Facevi l ie 6 Johnston Co.., No. Carolina 1 Luttmerding, H.A. , and P.N. Sprout. 1967. Soil Survey Preliminary Report No. 8, B.C. 2 Luttmerding, H.A. , and P.N. Sprout. 1967. Soil Survey Preliminary Report No. 9, B.C. 3 Day, J . H . , L. Farstad, and D.G. Laird. 1959. Soil Survey Report No. 6, B .C. ^ Singleton, G.A. Ph.D. Thesis, Department of Soil Science, UBC. (In preparation) 5 Personal Communication. 1978. Luis H. Rivera, U.S .D.A. , Puerto Rico. 6 Personal Communication. 1978. Hubert J . Byrd, U.S .D.A. , North Carolina. - 53 -Characterization of the Soil Samples The following determinations were performed on samples ground to pass a 0.50 mm sieve. Soi l pH was measured in water (1:1, Peech, p. 922, 1965), 0.01M CaCl 2 (1:2, Peech, p. 923, 1965), and IN KCl (1:2.5, Hesse, p. 30, 1971). Three different extractions of sesquioxides were made using: (i) sodium pyrophosphate, one overnight extraction at pH 10 (McKeague, 1976); ( i i ) acid ammonium oxalate, one four-hour extraction at pH 3.0 (McKeague and Day, 1966); and ( i i i ) sodium-citrate, -bicarbo-nate, -d i th ioni te , two fifteen-minute extractions (Mehra and Jackson, 1960). The extracted Fe and Al were determined by atomic absorption spectroscopy. Total so i l organic carbon was determined by Leco Analyser. Potentiometric Titrat ion Potentiometric t i trat ion for the determination of the ZPC was per-formed with a soi l to electrolyte ratio of 1:20 (gm:ml). The indifferent electrolyte was NaCl in concentrations of 0.001 N, 0.01N, 0.05N_, and 0.2N. In al l cases the fast-adsorption method of Blok and de Bruyn (1970) was used with a two-minute interval between additions of 0.1N_HC1 or NaOH. Stat is t ical Analysis Correlations between variables for the ten soils were analysed using the Pearson product-moment correlation coefficient r. Two-way analysis of variance tests were performed using the ZPC or values from the NaCl-saturated and organic matter extracted samples. This was done to determine whether these values came from the same or different popula-tions . - 54 -RESULTS AND DISCUSSION Untreated Samples The soi ls studied have a wide range of texture and sesquioxide content, in keeping with the variety of soi l types and sampling locations (Table 5.2). The particular form of the sesquioxide materials depends on the pedogenic processes and parent materials which contributed to the formation of each s o i l . The soi ls from Bri t i sh Columbia contain an appreciable amount of their extractable sesquioxides as organically com-plexed iron and aluminum; this is evidenced by the relatively large amount of these elements being extracted by pyrophosphate as compared to the other two extractants (McKeague et al_., 1971). In contrast, the sam-ples from Puerto Rico and North Carolina contain much of their extracta-ble iron in the form of crysta l l ine iron oxide, as indicated by the large difference between the amounts of Fe extracted by CBD and by oxalate (McKeague and Day, 1966). The pH and surface charge data for the ten soi ls are presented in Table 5.3. In a l l the samples except sample 9, the pH value measured in water was greater than that measured in KCl (pH(H20)>pH(CaCl2)> pH(KCl)), indicating that the ZPC of the untreated samples was located below the ZPT (a-j negative). Sample 9 has a higher pH in KCl than in water (pH(H20)<pH(CaCl2)<pH(KCl)), and the ZPC is located above the ZPT (a.j posit ive) . ' The ZPC values generally increase with increasing amounts of pyro-phosphate- and CBD-extractable sesquioxides; with increasing total C content, the a^ values become more negative, indicating a higher perma-nent cation exchange capacity. These trends are not constant for a l l of TABLE 5.2: Soil texture and extractable sesquioxides for the ten so i l s . PYROPHOSPHATE AMMONIUM OXALATE CITRATE-BICARBONATE -PITHIONITE SAMPLE SOIL TFYTI IDF F E A 1 F E + A 1 F E A 1 F E + A 1 F E A 1 F E + A 1 NUMBER CLASSIFICATION itAiUKt 1 TYPIC HAPLUMBREPT B21 SiL 0.46 0.46 0.92 0.89 0.60 1.49 1.54 0.61 2.15 2 TYPIC HAPLUMBREPT B21 SiL 0.41 1.26 1.67 1.52 3.38 4.90 2.38 2.31 4.69 3 TYPIC HAPLAQUEPT B2 SiL 0.34 0.57 0.91 0.42 1.31 1.73 1.05 0.84 1.89 4 SPODIC FERRUDALF B21 SiL 0.83 0.88 1.71 1.64 1.16 2.80 3.60 0.94 4.54 5 TYPIC UDIPSAMMENT B21 S 0.13 0.15 0.28 0.26 0.15 0.41 0.30 0.12 0.42 6 TYPIC HAPLORTHOD B21 S 0.28 0.14 0.42 0.36 0.17 0.53 0.44 0.17 0.61 7 TYPIC HAPLORTHOD B21 S 0.50 0.26 0.76 0.60 0.30 0.90 0.64 0.32 0.96 8 AQUIC HAPLORTHOD B21 S 0.34 0.29 0.63 0.48 0.29 0.77 0.48 0.28 0.76 9 TYPIC ACRORTHOX B22 C 3.14 0.97 4:11 0.33 0.24 0.57 21.5 3.17 24.7 10 TYPIC HAPLUDULT B22 SL 0.02 0.10 0.12 0.18 0.21 0.39 3.17 0.38 3.55 > TABLE 5 . 3 : pH , s u r f a c e charge c h a r a c t e r i s t i c s , and t o t a l ca rbon f o r t he ten s o i l s . SAMPLE SOIL UNTREATED NaCl-SATURATED 0 R ^ ^ A T J E R NUMBER CLASSIFICATION -n t X I K A U h D " pn o-j a-j o-j H 2 0 C a C l 2 KCl ZPC Jjjg. \ ZPC ^ \ ZPC \ 1 TYPIC HAPLUMBREPT B21 5.91 5.46 5.27 4 .83 - 3 . 8 6 . 4 5 3 .10 - 1 7 . 5 3 . 7 5 3 .20 - 1 0 . 0 0 .47 2 TYPIC HAPLUMBREPT B21 5.32 4 .80 4 . 7 3 5.02 - 0 . 2 4 .27 5 .03 - O J 3 .86 6.40 +4.0 1.05 3 TYPIC HAPLAQUEPT B2 5.38 4 .54 4 .33 4 .52 - 0 . 7 1.41 4 . 4 2 - 1 . 8 1.44 4 .60 - 1 . 8 0 . 3 8 4 SPODIC FERRUDALF B21 5.40 4 . 5 2 ^ 4 .34 4.51 - 0 . 4 2.80 4 .19 - 1 . 9 2.51 6.10 +2.0 0 .30 5 TYPIC UDIPSAMMENT B21 5.08 4 . 2 8 3 .90 3.94 - 0 . 5 0 . 4 7 u - 0 .34 u _ 0 6 TYPIC HAPLORTHOD B21 5 .02 4 .24 3 .77 3 .34 - 1 . 7 1.24 u - 1 .07 u _ 0 7 TYPIC HAPLORTHOD B21 5.27 4 .28 3 .97 3 .86 - 0 . 7 1 .82 3 .84 - 0 . 4 1 .69 7.23 +0.7 0 . 0 8 8 AQUIC HAPLORTHOD B21 5.02 4 . 1 5 3 .84 3 . 9 2 - 0 . 5 1 .61 3 . 7 5 -2 .1 1.44 6 .70 +0.3 0 .04 9 TYPIC ACRORTHOX B22 4.70 5.33 6 .13 6 .57 +0.9 1.07 7 .12 +1.9 1.07 8 .28 +6.0 0.'66 10 TYPIC HAPLUDULT B22 5.19 3 .94 3 . 7 5 3.86 - 0 . 4 0 . 5 3 u - 0 .54 3 .20 - 5 . 5 0 .28 u : Undefined - 57 -the samples, but correlation analysis reveals that the relationships are significant at the 0.05 level . NaCl-Saturated Samples As was discussed previously (Chapter Three) NaCl-saturation of the exchange complex of the samples prior to potentiometric t i trat ion results in a shi f t of both the ZPC and the a-j values (Table 5.3). For samples which had a a^ value of less than -0.2 meq/lOOg and ZPC values less than pH 5.0 when untreated, this shift was to lower ZPC and values; at a-values of -0.2 meq/lOOg or higher and ZPC values higher than pH 5.0 when untreated, the shift was to higher ZPC and an- values, as in samples 2 and 9. The t i trat ion curves for samples 5 , 6 , and 10 did not achieve crossover prior to discontinuation of the t i trat ion at pH 3.0. The changes in ZPC and an- are attributed to the substitution on the exchange complex of readily exchangeable Na + and Cl~ ions for strongly bonded ions such as A l 3 + and P O ^ 3 - . Although both of these reactions will occur in a l l of the samples, the balance between the amounts of A l 3 + and POi* 3 - exchanged wil l determine whether NaCl-saturation wi l l result in more positive or more negative exchange sites being made acces-sible during the potentiometric t i t r a t i o n . This in turn wi l l control the direct ion, and the amount, of the shi f t in ZPC and a . . Organic Matter Extracted Samples The data on the effect of organic matter extraction on the ZPC, a-j, and total organic carbon content of the ten soi ls are presented in Table 5.3. The amount of carbon extracted from the different samples was variable, indicating that to different degrees the organic matter was incorporated within the amorphous material and therefore was not - 58 -attacked by the NaOCl solution. The amount of carbon remaining after treatment ranges from 0% to 60% of the or ig inal . The extraction of organic matter has s ignif icantly altered the sur-face charge characteristics of the soil samples. The effect can best be evaluated by comparing the organic matter extracted samples with the NaCl-saturated samples, since they both have NaCl-saturated exchange complexes. In every case there is an increase in both the ZPC and De-values following the organic matter extraction treatment, indicating that the organic matter has a depressing effect on the ZPC and . To test these apparent relationships, analysis of variance tests were per-formed with the result that the relationships were found to be s i g n i f i -cant at the 0.05 level . This confirms the assumptions of previous researchers, who on the basis of indirect evidence concluded that the presence of organic matter has the effect of lowering the value of the ZPC and a i . CONCLUSIONS The present study confirms the results of Chapter Three in that, using a broader range of so i l types, similar shifts in the value of ZPC and a-j were measured as a result of NaCl-saturation. Although the NaOCl treatment was unable to extract al l of the organic carbon from a l l of the samples, there was a significant increase in the measured values of ZPC and a- following the treatment. The results therefore support the hypothesis that organic matter depresses the ZPC and a-j values of s o i l s . - 59 -LITERATURE CITED 1. Blok, L . , and P.L. de Bruyn. 1970. The ionic double layer at the ZnO/solution interface. I. The experimental point of zero charge. J . Col lo id . Interface Sc i . 32: 518-525. 2. Canada Soil Survey Committee. 1978. The Canadian System of Soi l Class i f icat ion. Agriculture Canada, Ottawa, Ont. (In press) 3. Ferreiro, E . A . , and A.K. Helmy. 1976. The point of zero charge of hydrous oxide. Z. Pflanzenernaehr. Bodenkd. 6: 767-775. 4. Gal lez , A . , A.S.R. Juo, and A . J . Herbil lon. 1976. Surface and charge characteristics of selected soi ls in the tropics. Soi l S c i . Soc. Am. J . 40: 601-608. 5. Hesse, P.R. 1971. A Textbook of Soil Chemical Analysis. John (Murray Publishers (Ltd . ) , London. 520 p. 6. Laverdiere, M.R., and R.M. Weaver. 1977. Charge characteristics of spodic horizons. Soil S c i . Soc. Am. J . 41: 505-510. 7. Lavkulich, L . M . , and J . H . Wiens. 1970. Comparison of organic matter destruction by hydrogen peroxide and sodium hypochlo-r i t e and i t s effect on selected mineral constituents. Soil S c i . Soc. Am. Proc. 34: 755-758. 8. McKeague, J .A . (Ed.) . 1976. Manual on Soil Sampling and Methods of Analysis. S . R . I . , Agriculture Canada, Ottawa, Ont. 9. McKeague, J . A . , J . E . Brydon, and N.M. Miles. 1971. Differentia-tion of forms of extractable iron and aluminum in so i l s . Soil Sc i . Soc. Am. Proc. 35: 33-38. 10. McKeague, J . A . , and J . H . Day. 1966. Dithionite and oxalate extrac-table Fe and Al as aids in differentiating various classes of . so i l s . Can. J . Soil Sc i . 46: 13-22. 11. Mehra, O .P . , and M.L. Jackson. 1960. Iron oxide removal from soils and clays by a dithionite-c i trate system buffered with sodium bicarbonate. Clay and Clay Minerals 5: 317-327. 12. Morais, F. I . , A .C. Page, and C S . Lund. 1976. The effect of pH, sa l t concentration and nature of electrolytes on the charge characteristics of Brazi l ian tropical s o i l s . Soil S c i . Soc. Am. J . 40: 521-527. 13. Peech, M. 1965. Hydrogen-ion act iv i ty . In C.A. Black (Ed.) . Methods of Soil Analysis. Part 2. Agronomy 9: 914-926. Am. Soc. Agron., Madison, Wisconsin. - 60 -14. Soil Survey Staff . 1975. Soil Taxonomy. Agriculture Handbook No. 436, Washington, D.C. 754 p. 15. Van R a i j , B . , and M. Peech. 1972. Electrochemical properties of some Oxisols and Alf i so ls in the tropics. Soil S c i . Soc. Am. Proc. 36: 587-593. - 61 -Chapter Six SUMMARY AND CONCLUSIONS The aim of this thesis was to examine the use of surface charge characteristics as a measure of pedogenic development in selected soils of the humid, temperate environment of southwestern Brit ish Columbia. A secondary objective was to gain further knowledge of the factors which influence the values of ZPC and o\j obtained in the laboratory, so that interpretations o f the results might be more clearly understood. Chapter One presents a brief discussion of the approach taken in this series o f experiments. The basic concept was introduced that soi l genesis proceeds at the interface between the so i l particles and the so i l solution. From this develops the realization of the importance of surface charge phenomena as a measure of the alteration o f the parent material, or in short, as a measure of so i l genesis. The f i r s t experiment, presented in Chapter Two, demonstrated that for three so i l s containing large amounts of clay minerals, the surface charge properties were strongly related to chemical and morphological measures of so i l development. The results indicated that as the pedo-genic development proceeded and the sesquioxide content increased, pH-dependent charge became dominant over pH-independent (permanent) charge. As a result , the an- values approached zero; the ZPC became more clearly defined and approached the pH of the s o i l . Chapter Three examined the effect that different potentiometric t i t rat ion procedures had on the measured values of ZPC and a,-. It was - 62 -found that the slow-adsorption method produced lower values of and higher values of ZPC compared with the fast-adsorption method. In the fast-adsorption method the reaction being measured is one between the surface proper and the potential determining ions H + and OH"; in con-trast , the slow adsorption includes in the measurement H + and OH" ions incorporated into the hydrous oxide coatings; i t is therefore not con-sidered to duplicate accurately a surface phenomenon. It was also found that NaCl-saturation prior to t i trat ion caused the ZPC and values to change compared to values obtained on untreated samples. For ZPC and values below pH 5.0 and -0.25 meq/lOOg respec-t ive ly , as measured on untreated samples, the NaCl-saturation resulted in lower values; for values above pH 5.0 and -0.25 meq/lOOg the change was to higher values. This change in surface charge characteristics is attributed to the displacement from the exchange complex of strongly bonded ions such as A l 3 + and PO^ 3". The fast-adsorption technique using untreated samples is considered by the author to be the most accurate measure of the surface charge characteristics in the f i e l d , since i t mea-sures the properties at the surface and involves the least alteration of the properties of that surface prior to t i t r a t i o n . In Chapter Four the fast-adsorption potentiometric t i trat ion method and untreated samples were used to study the changes in surface charge characteristics along a soi l chronosequence developed on sandy beach material. The variation of the organic matter contents at the seven sites resulted in poorly defined trends between ZPC and so i l age and between a- and soi l age. Partial correlation analysis revealed that i f the effect of organic matter was eliminated, both of these trends were - 63 -highly s igni f icant . The ApH value, defined as the difference between the ZPC and the pH measured in 1 N_ KCl , was found to correlate highly with soi l age, since the effect of organic matter cancelled out. In this chronosequence, ApH proved to be a good indicator of soi l genesis; the values decreased from the youngest to the oldest s i t e , indicating that the so i l part ic le surfaces are approaching a steady-state condition with respect to the so i l solution. Chapter Five re-examined the effect of NaCl-saturation on the ZPC and a.j values using ten so i l samples from southwestern Brit ish Columbia, Puerto Rico, and North Carolina. These samples were chosen because they presented a wide range of extractable sesquioxide contents and surface charge properties. The results confirm the conclusions presented in Chapter Three, that NaCl-saturation prior to t i trat ion changes the ZPC and a-j values. In the case of untreated samples with ZPC values below pH 5.0 and values less than -0.2 meq/lOOg, there was a shi f t to lower ZPC and values following NaCl-saturation; when the ZPC and were above pH 5.0 and -0.2 meq/lOOg respectively, the shift was in the oppo-site direct ion. The effect of organic matter was also evaluated and the removal of a portion of this material with NaOCl resulted in higher measured values of both ZPC and o\j.. This experiment has produced direct confirmation that organic matter has a depressing effect on ZPC and . Measurement techniques have a s ignif icant effect on the values of ZPC and an- obtained. The fast-adsorption procedure measures the fast reaction at the part ic le surface, while the slow-adsorption procedure measures the additional slow reaction resulting from the incorporation of H + and OH" ions into the structure of the oxide coatings. NaCl-satu-ration prior' to t i trat ion results in the exchange of strongly bonded - 64 -ions and thus alters the properties of the surface. The fast-adsorption method using untreated samples is therefore considered to be the most accurate means of measuring the surface charge properties as they exist in the f i e l d . Indirect evidence suggested that the presence of organic matter caused a decrease in the ZPC and a- values. This evidence is in the form of simple correlation coefficients between organic C and a.. The results of partial correlation analysis removing the effect of organic matter indicate strong relationships between ZPC and so i l age and also between a-j and soi l age. Evidence of this relationship is also supported by direct measurement. The removal of a portion of the organic matter using a pretreatment of NaOCl resulted in a significant increase in both the ZPC and values. Potentiometric t i trat ion is a useful measure of surface charge cha-racterist ics and soi l genesis. In soi ls with large amounts of clay mine-rals the ZPC and change as the sesquioxide content increases. In sandy s o i l s , the values of ZPC and a-j are only s ignif icantly related to so i l age when the effect of differences in the so i l organic matter con-tent has been eliminated. The ApH value, on the other hand, compensates for differences in organic matter and provides a good indication of the extent of so i l development. - 65 -APPENDIX The appendix consists of prof i le descriptions for those soi ls not described elsewhere in the thesis. Samples from the Cox Bay transect are used in the experiments described in Chapters Four and Five, and the morphological properties are presented here in tabular form. Five of the samples discussed in Chapter Five are described here in the form of f i e ld descriptions (Samples 1, 2, 3, 9, and 10); the others are i n -cluded in the descriptions in Chapter Two, or in the descriptions of the soi ls from Cox Bay. MORPHOLOGICAL PROPERTIES OF COX BAY TRANSECT NOTE: Texture of all mineral horizons is fine sand. SITE HORIZON^ Smi : , c o y o u R STRUCTURE = "fJSS^ N 0 - U.S CDN ( M o i s t ) TYPIC UDIPSAMMENT (Orthic Dystric Brunisol) 02 FH 6-0 5YR 2/1 - -B2 Bftf 0-10 5Y 4/3 wk. subangular blocky v. fr iable CI CI 10-25 5Y 3/2 v. wk. subangular blocky v. friable C2 C2 25-46 5Y 5/2 v. wk. subangular blocky loose C3 C3 46+ 5Y 5/2 v. wk. subangular blocky 1 oo se TYPIC UDIPSAMMENT (Orthic Dystric Brunisol) 02 FH 8-0 5YR 2/2 - -B2 Bm 0-15 2.5Y 5/4 v. wk. subangular blocky v. friable B3 BC 15-56 5Y 4/3 v. wk. subangular blocky 1 oose C C 56+ 5Y 4/3 wk. subangular blocky loose TYPIC HAPLORTHOD (Orthic Dystric Brunisol) 02 FH 10-0 5YR 2/1 - -A2 Aej 0-1 10YR 3/1 v. wk. subangular blocky loose B2ir Bm 1-38 10R 2/2 mod. subangular blocky v. fr iable B3 BC 38-69 2.5Y 4/4 mod. subangular blocky v. friable C C 69+ 5Y 4/3 v. wk. subangular blocky v. fr iable MORPHOLOGICAL PROPERTIES OF COX BAY TRANSECT (Cont'd) SITE NO. HORIZON U.S. . CDN DEPTH cm COLOUR STRUCTURE CONSISTENCE (Moist) 4 TYPIC HAPLORTHOD (Orthic Humo-Ferric Podzol) 02 FH 8-0 ~5YR 2/1 -A2 Ae 0-3 5YR 5/1 mod. subangular blocky v. fr iable B21ir Bf '3-18 10YR 3/4 mod. subangular blocky v. friable B22ir Bm 18-46 10YR 6/6 mod. subangular blocky firm B3 BC 46-91 2.5Y 4/4 v. wk. subangular blocky friable C C 91 + 5Y 4/3 v. wk. subangular blocky friable 5 TYPIC HAPLORTHOD (Orthic Humo-Ferri c Podzol) 02 FH 10-0 5YR 2/1 - -A2 Ae 0-5 10YR 6/2 mod. subangular blocky v. friable B21ir Bfl 5-26 7.5YR 3/2 st. subangular blocky firm B22ir B'f2"- 26-36 10YR 3/3 wk. subangular blocky f i rm B23ir Bm 36-58 2.5Y 5/6 wk. subangular blocky friable B3 BC 58-99 2.5Y 4/4 wk. subangular blocky friable C C 99+ 2.5Y 4/2 single grain loose CT) MORPHOLOGICAL PROPERTIES OF COX BAY TRANSECT (Cont'd) STRUCTURE CONSISTENCE (Moist) TYPIC HAPLORTHOD (Orthic Humo-Ferri c Podzol) 02 LFH 8-0 5YR 2/2 - -A2 Ae 0-5 10YR 5/2 wk. subangular blocky v. friable B21ir Bfl 5-10 2.5YR 3/2 wk. subangular blocky loose B22ir Bf2 10-20 2.5YR 3/2 mod . subangular blocky firm B23ir Bm 20-43 7.5YR 4/4 mod . subangular blocky firm B3 BC 43-74 TOYR 5/4 v. wk. subangular blocky v. friable C C 74+ 10YR 5/4 v. wk. subangular blocky loose AQUIC HAPLORTHOD (Orthic Humo-Ferri c Podzol) 02 LFH 20-0 2.5Y 2/0 - -A2 Ae 0-6 10YR 4/2 v. wk. subangular blocky friable A&B A&B 6-18 - mod . subangular blocky friable B21ir Bfl 18-30 10YR 2/1 wk. subangular blocky friable B22ir Bf2 30-56 10YR 2/2 mod . angular blocky - coarse platy f i rm B3 BC 56 + 10YR 3/3 wk. angular blocky f i rm FROM: G.A. Singleton. 1978. Ph.D. Thesis, Dept. of Soil Science, UBC. (In preparation) - 69 -FIELD DESCRIPTION OF MARBLEHILL SERIES Soil Class i f icat ion: TYPIC HAPLUMBREPT (Orthic Ferro-Humic Podzol) Locati on: On top of a ridge 1 km SE of Agriculture Canada experimental farm, Agassiz, B.C. HORIZON DEPTH cm DESCRIPTION 0 (LFH) 2--0 Deciduous l i t t e r in various states of de-composition . Al (Ah) 0--5 Very dark brown (10YR 2/2, m.); fine sandy loam; weak, fine granular; very f r iab le ; abundant, fine and medium roots; abrupt, smooth boundary to: B21 (Bhf) 5 -15 Brown (7.5YR 4/3, m.); s i l t loam; weak, fine granular; v. f r iab le ; abundant, fine and medium roots; abrupt, wavy boundary to: B22 (Bf) 15 -44 Brown (7.5YR 4/4, m.); s i l t loam; weak, f ine, subangular blocky; very fr iab le ; p l en t i fu l , fine and medium roots; clear wavy boundary to: B3 (BC) 44-80 Yellowish brown (10YR 5/5, m.); s i l t loam; moderate, f ine, subangular blocky; very fr iab le ; few, fine and medium roots; common, f ine, dist inct mottles (7.5YR 4/4, m.); diffuse boundary to: C (C) 80-150+ Brown (10YR 4/3, m.); s i l t loam; moderate, medium, subangular blocky; very fr iab le ; few, medium roots; common, f ine, dist inct mottles (7.5YR 4/4, m.) . - 70 -FIELD DESCRIPTION OF DURIEU SERIES Soi l Class i f icat ion: TYPIC HAPLUMBREPT (Sombric Humo-Ferric Podzol) Location: Durieu County, Bri t i sh Columbia. HORIZON D E P T H DESCRIPTION cm 0 (LFH) 3-0 Coniferous l i t t e r in various states of de-composition. Al (Ah) 0-13 Very dark brown (10YR 2/2, m.); s i l t loam; weak, fine granular; very f r iab le ; abun-dant f ine , medium, and coarse roots; abrupt, wavy boundary to: A2 (Ae) 13-14 Very dark grayish brown (10YR 3/2, m.); s i l t loam; fr iable ; few f ine, plentiful medium and few coarse roots; abrupt, wavy boundary to: B21ir (Bfl) 14-30 Dark reddish brown (2.5YR 3/4, m.); s i l t loam; moderate, medium, subangular blocky; f r iab le ; few fine, plentiful medium and few coarse roots; gradual, wavy boundary to: B22ir (Bf2) 30-50 Brown (7.5YR 4/4, m.); s i l t loam; moderate, medium, subangular blocky; f r iab le ; few f ine , plentiful medium and few coarse roots; d i f -fuse, wavy boundary to: B3 (BC) 50-90 Dark yellowish brown (10YR 3/6, m.); s i l t loam; moderate, medium, subangular blocky; f r iab le ; few f ine, plentiful medium, few coarse roots; clear smooth boundary to: C (C) 90-110+ Grayish brown (2.5Y 3/2, m.); sandy loam; strong, medium to coarse, subangular blocky; firm; weakly cemented; few medium and coarse roots; common, fine to medium, dis t inct mottles (7.5YR 5/6, m.). - 71 -FIELD DESCRIPTION OF UNNAMED ASSOCIATE OF DURIEU Soil Class i f icat ion: TYPIC HAPLAQUEPT (Orthic Humic Gleysol) Location: Durieu County, Bri t i sh Columbia. HORIZON DEPTH cm DESCRIPTION Al B2g (LFH) (Ah) (Bg) Clg (Cgl) C.2g (Cg 2) 1-0 0-12 12-22 22-50 501 Deciduous l i t t e r , raw to well-decomposed. Black (10YR 2/1, m.); loam; weak, fine to medium granular; very fr iab le ; abundant, fine and medium roots; abrupt, wavy boun-dary to: Brown (10YR 4/3, m.); s i l t loam; moderate, medium to coarse, subangular blocky; firm; few, fine and medium roots; common, medium, prominent mottles (5YR 5/8, m.); clear, wavy boundary to: Grayish brown (2.5Y 5/2, m.); loam; mode-rate, coarse, subangular blocky; firm; few, medium roots; common, medium, promi-nent mottles (5YR 5/8, m.); c lear, wavy boundary to: Grayish brown (2.5Y 5/2, m.); s i l t y clay loam; moderate, coarse, subangular blocky; firm; few, medium roots; many, medium, prominent mottles (5YR 3/3, m.); below water table. - 72 -FIELD DESCRIPTION OF NIPE SERIES Soil Class i f icat ion: Location: TYPIC ACRORTHOX (No Canadian Equivalent) Oeste SCD, Puerto Rico; 1 mile E of Mayaguez; 0.5 mile W of km marker 5.5 of highway 349. On top of a h i l l . HORIZON DEPTH i n . DESCRIPTION Ap Bl B21 B22 B23 B24 B25 0-11 11-18 18-28 28-38 38-48 48-62 62-80H Dark reddish brown (2.5YR 2/4, m.); clay; strong fine granular structure; fr iab le , nonsticky, s l ight ly p las t ic ; many fine roots; strongly acid; clear smooth boun-dary . Dark reddish brown (2.5YR 3/4, m.); clay; weak fine angular blocky structure; very f r iab le , nonsticky, s l ight ly plast ic; many fine pores; common fine roots; very strongly acid; clear smooth boundary. Dark red (7.5R 3/8, m.); clay; weak fine angular blocky structure; very fr iab le , nonsticky, s l ight ly p las t i c ; many fine pores; few fine roots; very strongly acid; diffuse smooth boundary. Dark red (7.5R 3/6, m.); clay; massive; firm, nonsticky, s l ight ly p last ic ; many fine pores; few fine iron concretions; strongly acid; diffuse smooth boundary. Dusky red (7.5R 3/4, m.); clay; massive; f r iab le , nonsticky, s l ight ly p las t ic ; many fine pores; few fine iron concretions; strongly acid; gradual smooth boundary. Dark red (7.5R 3/6, m.); clay; massive; f r iab le , nonsticky, s l ight ly plast ic; many fine pores; medium acid; diffuse smooth boundary. Dusky red (7.5R 3/4, m.); clay; massive; f r i a b l e , nonsticky and nonplastic; many fine pores; medium acid. FROM: Luis H. Rivera, Soil Conservation Service, U.S .D.A. , San Juan, Puerto Rico. - 73 -FIELD DESCRIPTION OF FACEVILLE SERIES Soil Class i f icat ion: TYPIC HAPLUDULT (No Canadian Equivalent) Location: About 6600 feet W (280° ) from the intersection of Johnston County Roads 1700 and 1715, 900 feet from the Neuse River, at the end of a logging road. HORIZON D ^ T H DESCRIPTION Ap 0-7 Dark brown (7.5YR 4/2); sandy loam; common medium faint brown (7.5YR 5/4) mottles; weak medium granular structure; very fr iable; common fine roots; common fine pores; strongly acid; clear wavy boundary, A2 7-10 Brown (7.5YR 5/4); sandy loam; common medium prominent dark red (2.5YR 3/6) mot-t les; massive; frrable; few fine roots; few fine pores; strongly acid; clear wavy boun-dary . B21t 10-50 Dark red (2.5YR 3/6); sandy clay; moderate medium subangular blocky structure; fr iab le ; few fine and medium roots; common thin d i s -t inct continuous clay films on faces of peds; strongly acid; diffuse smooth boun-dary. B22t 50-93 Dark red (2.5YR 3/6); sandy clay; few medium dist inet yellowish red (5YR 5/6) and few medium prominent yellowish brown (10YR 6/4) mottles; moderate medium subangular blocky structure; fr iable; common thin dist inct continuous clay f i lms; very strongly acid; diffuse smooth boundary. C 93-144 Red(2.5YR 4/6); clay loam; common medium prominent l ight gray (10YR 7/1) and pale brown (10YR 6/3) mottles; massive; fr iable; few thin faint clay films along root chan-nels; very strongly acid. FROM: Hubert J . Byrd, Soil Conservation Service, U.S .D.A. , Raleigh, North Carolina. 

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