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Phosphorus availability in two calcareous soils Gough, Neville Astor


A study was made of the influence of soil moisture tension and soil temperature on the availability of phosphorus from five different compounds in two calcareous soils. In the study of moisture tension, the top six inches of Machete stony sandy loam and of Nisconlith clay loam were treated with five phosphate carriers of varying water solubility, monoammonium phosphate, monocalcium phosphate, anhydrous dicalcium phosphate, calcium metaphosphate and hydroxyapatite, at a rate equivalent to 120 pounds of P₂O₅ per acre. The soils were placed in empty crocks and weighed. These weights were used along with information obtained from previously prepared moisture tension curves to maintain the moisture tension ranges. The tension ranges used were: 0.2 - 0.4, 0.2 - 0.8, 0.2 - 2.0 and 0.2 - 6.0 bars. Irrometers were used to measure and control moisture tension ranges of 0.2 - 0.4 and 0.2 - 0.8 bars. The moisture tension ranges of 0.2 - 2.0 and 0.2 - 6.0 bars were measured gravimetrically. The soils were seeded with alfalfa and placed in growth chambers built in the greenhouse. Three cuttings of the plants were made at the third blossom stage and the dried plant tissue was analysed for phosphorus. At the end of the first cutting, NaHCO₃ extractable phosphorus was determined in these soils and dicalcium phosphate activity at the end of the third harvest. Statistical methods were used to determine the significance of the experimental results. The yields from both soils showed that a moisture tension range of 0.2 - 2.0 bars gave the best growth for the tensions. On the Nisconlith soils the other tensions gave yields that were almost as high as those obtained at 0.2 - 2.0 bars tension. This moisture tension was also the most favourable for phosphorus uptake as indicated by the phosphorus content in plant tissue. The effect of phosphorus source proved to be of no significance on the yield of alfalfa. The phosphorus content in plant tissue from the Machete soil was related to the degree of water solubility of the phosphate compounds. A significant difference between the effectiveness of the water soluble compounds, such as monoammonium phosphate and the water insoluble or citrate insoluble compounds such as hydroxyapatite, was observed. Soil moisture tension had a significant effect on the phosphorus content in plant tissue at a 10% probability. NaHCO₃ extractable phosphorus, removed from both soils after the first cutting, was not directly related to the degree of water soluble phosphorus content of the compounds added. On the Machete soil, monocalcium phosphate treated soil released the greatest amount of NaHCO₃ extractable phosphorus, yet the monoammonium phosphate is ten times more soluble in water. Moisture tension had no effect on the amount of NaHCO₃ extractable phosphorus released. Dicalcium phosphate activity calculated from the calcium, magnesium and phosphorus concentrations in the soil solutions proved to be unsuitable for predicting the availability of phosphorus from the phosphate compounds. These determinations might have proved otherwise if the fertilizer trials were of a shorter duration. In the study of the effects of two soil temperatures 10°C and 24°C, on phosphorus availability, the same two soils, Machete stony sandy loam and Nisconlith clay loam were used. The two soils were potted and five phosphate compounds added at a rate equivalent to 120 pounds of P₂O₅ per acre. These pots were seeded with alfalfa and subjected to soil temperatures of 10°C and 24°C. The first temperature was maintained by the use of a temperature bath placed in the greenhouse and the second temperature was maintained by placing the pots on the greenhouse bench. It was found that at the higher temperature of 24°C higher yields of alfalfa were obtained than at 10°C. No one phosphate source was outstanding in its effect on alfalfa yield from both soils. There was a trend towards increased NaHCO₃ extractable phosphorus with an increase in temperature in both soils.

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