Some relationships between isotopically exchangeable phosphate, soil analysis and crop growth in the greenhouse

An experiment on perennial ryegrass in small pots compared yields of grass and responses to added superphosphate with the analyses of 15 soils and with their total isotopically exchangeable phosphate (‘A ’ value). the highest positive correlations between yields, and negative correlations between responses to added superphosphate, were obtained with ‘ A ’ values. the phosphate extracted by 0.5M‐NaHCO3, 0.5N‐acetic acid–sodium acetate, 0.3N‐HCl and 0.002N‐H2SO4 (reagents 1–4, respectively) also correlated well with yield. Yield and phosphate soluble in 0.5N‐acetic acid (reagent 5) were not significantly correlated.

The soils were mixed with 32P‐labelled superphosphate, stored for 30 days at field moisture capacity, air‐dried and extracted with the five reagents listed above. the amounts of 32PO43– that exchanged with soil phosphate during extraction were compared with the amounts that exchanged in soils similarly treated and cropped with ryegrass in the green‐house. More soil phosphate exchanged during extraction with reagents 3–5 in the laboratory than in the greenhouse experiment. These reagents dissolved soil phosphate, or exposed new surfaces on the soil, from which phosphate is not taken up by ryegrass. the soil phosphate that exchanged in reagents 1 or 2 was more closely correlated with, and nearly equal to, the ‘ A ’ value of the soils enriched with superphosphate. Reagent 1 did not increase soil phosphate surfaces in most soils, but reagent 2 dissolved some phosphate, which did not exchange in the greenhouse, from soils containing more than 4% CaCO3.