INORGANIC SOIL PHOSPHORUS

Abstract

Summary Sixteen soils and 4 soil preparations were cropped exhaustively with ryegrass in the glasshouse and monocalcium phosphate potentials (½pCa+pH 2 PO 4 = 1 ) were measured after each of 6 consecutive harvests. The amounts of phosphorus ( Q ) removed from the soils by ryegrass accounted for 95·1–96·6 per cent of the variance in 1 for 3 soils and 2 soil preparations ( P < 0·001), for 88·4–93·7 Per cent of the variance for 6 soils and 2 soil preparations (0·001 < P < 0·01), for 71·6–82·6 per cent of the variance for 3 soils (0·01 < P < 0·05) and for insignificant amounts of the variance for 4 soils. Values of Δ I /Δ Q ranged from 7 × 10 –4 to 431 × 10 –4 ½pCa+pH 2 PO 4 /ppm P removed from soil. Δ I /Δ Q tended to decrease (i.e. the soils were more buffered) with increasing clay contents and with increasing amounts of NaHCO 3 ‐soluble P and to increase (i.e. the soils were less buffered) with increasing amounts of CaCO 3 . Variations in organic C did not significantly affect Δ I /Δ Q . The following equation accounts for 81 per cent of the variance in Δ I /Δ Q for all soils except those in equilibrium with octacalcium phosphate: Δ I /Δ Q × (10 4 ) = 225·9–4·17(% clay)+8·01(% CaCO 3 )–1·38(ppm NaHCO 3 ‐soluble P).