Changes in soil phosphorus fractions following positive and negative phosphorus balances for long periods

The effect of phosphorus (P) balance (addition, in both fertilizers and farmyard manure (FYM), minus removal in crops) on eight soil P fractions determined by sequential extraction, was measured on archived soils from various long-term experiments run by Rothamsted Experimental Station in the United Kingdom. It has been established unequivocally that, for all the soils investigated, no one of the eight P fractions was increased or decreased during long periods of P addition or depletion, respectively. However, changes were mainly in the resin (2430%) and the inorganic (Pi) component of the four fractions extracted sequentially by 0.5 M NaHCO3, 0.1 M NaOH, 1.0 M NaOH, 0.5 M H2SO4 (41-60%). For the sandy loam there were also consistent changes in the organic (Po) fraction (25%), especially that extracted by bicarbonate, presumably because the soil contained only a little clay and presumably had low sorption capacity. When the soils were cropped without P addition the largest proportional change was in the P extracted by resin, 0.5 M NaHCO3 and 0.1 M NaOH, suggesting that the P in these fractions is readily available, or has the potential to become available, for crop growth. This was supported by changes in the overall P balance. On the heavier textured soils, 50-80% of the change in total soil P (PT) was in these fractions; on the sandy soil this increased to more than 90%. The change in the sum of the first five fractions accounted, on average, for 90% of the P balance. However these changes in the P in the plough layer frequently left large amounts of P unaccounted for in some of the excessively P enriched soils. The amount of Pi extracted by resin and bicarbonate (Pi(r+b)) ranged between 14 and 50% of the sum of the Pi fractions. Soils with the lower percentages were those known to be most responsive to P fertilizers. Pi(r+b) accounted for an average of 70% of the P balance (negative) in P depleting soils where crop offtake was not offset or exceeded by annual P additions (positive balance). The ratio between Pi(r+b) and Pi(sum) could be a guide in defining soils deficient in P and those which are excessively enriched.