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 (24 - 30%) and the inorganic (P-i) 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 (P-o) 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 (P-T) 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 P-i 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 P-i(sum) could be a guide in defining soils deficient in P and those which are excessively enriched.