THE MANAGEMENT OF PHOSPHATE FERTILIZATION IN WHEAT CROPS

The analytical soil and wheat yield data obtained from many long-term field experiments on phosphate fertilization have been analyzed. There were three P treatments. P0, without P fertilizer, P1 about 1.5 times the annual P output by crops; and P2 which is 2 x P1. Soil P status was determined by chemical extraction, according to the methods of Dyer (P-Dyer), Joret-Hebert (P-JH) and Olsen (P-Olsen) and also by means of an isotopic exchange procedure for determining the quantity of PO4-ions isotopically exchanged in 1 min (E(Pie1)). Wheat yields were transformed into a relative yield index (IR) defined as the ratio, expressed in percentage, of wheat yield observed in the P0 or P1 treatment to that measured in the P2 treatment considered as optimal condition of P nutrition to crops IR values and soil P status were correlated according to the following equations: IR = 100-81.4 x exp [-0.031 x (P-Dyer)] (18 trials); IR = 100-70.2 x exp [-0.036 x (P-JH)] (24 trials); IR = 100-36.4 x exp [-0.070 x (P-Olsen)] (22 trials); IR = 100-37.8 x exp [-0.53 x (E(Pie))] (11 trials). The soil P levels, expressed in mg P2O5 kg-1 soil, the above 95% of maximum grain yield in wheat and calculated by means of equations were: 90 for P-Dyer, 73 for P-JH, 28 for P-Olsen and 8.7 for E(Pie1). This approach allowed permitting the obtainment of to make P fertilizer recommendations for wheat crop to be made, preventing overfertilization without reducing crop yields.