Improved Corrective Fertilizer Recommendations Based on a Two-Step Alternative Usage of Soil Tests: III. The Bray-1 Test on Soils with Concretions

Six soils from temperate and tropical regions and of wide range in P fixation tendency were treated with five rates of P. Portions of each P-treated soil were equilibrated for 6-weeks and tested for Bray-1 P. Also, Bray-1 P was computed for each soil and P treatment from P fixation factors determined by quicktest recovery of added P in Bray-1 extract after a 2-h equilibration. Actual amounts of fertilizer P required to reach sufficiency level of available P for near-maximum yields (PSL = 20 µg·g⁻¹ were obtained from the added P vs. Bray-1 P relationship. Similarly, amounts of fertilizer P required for sufficiency levels in the soil were computed from constant 22.9% recovery for all soils, and from individual P fixation factors obtained by quicktest recovery of added P. The P-treated soils were also cropped to corn (Zea Mays) in a growth chamber. Yields and P contents of tissue were measured. The computed fertilizer P values for PSL = 20 from quicktest recovery of added P after 2-h equilibration were highly correlated with the actual amounts found by 6-week equilibration (r² = 0.831). Use of this second step, in which the quicktest-computed values are regressed to the equivalent of actual (6-week) values, make the average deviation from the actual values only 29 compared to 182% when computed on the basis of 22.9% recovery of P from all soils. When the Fe and Mn concretions in the Delicias soil (Oxisol) were crushed and the 2-h equilibration extended to 24 h, the r² value for computed vs. actual values was 0.983, and the average deviation of quicktest computed from the actual values for the six soils was only 15%. Quadratic regression showed that the Bray-1 P accounted for 81% of the variation in the yields, while linear regression showed Bray-1 P accounted for this same percent of the variation in P contents of the corn seedlings. When the soils were grouped into populations according to the factor for efficiency of recovery of available P, c₁, correlation of the yield component data [log(A-y)] with corresponding Bray-1 soil available-P component data (c̄₁x in the Mitscherlich-Bray equation gave r² = 0.884.