Improved Corrective Fertilizer Recommendations Based on a Two-Step Alternative Usage of Soil Tests: III. The Bray-1 Test on Soils with Concretions1
- S. Mostaghimi and
- E.O. McLean2
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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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