Results
Mehlich-3, Bray-1, and resin-extractable P explained 67, 57, and 62% of the variation in the optimum P fertilizer rate to be applied on the pooled soils where either kaolinite or gibbsite was the predominant mineral in the clay fraction.
The combination of Mehlich-3 with clay2 increased the proportion of the variation to 84%. Clay content and P-buffering coefficient, obtained after 2 hours of equilibration (R2 = 0.82), were the two best practical properties that could be combined with Mehlich-3 to improve the soil test for P. The same trend was observed with the other soil extractants.
Among the three soil P extractants used in this study, the resin seems to be the best when used in combination with estimates of the soil Pbuffering capacity. The combination of clay with Resin-P explained 95% of the variation in the optimum P rate for growing soybean in this group of soils.
The equation obtained to predict P fertilizer rate as a function of soil P extracted by resin and clay was as follows: P Rate = 31.6 + 0.0742(Resin-P) + 1.4903(clay) 0.0624(Resin-P)(clay). (1)
The Resin-P also showed a higher potential than the other extractants to improve the soil test interpretation when other estimates of the buffering capacity are used. The combination of the Resin-P with the Pbuffering coefficient obtained after 2 hours of equilibration was as good as that obtained for the combination of Resin-P and clay. This is shown by the good fit between the calculated (points) and predicted (lines) optimum P rates in Figure 1.
The predicted (lines) P rate in Figure 1 was obtained with the following equation:
P Rate = 372.8 7.41(Resin-P) 337.92(PBC) + 5.66(Resin-P)(PBC) (2) where PBC = P-buffering coefficient after a 2-hour equilibration period.
Conclusions
The models developed with the combination of Mehlich-3, Bray-1, and Resin-P with clay successfully predicted the P rate to be applied in the 27% clay soil cropped with soybean for 1 year to reach 90% of the maximum yield. However, none of the models reported here accurately predicted the P rate required to reach 90% of maximum yield for the 21% gibbsitic soil. This
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