A P-test Interpretation Model for Oxisols using Mehlich-3, Resin Extractants, and Estimates of the Capacity Factor
Ibere D. G. Lins, EMATER, Campo Grande, MS, Brazil Fred R. Cox, N. C. State University, Raleigh
A model developed to recommend P rate as a function of Mehlich-1 and buffering capacity was previously shown to be applicable only to kaolinitic soils. However, when Fe-containing solutions or a resin extractant were used in our exploratory greenhouse experiment, which included a wide range of soil conditions, it seemed reasonable to pool kaolinite and gibbsite when interpreting soil tests for the purpose of P fertilizer recommendations. It is unknown, however, if soils with either of these two minerals dominating will respond similarly in the field.
Objectives
1. To evaluate a mathematical approach developed to predict P
fertilization rates for soybean cropped in the field on soils with different mineralogy based on soil P extracted by Mehlich-3, Bray-, or
an anion-cation exchange resin and selected soil properties;
2. To validate the proposed models using data from two soils with
different chemical composition which were cropped with soybean for a
period of 1 year.
Procedures
Long-term P experiment data used in this study, as well as data used to validate the proposed model, are reported in the previous report.
Soil P was extracted with Mehlich-3, Bray-, and an anion-cation exchange resin. In addition to clay, estimates of the capacity factor were included, such as surface area, percentage of minerals in the clay fraction, P-absorption coefficients obtained by the Langmuir equation, and P-buffering coefficients. P-buffering coefficient estimates were determined by the slope of the linear regression equation obtained for the relationship between recovered P using the three extractants described above and applied P after
2 hours, 4 days, and 8 weeks of equilibration.
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