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Optimizing Phosphorus Fertilization Rates for Soybeans Grown on Oxisols and Associated Entisols¹

ABSTRACT

Although soils vary considerably in their P sorption characteristics, this factor is often not considered in a soil test interpretation. One soil property closely related to P sorption is clay content. Residual P studies were conducted for 4 yr on three tropical soils of similar clay mineralogy, two Oxisols of 63 and 27% clay, and a Quartzipsamment of 12% clay. The Mehlich 1 (1:10) extractable P level was described as a function of time, initial soil P level, and P fertilization rate. One crop of soybeans [Glycine max (L.) Merr.] was grown each year and the yield related to extractable P, which, in turn, was related to the initial P soil level and P fertilization rate for a period of 1 yr. Based on a soybean price of $0.23/kg and a fertilizer P price of $1.23/kg, rates of P were calculated for each of the three soils that would maximize net returns for various initial levels of extractable P. As these rates differed markedly with clay content, a soil test interpretation was created by multiple regression based upon both Mehlich 1 extractable P concentration and clay percentage to predict a recommended rate of fertilizer phosphorus (R) as follows: R = 80 – 2.57(soil P) + 0.01386(clay)² – 0.003281(soil P)(clay²). This function should be applicable to many soils with similar clay characteristics.

¹ Paper no. 9801 of the Journal Series of North Carolina Agricultural Research Service, Raleigh, NC 27695-7601. This work was partially supported by EMPAER (Agricultural and Livestock Research Enterprise of the state of Mato Grosso Do Sul), Brazil.

² EMBRAPA Fellow, Professor, and former Associate Professor, Soil Science Dep., NC State Univ., Raleigh, NC 27695-7619. Third author is now Associate Dean of the College of Agric., Director of Int. Agric., Professor of Agronomy, and Assistant Vice Chancellor for Res., Univ. of Illinois, Urbana, IL 61801.

Received for publication March 29, 1985. Accepted for publication June 27, 1985.