Wednesday, November 15, 2006

Which Extractant Most Efficiently Extracts Soil P and K from Fertilized Soils of Guatemala?.

Heather Hunsaker-Alcāntara, Von Jolley, Bruce Webb, Phil Allen, and Angela Ramptom. Brigham Young Univ, 275 WIDB, Provo, UT 84602

The inaccessibility and cost of soil testing reduce effectiveness of fertilizer use on small-scale subsistence farms, and inadequate funding promotes adoption of soil tests in developing countries with minimal validation. For example, the Mehlich-I P extraction used extensively in Guatemala may not be suitable for its broad range of soils. At least four alternatives are available but relatively untested [Bray 1, Mehlich III, Olsen and pressurized hot water (PHW)]. Pressurized hot water is simple and inexpensive but is not tested against other extraction methods under variable P or K fertilization. To determine whether PHW-extracted nutrients could be used to predict maize yield, nutrient content or uptake, soil, plant tissue and grain samples were obtained from calibration studies conducted using five rates of P and three rates of K on soils incubated without plants or cropped with maize in greenhouse and field conditions. In the two soils used in the greenhouse study, maize yield, vegetative P concentration and total P uptake were predicted by PHW extractable P (R2 = 0.72, 0.75 and 0.90, respectively). In the field experiment, grain yield was not improved by P or K application, but P content of maize leaf tissue related significantly to PHW-extracted P (R2 = 0.40) while Mehlich I did not. There were no yield responses to K application, but relationships defined between extractable K for all five K-extraction procedures and soil applied K were similarly significant. Comparing P extraction methods, PHW was as good as or better than Olsen, Bray 1 and Mehlich III for relating soil P extraction to the parameters measured, and these four alternative extraction methods were consistently better than Mehlich I. Mehlich I extraction should be replaced by one of the four alternatives tested, and PHW is the least expensive and most viable for use in Guatemala.