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Response of Alfalfa and Sorghum to Lime and P on Highly Weathered Soils¹

On highly weathered soils, crop response to low levels of lime is usually positive, corresponding to the neutralization of exchangeable Al and Mn, but response at high levels often becomes negative. Because the reasons for depressed yields when highly weathered soils are limed to near-neutral pH values have not been adequately elucidated, two Oxisols (Typic, Umbriorthox) having similar properties but responding differently to lime were used to further probe the mechanisms involved. Alfalfa (Medicago sativa L.) and sorghum (Sorghum sudanense) were used as test crops in greenhouse experiments in which the levels of P and lime were varied. The depressive effect of high rates of lime appears to be linked with the presence of toxic levels of exchangeable Al before liming. Although this depressive effect can be overcome by the application of large quantities of P, it is not proposed that the effect is due primarily to exchangeable Al. Rather the level of exchangeable A1 at a given pH reflects the reactivity of the aluminous surfaces which, in turn, govern the solubility of P.

Key Words: Overlimiting • Al toxicity • Oxisols • Mg

² Professor of agronomy, Univ. of Georgia, Athens.

Received for publication December 22, 1978.