Nutritional Complexities Associated with pH Variables in Humid Tropical Soils

Soil fertility problems associated with soil acidity in the humid tropics were identified by observing variations of some growth-controlling factors along response curves. Data for these curves were generated by liming an Oxisol and an Ultisol of Hawaii so that pH increased progressively, from <5 to > 7. Legumes were grown across the pH gradients. Liming the Oxisol depressed Al concentration in the pH range 4.8 to 5.5 and Mn in the pH range 4.8 to 5.7. Liming did not change solution Ca until pH 5.7 was reached, after which, solution Ca contents of saturation extracts increased exponentially. The beneficial effects of liming Leucaena leucocephala were linear from pH 4.8 to 7.0. Soybean (Glycine max L.) yields increased abruptly at approximately pH 5.9 which corresponded to increased Ca concentration in soil saturation extracts. Indigenous soil microvariability in the humid tropics, although sometimes disastrous to conventional field plot experimentation, provides opportunities for investigating soil-plant relations in ways that are difficult to duplicate by design. Such variability made possible an indigenous-function investigation using soybeans growing on an Alfisol of Nigeria. Increasing yield with increasing soil pH of the Alfisol was associated with increased N fixation, and this with decreased Mn in the foliage. Predicted N yield was zero at approximately 600 mg Mn kg⁻¹ leaf tissue. Vigorous soybean plants contained 1.5% leaf K, which was associated with at least 0.25 cmol K kg⁻¹ soil. Exchangeable K was positively correlated with cation exchange capacity, and cation exchange capacity with soil pH. Results of this study demonstrate the complex relationships that exist between soil pH and crop nutrition in the humid tropics. It is unlikely that such complex relationships can be covered by simple unqualified statements about liming such soils.