Agricultural productivity in the tropics and critical limits of properties of Oxisols, Ultisols, and Alfisols

The objective of this study was to establish critical limits and functional relationships for soil properties and yield of various crops on Oxisols, Ultisols, and Alfisols of the tropics. Relationships between relative yield and soil acidity, available P, exchangeable K, bulk density, and rooting depth were developed using published data of several experiments on liming and deep liming, fertilizer rates, traffic-induced compaction, and desurfacing. The results showed that critical limits of soil properties for maize (Zea mays L.) were 1.08% for soil organic C, 5.0 for pH, 23.5% for Al-saturation, 7.6 mg kg -1 for P (Bray-1), 0.83 mmol (K + ) kg -1 for exchangeable K, 1.50 Mg m -3 for bulk density, and 23 cm for rooting depth. Grain legumes [cowpea (Vigna unguiculata (L.) Walp.), soya bean (Glycine max (L.) Merr.), and groundnut (Arachis hypogaea L.)] had similar critical limits, except higher P requirements. Cassava was less sensitive to acid soils than grain crops. The functional relationships developed can serve as a guideline for assessing soil degradation and determining response trends to inputs.