ALUMINUM AND CALCIUM CONSTRAINTS TO CONTINUOUS CROP PRODUCTION IN A BRAZILIAN AMAZON OXISOL

Acidity constraints in Oxisols of the Amazon may entail both Al toxicity and Ca deficiency. A field study was conducted in a Xanthic Hapludox near Manaus, Brazil, to evaluate lime and Ca requirements for corn (Zea mays L.), cowpea (Vigna unguiculata L.), soybean [Glycine max (L.) Merr.), and peanut (Arachis hypogea L.) during five consecutive years. Calcitic lime was applied once at rates of 0, 0.6, 1.2, 2.3, and 4.6 t of CaCO3 equivalent per hectare. The effects of 1 t ha-1 of gypsum, applied to four crops, were evaluated with lime rates of 0, 1.2 and 2.3 t ha-1. During the two initial years, maximum yields of corn and soybean occurred with 1.2 t of lime ha-1. In subsequent years, maximum corn yields occurred to 4.6 t of lime ha-1. Liming only increased cowpea yields in the third year. Peanut yields increased with liming to the highest lime rate in the fourth and fifth year. Gypsum increased yields for all crops to which it was applied. When averaged across time gypsum increased exchangeable Ca and reduced exchangeable Al by 0.41 and 0.20 cmol(c) kg-1, respectively. Increased peanut shelling percent and peanut and cowpea leaf Ca concentrations with lime, gypsum, or both were indicative of improved soil Ca supply. Maximum corn and soybean yields occurred at 27% Al saturation, whereas the critical level for peanut was 54%. Regressions of relative yield for all lime and gypsum treatments on the exchangeable Ca:Al ratio gave R2 values equal to those achieved with Al or Ca saturation. Critical Ca:Al ratio values would provide a useful index to ensure that lime recommendations based on Al saturation provide an adequate Ca supply.