EFFECTS OF GREEN MANURE AND CORAL LIME ON CORN GROWTH AND CHEMICAL-PROPERTIES OF AN ACID OXISOL IN WESTERN-SAMOA

Effects of local green manure (GM) and lime on soil productivity in a low-input agricultural system were evaluated by growing three successi ve crops of sweet corn (Zea mays) on an acid Oxisol (Typic acrorthox, Togitogiga series) in Western Samoa, The soil was amended with coral l ime at 0, 5, and 10 Mg ha(-1) and with cowpea GM at 0, 7.5, and 15 Mg ha(-1). Commercial NPK fertilizers at 50 kg ha(-1) each of N, P, and K were included for comparison. The amendments were applied only once p rior to planting of the first crop. Response parameters measured inclu ded nutrient composition of leaves at tasseling and grain yield of eac h crop, and selected soil chemical properties at each planting. Yields of the first crop were nearly tripled with GM additions and doubled w ith lime additions. Such yield increases were caused mainly by better K nutrition and to a lesser extent by enhanced P nutrition. Yields of subsequent crops were much lower than those of the first, and the decl ines were much steeper for the GM treatments than for the lime treatme nts. Thus, the enhancement effect on K nutrition did not last beyond o ne crop. Poor growth of the second and third crops was caused by K def iciency; probably coupled with Mn toxicity. Significant yield reductio ns were found when Mn-to-K ratios in leaves exceeded 0.010. As for eff ects on soil, soil pH was increased significantly by lime but only sli ghtly by GM. Given the variable charge property of this Oxisol, each u nit pH increase corresponds to a cation exchange capacity (CEC) increa se of 5 cmol(c) kg(-1). Having greater CEC, the amended soil retained K more effectively, thereby causing yield increases, especially of the first corn crop, which required at least 0.75 cmol(c) kg(-1) of excha ngeable soil K or 7% of CEC for adequate growth.