PEANUT GROWTH AS AFFECTED BY LIMING, CA-MN INTERACTIONS, AND CU PLUS ZN APPLICATIONS TO OXIDIC SAMOAN SOILS

Effects of coralline lime, in combination with 3 kg Cu ha(-1) plus 3 kg Zn ha(-1), on yield and nutrient uptake by peanut (Arachis hypogea) were studied at three locations in Western Samoa. Coarse (0-10 mm) coralline lime material containing 31.1% Ca and 1.7% Mg was used as lime at 0, 555, 2222 and 5000 kg ha(-1). In the Togitogiga soil, which had the lowest level of exchangeable Ca, peanut yield increased by 6 fold after liming with 555 kg ha(-1), relative to the unamended control. This yield increase was associated with reduced Mn toxicity as well as reduced Ca deficiency. The alleviation of Mn toxicity was not likely due to decreased Mn solubility because the lime application (555 kg ha(-1)) increased soil pH by < 0.1 unit. Rather it was the increased Ca availability which reduced the Mn toxicity through a Ca/Mn antagonism. The critical range of exchangeable Ca for peanut growth was found to be about 1.5-1.6 cmol 1/2 Ca2+ kg(-1). A Ca/Mn-ratio >80 was required for a desirable Ca/Mn balance in peanut tissue. On the other two locations (with exchangeable Ca levels of 1.5-1.6 cmol 1/2 Ca2+ kg(-1)), liming increased peanut yields by 15-20%. Additions of Cu plus Zn also increased the yields, although the increases were small (7%) and not significant at the 95% probability level.