Manganese toxicity in a Hawaiian oxisol affected by soil pH and organic amendments

Manganese toxicity is a serious constraint to many crops grown on acid soils in Hawaii. To develop management strategies to deal with the Mn problem, four experiments were conducted. First, to study soil pH effect, a pH gradient from 4.7 (unamended) to 6.0 was established in a high-Mn Oxisol (Wahiawa series), using combinations of Ca(OH)(2) (lime) and CaSO(4). 2H(2)O (gypsum); soybean [Glycine max (L.) Merr. cv. Kahala] was grown as a test crop. Second, effects of Ca, and particularly SO(4), on ameliorating Mn toxicity to soybean were subsequently evaluated. Third, soil Mn solubility by organic molecules was studied in the laboratory as a function of chemical structure, pH, and equilibration time. Fourth, soybean responses to green manure and biosolids applied at 5 and 10 g kg(-1) to the Wahiawa soil were compared with those of the unamended control and CaCO(3) treatments. Manganese concentration in the saturated paste extract of the first experiment increased 100-fold for each pH unit decrease. A combination of gypsum and lime was more effective in correcting Mn toxicity than either amendment alone. Soybean growth was better correlated with leaf Ca/Mn ratio than with leaf Mn concentration. Increased SO(4) concentration alleviated Mn toxicity. Organic molecules or ions containing OH-OH in the ortho position or SH groups, such as catechol, tannic acid, and cysteine, were more effective in dissolving soil Mn than molecules or ions not containing these functional groups. Application of green manure and biosolids generally increased Mn toxicity.