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 soil s in Hawaii. To develop management strategies to deal with the Mn problem, four experiments were conducted. First, to study soil pH effect, a pH gradi ent from 4.7 (unamended) to 6.0 was established in a high-Mn Oxisol (Wahiaw a series), using combinations of Ca(OH)(2) (lime) and CaSO4. 2H(2)O (gypsum ); soybean [Glycine max (L.) Merr. cv. Kahala] was grown as a test crop. Se cond, effects of Ca, and particularly SO4, on ameliorating Mn toxicity to s oybean were subsequently evaluated. Third, soil Mn solubility by organic mo lecules was studied in the laboratory as a function of chemical structure, pH, and equilibration time. Fourth, soybean responses to green manure and b iosolids applied at 5 and 10 g kg(-1) to the Wahiawa soil were compared wit h those of the unamended control and CaCO3 treatments. Manganese concentrat ion in the saturated paste extract of the first experiment increased 100-fo ld for each pH unit decrease. A combination of gypsum and lime was more eff ective in correcting Mn toxicity than either amendment alone. Soybean growt h was better correlated with leaf Ca/Mn ratio than with leaf Mn concentrati on. Increased SO4 concentration alleviated Mn toxicity. Organic molecules o r ions containing OH-OH in the ortho position or SH groups, such as catecho l, tannic acid, and cysteine, were more effective in dissolving soil Mn tha n molecules or ions not containing these functional groups. Application of green manure and biosolids generally increased Mn toxicity.