Although less widespread than Al-toxic acid soils, Mntoxic acid soils do exist in Hawaii (Hue et al., 1998a), Manganese toxicity is a serious constraint to many crops grown Brazil (R.S. Yost, personal communication, 1999), and on acid soils in Hawaii. To develop management strategies to deal with the Mn problem, four experiments were conducted. First, to the Philippines (Hue, 1999). For example, a major porstudy soil pH effect, a pH gradient from 4.7 (unamended) to 6.0 was tion of agricultural land on Oahu, Hawaii, consists of established in a high-Mn Oxisol (Wahiawa series), using combinations soils with 10 to 40 g kg Ϫ1 total Mn concentration (Fujiof Ca(OH) 2 (lime) and CaSO 4 • 2H 2 O (gypsum); soybean [Glycine moto and Sherman, 1948). These soils, mostly Oxisols max (L.) Merr. cv. Kahala] was grown as a test crop. Second, effects of basaltic origin, are often located in areas of low to of Ca, and particularly SO 4 , on ameliorating Mn toxicity to soybean moderate elevations (70-250 m above sea level) and were subsequently evaluated. Third, soil Mn solubility by organic with moderate annual rainfall (50-150 cm) (Swindale molecules was studied in the laboratory as a function of chemical and Uehara, 1966). The total Mn contents of these structure, pH, and equilibration time. Fourth, soybean responses to Hawaii Oxisols are about 10 times greater than the green manure and biosolids applied at 5 and 10 g kg Ϫ1 to the Wahiawa average soil Mn content worldwide (Kabata-Pendias soil were compared with those of the unamended control and CaCO 3 treatments. Manganese concentration in the saturated paste extract and Adriano, 1995). However, total soil Mn only indiof the first experiment increased 100-fold for each pH unit decrease. cates the potential toxicity. Actual Mn toxicity is associ-A combination of gypsum and lime was more effective in correcting ated with forms that are either water soluble or easily Mn toxicity than either amendment alone. Soybean growth was better reducible. Adams (1984) suggested a reducible Mn correlated with leaf Ca/Mn ratio than with leaf Mn concentration. range of 50 to 100 mg kg Ϫ1 , above which Mn toxicity Increased SO 4 concentration alleviated Mn toxicity. Organic molewould occur. To avoid Mn toxicity, Hue et al. (1998b) cules or ions containing OH-OH in the ortho position or SH groups, proposed to keep Mn concentrations in the saturated such as catechol, tannic acid, and cysteine, were more effective in paste extract below 0.5 mg L Ϫ1. This value agrees well dissolving soil Mn than molecules or ions not containing these funcwith the critical toxic levels of 5 to 10 M Mn (0.27-0.55 tional groups. Application of green manure and biosolids generally mg L Ϫ1) in nutrient solutions that contained some Si increased Mn toxicity. (0.75-40 mg Si L Ϫ1) (Horst and Marschner, 1978). The same authors reported that Mn toxicity in bean (Phaseolus vulgaris L.) was observed at 0.5 M when the nutri
