Manganese Toxicity in a Hawaiian Oxisol Affected by Soil pH and Organic Amendments

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 kg21 total Mn concentration (Fujiof Ca(OH)2 (lime) and CaSO4 · 2H2O (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 SO4, 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 kg21 to the Wahiawa average soil Mn content worldwide (Kabata-Pendias soil were compared with those of the unamended control and CaCO3 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 associA 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 kg21, above which Mn toxicity Increased SO4 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 L21. This value agrees well dissolving soil Mn than molecules or ions not containing these funcwith the critical toxic levels of 5 to 10 mM Mn (0.27–0.55 tional groups. Application of green manure and biosolids generally mg L21) in nutrient solutions that contained some Si increased Mn toxicity. (0.75–40 mg Si L21) (Horst and Marschner, 1978). The same authors reported that Mn toxicity in bean (Phaseolus vulgaris L.) was observed at 0.5 mM when the nutriM (usually present as Mn21 in the soil ent solution was free of Si. solution) is an essential nutrient that can be toxic Different plant species or even varieties within a speto crops when occurring in excess (Marschner, 1995). cies have different degrees of tolerance to Mn (Foy et Levels of Mn in the soil solution are controlled mainly al., 1988). For example, adverse effects were observed by a soil’s Mn reserve, pH, and the availability of elecwhen leaf Mn (in mg kg21) exceeded 150 in bean, 650 in trons (e) as illustrated by the following reaction (Adclover (Trifolium subterraneum L.), and 5000 in lowland ams, 1981; Sparrow and Uren, 1987). rice (Oryza sativa L.) (Hannam and Ohki, 1988). Also, MnO2 1 4 H 1 2 e ⇔ Mn21 1 2 H2O [1] Mn toxicity was alleviated by high levels of other nutrients, such as Ca (Horst, 1988), Mg (Lohnis, 1960; Goss Thus, soils with high Mn reserves may be Mn toxic when et al., 1991), and Si (Horst and Marschner, 1978). soil pH is below 6.0, a pH level at which soil Al remains The objective of this study was to determine the exvirtually insoluble (Hue et al., 1987). In an electron-rich tent to which Mn solubility and toxicity in a high-Mn environment (reducing conditions) caused by overwatOxisol was affected by soil pH, Ca, and SO4 sources, and ering, poor drainage, or heavy applications of organic by organic molecules and amendments, so that proper materials, Mn toxicity can occur even at alkaline pH management strategies can be developed. (Hue, 1988). Several organic molecules can dissolve solid Mn oxides via e transfer (reductive processes) MATERIALS AND METHODS (Stone and Morgan, 1984; Laha and Luthy, 1990). For example, hydroquinone can dissolve solid Mn as follows A high-Mn soil (Wahiawa series [clayey Kaolinitic Isohy(Stone and Ulrich, 1989): perthermic, Rhodic Eutrustox]) from central Oahu, HI, was selected for this study because several watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus] crops grown on this soil had failed due to severe Mn toxicity (Hue et al., 1998a). In the unamended state, the soil had a pH of 4.7, 17 g [2] kg2 total Mn, and 540 mg kg2 Mn as extracted by the Mehlich-3 solution. Its KCl-extractable Al was only 10 mg kg2, N.V. Hue, S. Vega, and J.A. Silva, Dep. of Tropical Plant and Soil Science, Univ. of Hawaii, 1910 East-West Road, Honolulu, HI 96822. Abbreviations: AA, atomic absorption; EDDHA, ethylenediaReceived 8 Nov. 1999. *Corresponding author ([email protected]). minedi(o-hydroxy phenylacetic) acid; EDTA, ethylenediaminetetraacetic acid; ICP, inductively coupled plasma; UV, ultraviolet light. Published in Soil Sci. Soc. Am. J. 65:153–160 (2001).