Manganese Toxicity in Watermelon as Affected by Lime and Compost Amended to a Hawaiian Acid Oxisol

Manganese (Mn) toxicity in crops is a serious problem in Hawaii, especially Kauai and Oahu, where most soils are highly weathered. To devise a management strategy to control Mn toxicity, a greenhouse experiment was conducted on an acid (pH 4.4) Oxisol (Wahiawa series) having 15g·kg total Mn. Factorial combinations of lime (0, 2.0, and 4.0 g·kg CaCO3) and two composts (made from chicken manure and from sewage sludge at 0, 5, and 10 g·kg) were applied to the soil, which was subsequently planted to watermelon (Citrullus lanatus Thunb. ‘Crimson Sweet’). Our preliminary results showed that: 1) liming reduced Mn extractability and phytoavailability, but the reduction in Mn per unit increase in pH was much less than predicted by theory; 2) for good watermelon growth, soluble Mn, as extracted by the saturated paste method, should be <2.0 mg·L corresponding to a soil pH >5.7; 3) unlike the saturated-paste extractable Mn, the Mehlich3extractable Mn varied less with pH in a given soil series than between soil series; 4) effects of composts on Mn toxicity varied with compost properties, especially their Ca content and pH altering capacity; and 5) the diagnostic criteria for Mn toxicity in watermelon are tentatively proposed as: leaf Mn >1000 mg·kg–1 and leaf Ca/Mn ratio (g·g–1) <25. Manganese (Mn) is an essential plant nutrient at low levels, but is toxic when in excess (Marschner, 1995). In fact, Mn toxicity is often more common than Mn deficiency in some regions, such as the humid tropics, where most soils are highly weathered and acidic (Hue et al., 2001). This is because the solubility and availability/toxicity of soil Mn are increased many fold by low pH, and the abundance of electrons (e) as shown below (Adams, 1981; Sparrow and Uren, 1987):