Manganese toxicity in cowpea as affected by soil pH and sewage sludge amendments

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Abstract
Manganese toxicity in cowpea, an important grain legume for the Tropics, was studied in a greenhouse experiment on a manganiferous Oxisol of Hawaii. Treatments consisted of three target pH levels (4.0, 5.5, and 6.1) superimposed on four sludge rates (0, 5,10, and 20 g/kg). In the early stages of growth, low soil pH and high sludge addition caused severe Mn toxicity. Visual symptoms included chlorosis resembling iron deficiency coupled with a freckling (brown spots) on the unifoliate and the first trifoliate leaves. Plant height, dry matter weight, and plant Mn concentration, measured 28 days after seeding, were highly responsive to the excess Mn. Reductions of 10 and 50% in dry‐matter yield can be expected when Mn levels were 620 and 3000 mg/kg in plant, 1.54 and 35.8 mg/L in soil solution, and 306 and 475 mg/kg in soil as extracted with a DTPA solution.