Aluminum toxicity in corn at near neutral soil pH levels

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Abstract
Al toxicity to plants is often responsible for yield reductions under acid soil conditions. Despite the fact that Al becomes insoluble in the slightly acid to neutral pH range, there are indications that it can still be taken up and become toxic to plants. Corn plants were grown in the greenhouse on 3 highly weathered soils (Oxisol and Ultisols) containing substantial quantities of exchangeable Al. The soils were limed at various rates up to pH values near or above neutrality. On all soils, a positive yield response to lime occurred at low pH values, while on 2 of the soils, yields decreased significantly as the pH approached neutrality. Yield and Al content of the tissue were exponentially related irrespective of the level of phosphate applied. The mechanism by which all becomes increasingly available to the plant as the pH approaches neutrality is not clear and needs further investigation. Strong Al-Mg and Al-P antagonisms occur at the high and low pH values. The existence of Al toxicity at near-neutral pH values can explain many of the anomolous results concerning yield depressions previously ascribed to such factors as micro-element or P deficiencies.