Effect of pH on soil magnesium and its absorption by corn

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Abstract
Lime induced depression of exchangeable Mg had been observed previously and it was postulated that yield decreases, which frequently occur at high pH levels may in some instances be due to Mg deficiency. This study was conducted to evaluate the influence of pH on exchangeable Mg in a wide range of acid soils and to determine the effects on recovery of Mg by corn (Zea mays L.) in the greenhouse. Treatments consisted of five pH levels, three P levels, and three Mg levels. Soil Mg was extracted with two neutral salt solutions and double acid. Salt extractable Mg decreased markedly at near neutral pH values, but double acid extractable Mg remained essentially unchanged. In several of the soils where yields were depressed at the highest pH levels, plant Mg content was also significantly lower. However, yield depressions were not ameliorated by Mg applications and could not be ascribed to induced Mg deficiency. Decreased plant uptake of Mg at both low and high pH levels was considered to have resulted from competitive effects between Mg and Al. The evidence suggested that salt extractable Mg did not accurately reflect plant availability. Because soil reaction has profound effects on the availability of aluminum and many elements essential to plants, liming should be practiced primarily to optimize the utilization of the nutrient resources of soil. While pH values near neutrality are regarded as optimal in many areas of the world, particularly where legumes have traditionally been part of the rotational system, yield depressions have sometimes been reported at such pH levels on highly weathered acid soils of tropical and subtropical origin.