Soil magnesium level, corn (Zea maysL.) yield, and magnesium uptake

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Abstract
The exchangeable soil Mg level recommended for agronomic crop production by different soil testing services ranges from 25 to over 180 ppm (?10% saturation of the cation exchange capacity). In some cases the justification for high soil Mg recommendations is to ensure that forage Mg concentrations are high enough to prevent hypomagnesemia in ruminants consuming it. There has been very little research on determining the minimum amount of soil Mg needed to produce optimum corn (Zea mays L.) grain yields. Some soil testing services recommend adding Ca to soils having high exchangeable Mg contents in spite of little published evidence to support the efficacy of this practice. In 1980, field experiments were initiated on two low and one high exchangeable Mg soils in Pennsylvania to determine: 1) the high and low critical soil Mg levels for corn grain production, 2) how well several soil test methods for determining Mg availability predicted Mg uptake by corn, and 3) the exchangeable soil Mg levels needed to ensure 0.2% Mg in corn silage grown in that soil. Corn grain yields were unaffected by a wide range of exchangeable Mg levels in the experimental soils. Since there was no reduction in yield at the highest (28.8% Mg saturation, exchangeable Ca/Mg = 1.8) or lowest (1.8% Mg saturation, Ca/Mg = 36.9) soil Mg level, it was not possible to identify critical limits. It was apparent from these and results in the literature, though, that a lower limit of 5% Mg saturation should be adequate for corn grain production and that Mg toxicity in corn will not occur in soils that have an exchangeable Ca/Mg equivalent of 1.0 or higher. As 1.0 is the ratio found in pure dolomite, it is unlikely that Mg toxicity will occur in normal agricultural soils. Four of the tested Mg availability indexes (exchangeable Mg, Mg saturation, exchangeable Mg/K ratio and Baker test pMg) were well correlated (|r| = 0.84 to 0.93) with corn ear leaf and silage Mg concentrations and total Mg uptake. At least 10% Mg saturation was required to obtain 0.2% Mg in the corn silage grown on those soils. It was also found that the Mg concentration in ear leaves at silking could be as low as 0.1% with no decrease in grain yield.