Magnesium Accumulation by Corn (Zea mays L.) as a Function of Potassium-Magnesium Exchange in Soils

Low magnesium (Mg) in forages is important in hypomagnesemic tetany of pastured cattle and generally accentuated by an imbalance among basic cations in soil but has not been predicted well with various indices of availability for Mg in soil. A greenhouse experiment was conducted to test different methods of predicting Mg availability for soils in which the levels of Mg, K, and Ca are generally above the individual critical amounts as determined from existing soil tests. Eleven Ap soils collected from various parts of Pennsylvania were used. Variables included low and high Mg-accumulating corn hybrids (Zea mays L.), and soil treatments included 0.2 meq of K as KNO₃ and 4.0 meq of Ca as precipitated CaCO₃ per 100 g of soil for comparison with untreated soil. Concentrations of Mg in plants were poorly correlated with 1N NH₄OAc-exchangeable Mg, % CEC saturated with Mg, and pMg and pMg — pCa by the Baker method. A correlation was found between Mg concentrations in both hybrids and ½pMg-pK, an expression for the relative availability of Mg and K by the Baker method. This relationship was markedly improved by excluding data from one very acid soil, so its use may be restricted to soils with nearneutral pH and optimum Ca availability. Magnesium concentrations in both hybrids also were correlated with [(Mgₛₒᵢₗ × CEC)½/Kₛₒᵢₗ], an index of relative availability of K and Mg derived from a modification of Vanselow's exchange equation. This relationship was markedly improved by excluding data for a high Mg, high pH soil so its use may be of limited value for soils of high pH which are also very high in Mg. The two methods of measuring the relative availability of Mg and K were unsuitable for all soils; ½pMg − pK could be used for soils with optimum pH and Ca availability, and [(Mgₛₒᵢₗ × CEC)½/Kₛₒᵢₗ] could be used for soils not substantially different with respect to their bonding energies for exchangeable Mg. While it has been shown by Stout and Baker that corrections for soil buffer effects can improve interpretations, the results of this investigation indicate that soil test results calculated from data by existing methods with no corrections for differential bonding of cations may be used to improve predictions of Mg availability for many soils. In addition, the data indicate that plant uptake of Mg in these soils was more a function of K availability than of Mg availability.