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Influence of Soil Moisture on Soil Solution Cation Concentrations and the Tetany Potential of Wheat Forage¹

Outbreaks of grass tetany occur more frequently when cattle are gazing coolseason forages growing on soils that are nearly water saturated. Therefore, we conducted two experiments to study the interaction between soil moisture, cation availability, and cation uptake by winter wheat (Triticum aestivum). Equilibrium soil solution was extracted from three soil types and analyzed for K, Ca, and Mg concentrations. Wheat was then grown in growth chambers on these soils under wet (partially saturated) and dry (below field capacity) moisture regimes to determine if changes found in the cation concentrations in the equilibrium soil solution would be reflected in the cation composition of the wheat. The potential for outbreaks of grass tetany in cattle consuming the forage was estimated from the equivalent ratio K/(Ca + Mg).

Soil solution K, Ca, and Mg concentrations increased as the soil water content decreased from approximately 40 to 10 volume percent. Changes in cation concentraions were nonproportional as predicted by the principles of thermodynamics. Potassium concentrations did not change as much as the Ca and Mg concentrations as the soil water content decreased. The K/(Ca + Mg) molar ratio in the extracted soil solution and the equivalent ratio in the wheat forage showed similar changes as soil water content increased in both experiments. These studies showed that, in addition to the effects of increased soil water content on aeration and ion transport mechanisms, changes in cation concentrations in the soil solution may influence the composition of wheat forage and thus increase its potential for inducing grass tetany in grazing cattle.

Key Words: Hypomagnesaemia • Forage quality • Donnan equilibrium • K • Ca • Mg • Triticum aestivium

² Soil scientist, Coastal Plains Soil and Water Conservation Res. Ctr. Florence, SC 29502 (Former research assistant, Dep. of Agron. Kansas State Univ.) research soil chemist and soil fertility research agronomist, Dep. of Agronomy, Kansas State Univ., and soil scientist, U.S. Plant, Soil, and Nutrition Laboratory, Ithaca, NY 14853, respectively.

Received for publication November 8, 1978.