Cation Composition of Tall Fescue Forage as Affected by K and Mg Fertilization

Grass tetany has been observed in cattle grazing cool-season grasses during the late winter and early spring in the southeastern USA. It has been suggested that Al is involved when this problem occurs on wet sites. Tall fescue (Festuca arundinacea Schreb.) experiments were located at Knoxville (2 years) and Chapel Hill, Tenn. (1 year) to determine fertilizer effects on K, Mg, Ca, and Al concentrations of forage grown on soils subject to flooding. The soils were Lindside loam (fine-silty, mixed, mesic Fluvaquentic Eutrochrepts) and Roellen silty clay loam (fine, mixed, thermic Vertic Haplaquolls). Treatments consisted of a control and fall applied Sul-Po-Mag (2 MgSO₄ • K₂SO₄), Epsom salts (MgSO4 • 7 H₂O) + potassium sulfate (K₂SO₄), Epsom salts alone, or potassium sulfate alone to supply 112 kg Mg and/or 181 kg K ha⁻¹. The forage was sampled from February through early May while these soils were usually saturated with water and at some times while there was water standing on the surface. Magnesium fertilization had little effect on forage Mg while K fertilization usually increased forage K. As growth increased, forage K increased in early spring. Equivalent ratios of K/(Ca + Mg) closely followed trends of forage K and increased potential for tetany but remained below 2.2. Magnesium concentrations remained low (< 2 g kg⁻¹) in the forage through early May. Aluminum and Ca concentrations were usually unaffected by fertilization treatments. There was no apparent benefit from fertilizing these soils with Mg sources. Concentrations of Al dropped considerably when the plant growth was not close to the wet soil surface and when standing water was not present. Apparently, soil Al adhered to the leaves in contact with wet soil or standing water since Al uptake through the roots would be very limited with the soil pH about 6.0.