Effect of 20 Years of Low N Rate Pasture Fertilization on Soil Acidity

Abstract

Soil acidity resulting from nitrification of ammonium in fertilizer can limit plant growth. In this study on weeping lovegrass (Eragrostis curvula (Schrad.) Nees) and Caucasian bluestem (Bothriochloa caucasica (Trin.) Hubb) pastures on sandy soil in northwestern Oklahoma, 20 years of N fertilization at an average rate of 37 kg N ha-1 yr-1 reduced the pH of the surface 5 cm of soil from 6.7 to 5.3. Sweetclover (Melilotus officinalis (L.) Lam.) and alfalfa (Medicago sativa L.) had symptoms of manganese toxicity when grown in the acidified soil in a glasshouse. The growth of warm-season grasses was not adversely affected when grown in the acidified soil. The lime requirement of the acidified soil was 896 kg CaCO3 ha-1 greater than the lime requirement of adjacent unfertilized pastures. The lime requirements in relation to the amount of acid producing N fertilizer applied was similar to or less than lime requirements reported in the literature for larger N applications to farmlands. Continued use of N fertilizer at low rates will eventually require that once near-neutral soils be limed if species sensitive to acid soil are grown.