Measurement of Exchangeable NH₄⁺ in Tropical Rice Soils^†

Abstract

Measurement of exchangeable NH₄⁺ by two methods—steam distillation of the filtrates of KCl extracts and of soil-KCl suspensions with MgO—was compared for 17 tropical soils covering a wide range in texture, pH, and organic matter. Distillation of soil suspensions gave significantly higher values for NH₄⁺ both for aerobic and anaerobic soils, but the differences were much greater in the anaerobic soils. The higher values obtained by direct distillation of KCl-soil suspensions were probably due to hydrolysis of soil organic matter at the high pH values (9.9 to 10.7) brought about by the boiling MgO suspensions. We recommend the use of a filtered KCl extract instead of a soil suspension to measure exchangeable NH₄⁺ in tropical rice soils.