Abstract

Ammonia injection into soils is used both to supply N to field crops and to reduce soil-borne pests and weeds. Ammonia efficacy depends on its persistence in the soil environment following application. Hence, the extent of ammonia binding in six thermic, xeric Mediterranean soils, varying widely in texture, was evaluated at three moisture contents. Fitting the experimental retention data for each of the six oven-dry soils to the Langmuir isotherm yielded apparent maximum retention capacities (SMAX) ranging from 440 to 1730 mg NH3-N kg−1, and Langmuir binding coefficients (k) ranging from 0.08 to 4.31 L mg−1. The SMAX correlated strongly and linearly with clay content (r2 = 0.946, p < 0.01). The SMAX of the six soils increased with moisture content, likely due to ammonia dissolution in the liquid phase and the adsorption of its cationic derivative. The expected reduction in ammonia binding to the solid phase upon wetting, due to competition with water, was reflected in a sharp, unanimous decrease in the Langmuir model coefficients. A reduction in ammonia retention (but not in SMAX) due to moistening occurred only in the very-fine clayey montmorillonitic soil. While anhydrous ammonia may act as an effective slow-release N source in heavier-textured soils, its tendency to remain in the gas phase in sandy soils is advantageous for pest control.

IPC Classification

Keywords