Simulation Model of Banded Ammonia in Soils

The efficient use of anhydrous NH 3 as a fertilizer depends on factors such as N rate, knife spacing, depth of application, and soil properties like NH 3 ‐retention capacity. We developed a computer model that predicts some characteristics of an NH 3 ‐retention zone in soil. Soil titratable acidity to pH 9 using NH 4 OH as the base was an input to the model used to estimate the NH 4 ‐N (NH 3 and NH 4 ) concentration at the line of injection 24 h after NH 3 application. Data for the model were collected using a laboratory apparatus that applies anhydrous NH 3 in a band. Ammoniacal‐N concentration with distance away and normal to the line of injection was predicted using a nonlinear equation and values of N rate, band spacing, and soil bulk density. Soil pH was calculated as a linear function of the ammoniacal‐N concentration in soil with ammoniacal N at pH 9 equal to titratable acidity. Direct losses of NH 3 by volatilization were assumed to occur because of insufficient depth of application or rates of N application that were too high. Concentrations of NH 4 and NH 3 in solution were calculated using the Gapon cation‐exchange and Henderson‐Hasselbach equations, respectively. Observed vs. predicted ammoniacal N and pH at different distances from the injection line for seven soils and 13 rates (1.6–8.4 g N m −1 ) correlated at P < 0.01 ( r 2 = 0.85 and 0.84, respectively). The model provided reasonable estimates of direct NH 3 loss and concentration of ammoniacal species in solution. This tool could help to better understand the behavior of NH 3 in soil and, therefore, to manage its application to reduce N losses. Information on ammoniacal‐N distribution and on soil pH in an NH 3 band could also be used for simulating transformations of banded N.