The SAHGA model to calculate the Spatial Ammoniacal Heterogeneity at the soil surface after fertiliser Granule Application

After dissolution of fertiliser granules, a high nitrogen concentration is recovered in the immediate vicinity of granules, which may enhance damaging processes like nitrite accumulation or ammonia volatilisation. Based on the diffusion equations of Cranck, the granule-soil microsite was modelled to obtain the actual fertilised surface plot and the effective rate of N application on this surface. Parameterisation of the diffusion coefficient of solutes consisted of a temperature and soil texture correction. The model was tested against an experimental data set obtained from soil incubations at two soil water contents (21.2% m³ m^–3 and 28.3% m³ m^–3) and two temperatures (4°C and 25°C) by comparing NH₄⁺ recovery at various distances from the granules. The simulated radius of the granule-soil microsite was more affected by the water content than by the temperature. The model is very accurate because 95–100% of total NH₄⁺ applied was recovered in the modelled surface depending on the experimental conditions (temperature and water content). The model was simple enough to be easily integrated into larger models dealing with surface-applied granule fertilisers.