Nitrogen Distributions in Soil Under Fertigation From a Point Source

Simultaneous distributions of water, nitrate-N and ammonium-N in a sandy loam soil from a point source discharging an ammonium nitrate (NH₄NO₃) solution were measured using the gravimetric method. The variables affecting water and solute distributions, including application rate varying from 0.6 to 7.8 L/h, input concentration from 100 to 700 mg/L, and applied volume from 6 to 15 L, were investigated and their effects on the distributions of nitrate-N and ammonium-N were presented in this article. A uniform distribution of nitrate-N concentration in the soil was found a radius of 17.5 cm around the point source for a given input concentration. For any input concentration, the accumulation of nitrate at the boundary of the wetted volume was observed. The nitrate-N concentration in the soil was primarily affected by the input concentration. There was an increased nitrate-N concentration with a higher input concentration. The effects of application rate and applied volume on nitrate distribution were not significant up to a radius of 15 cm, beyond which either a higher application rate or a smaller applied volume resulted in a higher nitrate concentration in the soil. The results of ammonium-N distribution measurements indicated that there existed an extremely high ammonium-N concentration in the proximity of the point source (about 2.5 to 7.5 cm from the source). An increased input concentration produced a higher ammonium concentration around the point source. Results also demonstrated that the influence of fertigation on the ammonium-N distribution was restricted in a small volume, about 10 cm around the point source. Beyond this range, input concentration, application rate, and total applied volume had insignificant effects on ammonium-N distribution. The information obtained from this research is useful in the design, operation, and management of a fertigation system with drip irrigation.