Wetting patterns and nitrogen distributions as affected by fertigation strategies from a surface point source

Water and solute dynamics in the soil are important for the design, operation, and management of fertigation system by drip irrigation. Experiments were conducted to study the effects of fertigation strategies on wetting patterns and nitrogen distribution in the soil from a surface point source of nitrate ammonium (NH4NO3). A 15° wedge-shaped plexiglass container was used to represent one twenty-fourth of the complete cylinder. Monitoring of the wetting front movement in two soil textures, a sand and a loam, revealed that the increase in the surface wetted radius and in the vertical plane with water volume applied can be represented by a power function with power values of about 0.3 and 0.45, respectively. Similar values have been reported for a clay soil which suggests that these relations can be applied to a wide range of soil textures, from sand to clay. Analyses on the effects of application rate on the water distribution pattern demonstrated that increasing the water application rate allows more water to distribute in horizontal direction, while decreasing the rate allows more water to distribute in vertical direction for a given volume applied. The distributions of ammonium and nitrate concentrations in the soil were measured under different fertigation strategies that varied the order in which water and nutrient were applied. An extremely high ammonium concentration existed in the proximity of the point source because ammonium is absorbed by soil. Consequently fertigation strategy had an insignificant influence on the peak value. Accumulation of nitrate toward the boundary of the wetted volume was observed for each fertigation strategy, which suggests that flushing of the remaining fertilizer solution in the drip pipeline system should be as short as possible after fertilizer application is finished to avoid the potential loss of nitrate from the root zone. The strategy of first applying water for one-fourth of the total irrigation time, then applying fertilizer solution for one-half of the total irrigation time, followed by applying water for the remaining one-fourth of the total irrigation time left the most nitrate close to the source and was therefore recommended.