Water and nitrate distributions as affected by layered-textural soil and buried dripline depth under subsurface drip fertigation

Laboratory experiments were conducted to investigate the distributions of water and nitrate from a buried dripline discharging an ammonium nitrate solution in uniform and layered-textural soils. Two layered soils, a sandy-over-loam soil (SL) and a loam-sandy-loam soil (LSL), and two uniform soils of sandy (S) and loam (L) were tested. The experimental results demonstrated that dripline depth and layered-textural soil greatly affected water and nitrate distribution. Wetted depth increased with dripline depth and initial soil water content for both uniform and layered soils. The distribution pattern of water in the layered soils was controlled by the layering sequence and the dripline position relative to the interface between two soil layers. Water accumulation occurred in the fine-textural layer of soil for the layered soils. For the sandy-over-loam soil (SL), positioning the dripline below the interface led to much water (89%) moving to the sublayer of loam soil than positioning the dripline above the interface (73%). For the loam-sandy-loam soil (LSL), positioning the dripline in the top layer of loam soil resulted in 77% of water applied distributed in the top layer, while positioning the dripline in the bottom layer of loam soil resulted in 93% of water applied distributed in the bottom layer. Measurements of nitrate distribution showed that nitrate concentration in the proximity of the dripline and of the water accumulation zone approximated the input concentration while nitrate accumulated at the boundary of the wetted volume for both uniform and layered soils tested. The results from this study suggest that the dripline depth should be carefully selected in the design of subsurface drip irrigation systems for layered soils to obtain a target distribution of water and nitrate.