Wetting Patterns and Nitrate Distributions in Layered-Textural Soils Under Drip Irrigation

Laboratory experiments were conducted in different sequence and thickness of the soil layers to investigate the effects of layered-textural soils on wetting patterns and water and nitrate distributions from a surface point source under various combinations of application rate and applied volume. Three layered soils, including a sandy-over-sandy loam (SL), a sandy loam-over-sandy (LS), and a sandy loam-sandy-sandy loam (LSL), and two uniform soils (a uniform sandy loam and a uniform sandy soil) were tested. In the experiments, the application rate was varied from 0.69 to 3.86 L h-1 and the applied volume from 5.7 to 12.1 L. The experimental results demonstrated that the wetting patterns and water and nitrate distributions were greatly affected by the sequence and thickness of soil layers as well as the application rate and volume applied. An interface existing in the layered soils, whether a fine-over-coarse or a coarse-over-fine, had a common feature of limiting downward water movement and of increasing horizontal water movement. For the fine-over-coarse layered soils of LS and LSL, water and nitrate were uniformly distributed at a given depth in the top layer soil. For a coarse-over-fine layered soil of SL, however, water accumulated in the sublayer soil underneath the interface and a zone of lower nitrate concentration was observed. The effect of application rate on water distribution pattern was dependent upon soil layering. A minor influence of application rate on water distribution for the fine-over-coarse layered soils (LS and LSL) than for the uniform soils was found. To obtain a greater wetted depth through selecting the emitters having a smaller application rate, which is a common method in the system design for a uniform soil, may not be necessarily applied for the layered soils. Measurements of nitrate distribution showed that nitrate accumulated toward the boundary of the wetted volume for both the uniform and the layered soils. This suggests the importance of optimal management of drip fertigation because nitrate is susceptible to the movement out of the root zone by mismanagement of fertigation. The information obtained from this research is useful in the design, operation, and management of a drip fertigation system.