Depth and Width of the Wetted Zone in a Sandy Soil after Leaching Drip‐Irrigation Events and Implications for Nitrate‐Load Calculation

Abstract

The quantitative assessment of nitrate‐nitrogen (NO3‐N) leaching below the root zone of vegetable crops grown with plasticulture (called load) may be done using 150‐cm‐deep soil samples divided into five 30‐cm‐long subsamples. The load is then calculated by multiplying the NO3‐N concentration in each subsample by the volume of soil (width×length×depth, W×L×D) wetted by the drip tape. Length (total L of mulched bed per unit surface) and D (length of the soil subsample) are well known, but W is not. To determine W at different depths, two dye tests were conducted on a 7‐m‐deep Lakeland fine sand using standard 71‐cm‐wide plasticulture beds. Dye tests consisted of irrigation lengths of up to 38 and 60 h, digging transverse sections of the raised beds at set times, and taking measurements of D and W in 30‐cm‐deep increments. Most dye patterns were elliptically elongated. Maximum average depths were similar (118 and 119 cm) for both tests despite differences in irrigation duration and physical proximity of both tests (100 m apart in the same field). Overall, D response (cm, both tests combined) to irrigation volume (V, L/100 m) was quadratic (Dcomb.avg=−2×10−7 V2+0.008 V+34), and W responses (using maximum and mean values at each 30‐cm increment depth, Wmax and Wmean, respectively) to D (cm) were linear (Wmax=−0.65D+114 and Wmean=−0.42D+79). Predicted Wmax were 104, 84, 64, 44, and 25 cm in 30‐cm depth increments. Load calculations using NO3‐N concentrations of 7.2, 5.0, 3.9, 3.0, and 2.9 µg/kg for the 15, 46, 77, 107, and 137 cm depths, respectively, were 21.2, 37.6, 28.2, and 39.1 kg/ha for W values of 40 cm, bed width (71 cm), Wmean, and Wmax, respectively. These load calculations ranged from simple to double based on the choice of W estimate used, which illustrates the importance of knowing W accurately when load is calculated from field measurements. These Wmax and Wmean values may be used for load calculations on sandy soils but are likely to overestimate load because they were determined without transpiring plants and may need to be adjusted for different soil types.