Irrigation and tillage management effects on solute movement

In developing management practices to reduce chemical leaching below the root zone, tillage and irrigation management are important considerations. Two studies were performed to evaluate the movement of bromide in tilled and non-tilled soils under sprinkler versus flood irrigation. In each study, bromide was applied either with an irrigation or presprayed to the soil surface followed by periodic soil sampling to monitor the bromide movement. Tillage was observed to reduce the mean depth of chemical penetration under both irrigation treatments and reduce the spatial variation of bromide concentration under flood irrigation. For example, after 30 days of periodic flood irrigation, 25% of the applied bromide remained in the upper 0.2 m of a tilled soil while in the companion non-tilled soil virtually no bromide remained above this depth. The most rapid bromide movement was observed in non-tilled, flood irrigated soil, particularly when the solute was added with the irrigation. We speculate that the tillage effect of reduced leaching results from the alteration of pore continuity and creation of diffusional sinks and not increased evaporative water loss in the tilled soil. The Root Zone Water Quality Model was calibrated using site-specific hydraulic property measurements and used to predict the solute movement. The model predictions were fairly accurate for the sprinkler irrigated soil but less satisfactory for the flood irrigation studies. In comparing the effect on chemical leaching of the treatments imposed, we found that tillage and the timing of the chemical application had greater impact on reducing leaching than did the method of irrigation.