The effect of irrigation practice on drainage and solute leaching under spray irrigation on a stony soil

Abstract

Research was conducted to assist in the improvement of irrigation efficiency on the shallow stony soils of Canterbury and identify the effect of current irrigation practice on the redistribution of solute within the soil profile. Irrigation treatments were measured using twenty-four undisturbed monolith lysimeters containing a stony Eyre shallow silt loam soil. Treatments included 15/10, 15/15, 30/20, 30/30, 60/40 and 60/60, being the respective soil deficit irrigation trigger/irrigation depth combinations (mm). The trial was split into three experiments. Experiment 1 began with a surface application of bromide tracer before exposing the lysimeters to the irrigation treatments for three months. Experiment 2 and 3 were designed to examine how the bromide had been redistributed within the pores during Experiment 1. Experiment 2 irrigated 250 mm depth continuously at 50 mm/hr to drain bromide in the macropores, while Experiment 3 irrigated 500 mm depth continuously at 2 mm/hr to drain bromide in the soil matrix. Over the three experiments, leachate was collected regularly and analysed for bromide. Preferential flow dominated solute leaching, occurring in the first drainage event irrespective of the application volume and frequency of irrigation, with leached bromide moving predominately through the macropore fraction of the soil. Treatments with greater irrigation quantities corresponded with more extensive preferential flow, drainage and for the most part, leaching in Experiment 1. Treatments irrigated to field capacity (FC) had greater leaching and drainage as well, as uniform irrigation of lysimeters in a treatment meant soil heterogeneity caused some lysimeters to exceed FC before others. Generally, there were no significant treatment effects on the cumulative bromide leached across the experiments, reflecting the dominance of preferential flow under the irrigation conditions studied. There was evidence that bromide distribution in the profile at the end of Experiment 1 was affected by treatments, with moisture status after irrigation having an effect on the bromide peak mass readings in Experiment 2, while the moisture deficit irrigation trigger influenced the bromide peak mass and cumulative mass readings in Experiment 3. However, effects were not consistent across treatments and experiments, making interpretations difficult. The results indicate that irrigation practices on Eyre shallow silt loam soils at 50 mm/hr needs to be adjusted for preferential flow, which has a dominant influence on solute distribution within the soil profile. Results imply that the 15/10 treatment had the least leaching as less extensive preferential flow means solute remains within the profile and has a greater opportunity to be immobilised.