Application of electro-kinesis for saline-sodic irrigation management

Application of electro-kinesis for saline-sodic irrigation management

The increasing demand for food production, and scarcity of good quality water with which to address this, has led to the use of marginal quality water, such as saline-sodic groundwater, for irrigation purposes, particularly in the arid and semi-arid areas. Long-term use of saline–sodic water for irrigation may adversely affect soil properties and crop yields. Traditionally, larger amounts of irrigation water are applied to leach salts from the root zone in order to maintain the root zone concentration of salts below the threshold for plant damage. However, the addition of excess water for salt leaching may lead to nitrate leaching out of the root zone, degrading groundwater quality and reducing crop production. To reduce these issues, this research has evaluated the potential for electro-kinesis (EK) to manage nitrate and salts within a simulated root zone depth (sand column) irrigated with saline-sodic water.

While previous research has been undertaken to evaluate the use of electro-kinesis (EK) under a narrow range of soil conditions, there is a lack of data on the ability to simultaneously control salt and nitrate movement in soil using EK, and insufficient evidence to understand the soil, water and management conditions under which EK may be usefully applied to improve root zone management. Therefore, three laboratory trials were conducted using sand columns (uncharged particles) investigate the effect of EK on salt distribution, nitrate movement and management options for the use of EK in combination with saline-sodic irrigation. A combined soil-water and EK model was subsequently developed and validated. The model was then used to predict nitrate and salt distribution under a range of EK and irrigation treatments.

The first study evaluated the effect of applying EK to irrigated sand columns (30 and 50 cm) with two different electrode separate distances (20 and 40 cm, where the anode was installed at the top while the cathode sheet was installed at the bottom of the sand column). The results showed that EK application significantly affected both soil solution ion movement within the column and the soil pH near the electrodes. Anions (i.e. chloride) were moved upwards in the sand column and held in the upper layers against the downward flow of applied irrigation water. However, the magnitude of the effect varied as a function of the EK application period, rate of EK application and the distance between the electrodes.

The second trial evaluated nitrate movement with three EK treatments (0, 0.005A, 0.01A and 0.02A, for 24h) applied to sand columns under various irrigation rates (0, 10, 20 and 30 mL h-1 for 9h). The results showed that application of EK resulted in a significant increase in nitrate concentration near the anode, and a significant decrease near the cathode. At the same time, the EC was significantly increased and the solution pH became more acidic (<6.5 initial condition) near the anode and alkaline (~ pH 10) near the cathode. Irrigation significantly decreased the nitrate concentration throughout the sand columns with leaching increasing as high irrigation rates and longer irrigation durations were applied.

The third experiment evaluated the effect of applying 0.01A EK for 24 h on both sodium and nitrate movement in a sand column. The results demonstrated the potential to move nitrate upwards in the sand column while simultaneously moving sodium downwards. Substantial changes in nitrate concentration were observed, with increases recorded near the anode and a reduction observed near the cathode. The sodium concentration increased near the cathode and decreased near the anode.

The sequential application of EK, irrigation and fertigation was shown to be an effective strategy for managing the movement of water and solution in a sand column. In general, nitrate moved upwards and sodium moved downwards during EK application. The application of irrigation and fertigation buffered the effects of EK on pH and EC. Periods of drainage also allowed for redistribution within the column by both diffusion and hydraulic movement under gravity. However, the dynamic responses and time-lag associated with changes at each depth in the root zone make it difficult to identify optimal management strategies. Hence, a numerical model incorporating the effect of both soil-water movement and EK on the movement and distribution of ions would be beneficial.

A combined 'Diffusion Convection Electro-osmosis Electro-migration Model' (DCEEM) was developed and validated using experimental data. The model was shown to accurately predict soil-water content, nitrate movement and salt concentration. This model was then used to simulate alternate EK and irrigation management strategies and demonstrate the effect of these management practices on EC changes, soil-water and nitrate movement within irrigated sand columns. Although the current version of the model has limitations including the selection of system design and management options, the results confirmed the utility of the model to better understand the relative scale and nature of the soil-water and ion movement interactions within the column. Recommendations to improve the model and thus the ability to determine best management practices for a range of system requirements were provided.