UC Davis

Numerous studies have reported reductions in soil permeability resulting from the use of marginal recycled water for irrigation due to increased Exchangeable Sodium Percentage (ESP). The increase in soil ESP causes the breakdown of soil aggregates, followed by the swelling and dispersion of clay particles which leads to soil crusting, loss of porosity, and reduced permeability. If the sodium adsorption ratio (SAR) and the EC of the irrigation water are known, the effect of using recycled water on soil permeability (as a proxy for structure) can be predicted using empirical relationships. There is a vast body of literature on the negative impacts of sodium on soil structure. However, emerging research has shown that potassium and magnesium can also impact soil structure negatively and need to be included in irrigation water quality assessment criteria. A new criterion known as the Cation Ratio of Structural Stability (CROSS) is being advocated as a better predictor of potential soil permeability problems arising from the use of recycled water for irrigation. This is because CROSS accounts for the dispersive effects of exchangeable potassium and magnesium. The study objectives were to: 1. Conduct a literature review to document the state of knowledge. 2. Evaluate recycled water quality requirements to minimize long-term impacts on soil structure. 3. Conduct experiments investigating crop/soil/recycled water interactions in greenhouse and laboratory environments. 4. Evaluate the long-term impact of recycled water use for irrigation on commercial fields. 5. Investigate remediation options for sodium-affected soils. 6. Summarize project findings in a final WRF report Key Findings: 1. Sodium Adsorption Ratio (SAR) and Cation Ratio of Soil Structural Stability (CROSS) were assessed in long-term field experiments, greenhouse studies, and soil columns to determine their effectiveness in predicting the negative impacts of using recycled water for irrigation on soil sodicity. 2. Overall, CROSS proved to be a more reliable tool for predicting the impact of recycled water reuse on soil sodicity and permeability. 3. Continuous irrigation with recycled water having a high CROSS value led to reduced strawberry productivity, and the salt uptake aligned with the cation composition of the water. 4. To reclaim sodic soils effectively, it is recommended to employ a combination of management strategies, such as using chemical amendments (e.g., gypsum) along with soil profile modification or implementing phytoremediation techniques. These approaches enhance the effectiveness of sodic soil reclamation.