Treated wastewater irrigation: Soil variables and grapefruit tree performance

Soil degradation and declining tree performance following long term irrigation with treated wastewater (TWW) have been reported recently in orchards grown on clay soils. In an attempt to reverse this situation our research objectives were to quantify the effects of replacing TWW irrigation with fresh water (FW) on water uptake, water and mineral status, growth and yield of citrus trees in relation to soil physical and chemical properties. A field experiment was carried out in a commercial grapefruit orchard in a clay soil with a history of TWW irrigation. Changing irrigation water quality from TWW to FW significantly decreased soil solution electrical conductivity (EC), Na and Cl concentration, sodium adsorption ratio (SAR), exchangeable sodium percentage (ESP) and improved aggregate stability (AS) of the soil. The concentrations of Na and Cl in leaves and roots were lower in FW-irrigated trees than in TWW-irrigated ones. Fruit yield, shoot and root growth, leaf area, water status and water uptake were all significantly and favorably affected by replacing TWW with FW. Although fruit yield increased by replacing TWW with FW irrigation, it was not significantly associated with any single or group of the studied soil attributes. However, in a stepwise regression analysis a correlation was established between fruit yield and leaf Cl and soil AS. Our findings indicate that the negative effects of irrigation with TWW are (i) through damage to soil structure leading to reduced water uptake and (ii) via accumulation of Na and Cl in roots and leaves of grapefruits to toxic levels. The positive effects of alternating poor quality water (TWW) with water of high quality (FW) occur in a relatively short time span, i.e. several months to two years, thus promoting the viability of this management practice.