Management of soil salinity associated with irrigation of protected crops

Long-term evaluation of soil chemical changes in the soil is required to optimize irrigation of protected crops and to control associated environmental issues. In this study, the multi-component solute transport module UNSATCHEM of HYDRUS-1D was used to assess the effects of long-term (2018-2050) irrigation on salt build-up in the soil under unheated greenhouse conditions. Blended water (recycled water, artificially recharged groundwater and harvested rainwater) was used to irrigate tomato, cucumber, capsicum, and eggplant. Irrigation management included four leaching fractions (LF), i.e., accounting for 0, 15, 20, and 30% of applied excess water. The effects of four amounts of annual gypsum application, i.e. 0, 1.7, 2.6, and 3.4t/ha, were also simulated. Model-simulated annual root water uptake by cucumber, tomato, capsicum, and eggplant was 303, 476, 642, and 649 mm, respectively, for an irrigation schedule based on outdoor temperature thresholds. Annual drainage was 4.1-6.1% of irrigation for these crops. Average salinity in the soil solution (ECsw) at the end of the simulation (year 2050) reached 6.5 dS/m for cucumber, 7.6 dS/m for tomato, 8.7 dS/m for capsicum, and 9.3 dS/m for eggplant. Soil salinity was building up at a nearly constant rate over 33 years. Exchangeable sodium percentage (ESP) at the end of the simulation was 30.8% for tomato, 27.1% for capsicum, 33.2% for eggplant, and 31.4% for cucumber. These results indicate that both salinity and sodicity will exceed critical threshold values and thus must be managed to maintain sustainability of irrigated horticulture. Modeling showed that higher irrigation (up to 15-20%) coupled with an annual addition of gypsum of 1.7 t/ha kept both salinity and sodicity (SAR, ESP) below critical thresholds while pH was reduced from 8.7 to 7.8, thus creating a favorable soil environment for long-term sustainable vegetable production under greenhouse conditions. The new gypsum application module implemented in the UNSATCHEM model has broad applicability in evaluating soil amendment scenarios to address sodicity hazards linked to long-term irrigation.