Effect of salt ion composition of irrigation water on parameters of soil water and salt movement

Abstract: Shortage of freshwater resources restricts agricultural production in arid and semiarid region. Irrigation with saline water provides an effective way to make up for the deficiency of freshwater resources. However, it is unknown about influence of irrigation with different salt ion compositions on soil physical and chemical properties. In this study, the effect of different salt ion compositions of irrigation on soil water and salt movement was investigated. The soil samples were collected from orchard calcareous soil in Qin'an County of Gansu Province. The soil electrical conductivity was 0.43 mS/cm, pH was 8.54 and soil organic matter was 10.52 g/kg. A total of 4 treatments were included: NaCl, NaCl:MgCl2 with the ratio of 1∶1, NaCl:CaCl2 with the ratio of 1∶1, NaCl:KCl with the ratio of 1∶1. The concentration of all the treatments was 3 g/L. Soil water diffusivity and salt ions movement along the horizontal distance were studied by the method of horizontal soil column infiltration. Soil pH and sodium adsorption ratio were determined. The Boltzmann function was used for simulation of soil salt movement. The relationship between soil water diffusivity and water content was simulated by an power function. The results showed that the soil water diffusivity capacity at water content of 0 was the highest in the treatment of NaCl, followed by NaCl:MgCl2, NaCl:KCl and NaCl:CaCl2. The change rate of diffusivity with water content was the highest in the treatment of NaCl:CaCl2 followed by the NaCl:KCl, NaCl:MgCl2 and NaCl only. The salt ion composition of irrigation water and soil water content both significantly affected the soil water diffusivity (P<0.01). For all the treatments, the soil salt content, calcium ions, potassium ions, sulfate ions and sodium ions all increased with the infiltration distance but the bicarbonate ion decreased with the distance. The obvious trend wasn't found for the chloride and magnesium ions. Soil pH decreased with infiltration distance for the treatment of NaCl only. The decrease trend of soil pH fluctuated for the other treatments. The soil pH at distance of 8 and 32 cm was significantly different (P<0.05). The salt ion composition did not significantly affect the soil pH. The sodium adsorption ratio decreased obviously with the infiltration distance in the treatment of NaCl only. The sodium adsorption ratio in the treatment of NaCl was higher than the other treatments. The sodium adsorption ratio increase trend fluctuated for the other treatments. The salt ion composition of irrigation water significantly affected the sodium adsorption ratio (P<0.05). Based on the Boltzmann function fitness, the initialization value and final value were influenced by the salt ion composition of irrigation water and the treatment NaCl was higher than the other treatments. The center was similar for all the treatments (28.488-28.700). The simulation was well with the determination coefficient higher than 0.89. The study indicated that adding calcium ions in irrigation water could inhibit water movement and decrease salt accumulation in the process of irrigation with brackish water. The irrigation at one end of plot was not recommended since it may cause salt accumulation and we suggested to irrigate at both ends of the plot. The study could guide the irrigation with brackish water in arid or semiarid regions.