Efficacy of Monsoon Rainfall in Salt Leaching through a Field Soil after Wheat Cultivation under Saline Water Irrigation

抄録

Characteristics of rainfall-induced salt leaching was investigated in a field after wheat (Triticum aestivum L.) cultivation under irrigation by synthetic saline water, which was prepared by mixing sodium chloride (NaCl) with fresh water pumped from a borehole well. The treatments of the experiment — I₁: root zone soil with salinity 0.12 dS m ⁻¹, and I₂, I₃, I₄ and I₅: root zone soils with salinity 0.39, 0.51, 0.69 and 0.86 dS m ⁻¹, respectively — had three replications. The soil salinities were developed due to irrigation by fresh water (of 0.385-dS m ⁻¹ background salinity) for I₁ and by saline water of 4, 7, 10 and 13 dS m ⁻¹ salinity for I₂, I₃, I₄ and I₅, respectively. Soil samples were collected from each plot from 0–20, 20–40 and 40–60 cm soil layers at 15 (in a few cases at 30) days interval during rainy season (April–September 2011) after harvesting of the crop. By measuring electrical conductivity (EC) on saturation extracts of the samples salt dynamics at the three soil layers under the five irrigation treatments were characterized. Variation of soil pH due to the leaching process was also evaluated by measuring pH of the soil extracts. At the end of rainy season, the salt that accumulated in the soil due to irrigation by saline water leached almost completely out of the top 0–20-cm soil layer in all treatments. The salt from the upper layer leached down and initially elevated salinity of the lower layers. Salinity in the bottom layer of I₂–I₅ treatments did not reduce to pre-irrigation non-saline level, implying that rainfall, although a substantial amount (1673 mm) could not wash out the imposed salinity from the root zone soil. At the early period of leaching, both the frequency and quantity of rainfall were relatively small, but the rate of reduction of EC in the soil profile was quite substantial and rapid, especially in the soil with high salinities (e.g., I₄ and I₅). At the later period, on the other hand, the rate of EC reduction slowed down although the frequency and quantity of rainfall were higher than at the early period of leaching. The salt removal rate was thus inversely related to its concentration in the soil. It is concluded that, irrigation by saline water of EC 4, 7, 10 and 13 dS m ⁻¹ caused accumulation of salt in the soil in excess of that washed out by the monsoon rainfall.