Impacts of soil, water source and agro-climatic conditions on exchangeable sodium in rice-cultivated lowland soils of Sri Lanka

Rice is sensitive to salt stress, commonly caused by high concentrations of sodium (Na) in soils. However, the concentration and spatial variability of exchangeable-Na in lowland paddy fields of Sri Lanka are not known. This study examined the interactive effects of the agro-climatic zones (ACZs), soil orders and water sources on exchangeable-Na in lowland paddy fields in Sri Lanka using 8,566 soil samples. Exchangeable-Na was extracted using 0.01 M CaCl2 solution and detected using inductively coupled plasma-mass spectrophotometry. Exchangeable-Na concentration ranged from 0.05 to 4,814 mg kg−1 with a mean concentration of 140.7 mg kg−1, and these values were within the optimal range reported for tropical paddy soils. The distribution of exchangeable-Na was right skewed with 44%, 35%, and 9.3% of samples falling into 0-100, 100-200, and 200-300 mg kg−1, respectively. Samples from the Low country Dry zone had the highest (157 mg kg−1) Na concentration while that in the Upcountry Intermediate zone was the lowest (13 mg kg−1) (p<0.05). Vertisols recorded the highest (255 mg kg−1) and Ultisols recorded the lowest (81±5.0 mg kg−1) among soil orders. Soils receiving supplementary irrigation in Low country Dry zone had lower Na than that in rainfed systems. Exchangeable-Na concentration was positively correlated with soil pH (p<0.05). Even though the concentrations of Na in soil samples were within the optimal range, agronomic decisions based on ACZ, soil order, and water source need to be made to minimize the potential development of saline-sodic soils in the study areas.