Distribution of Exchangeable Magnesium in Lowland Rice-Cultivated Soils of Sri Lanka as Affected by the Differences in Climate, Soil, and Water Source

Magnesium (Mg) is an essential macronutrient for plants. Even though Sri Lankans consume rice as the staple food, the Mg status in Sri Lankan paddy soils as affected by its climate, soils, and water sources used are not well understood. This study was conducted to (i) determine the distribution of exchangeable Mg (ex-Mg) concentration, and (ii) examine the interactive effects of agro-climatic zones (ACZs), soil order, and water source in determining the ex-Mg concentra-tion in lowland paddy fields of Sri Lanka. A total of 9,038 soil samples representing six ACZs, six soil orders, and three water sources were collected using a stratified random sampling approach. The ex-Mg concentration was determined after extracting in 0.01 M CaCl2 and detected using inductively coupled plasma-mass spectrometry. The range of ex-Mg was within 0.01-1,610 mg kg-1, with a mean of 210.4 mg kg-1. From the tested soil samples, 66% were Mg-deficient (<240 mg kg-1), 32% were Mg-optimal (240-730 mg kg-1) and 2% were Mg-excessive (>730 mg kg-1). Among the ACZ, the Dry zone Low country had the highest ex-Mg concentration (p<0.05). Among the soil orders tested, Vertisols had the highest and Histosols had the lowest ex-Mg concentration (p<0.05). Irrigated rice fields had higher ex-Mg than the rainfed systems (p<0.05). Soil ex-Mg concentration was positively correlated with soil pH (p<0.05) and crop productivity (p<0.05). As most rice-growing soils of Sri Lanka are Mg-deficient, it is important to implement strategies specific to ACZs, soil orders, and water sources to improve the soil-Mg status.