Abstract
Soil salinity and sodicity are major factors limiting agricultural productivity in irrigation schemes located in semi-arid areas. A study was conducted to assess the quality of irrigation water used in Mutema Irrigation Scheme located in south-east Zimbabwe to understand how irrigation water quality is related to the chemical quality of soils in the scheme. Irrigation water samples were collected from groundwater and surface sources in 2012 and their hydrochemistry determined while soil samples were collected from irrigated and non-irrigated parts of the scheme in 2006 and 2012 and analysed for selected chemical properties. The results indicated that the groundwater had high concentrations of Na+ (4.35 mg/l), Mg2+ (4.75 mg/l), Cl- (3.6 mg/l) and Electrical Conductivity (EC) (1729 Msm/cm) compared to the surface irrigation water source which had 0.72 mg/l Na+, 2.25 mg/l Mg2+, 0.78 mg/l Cl- and 594 Msm/cm EC. The soils in the scheme had higher levels of pH, Sodium Adsorption Ratio (SAR), Exchangeable Sodium Percentage (ESP) and EC which in some blocks exceeded the threshold requirements for cropping. It was found that pH, SAR and ESP were significantly higher (p ) in irrigated blocks compared to non-irrigated areas of the scheme, indicating an influence of irrigation water on soils characteristics in irrigated plots. Mg2+ and Ca2+ in the soils positively correlated with Na+ (r2 = 0.67 and r2 = 0.57 respectively). The results indicated that the groundwater presents a chloride hazard to the soils in the scheme which are becoming saline and therefore require proper management and choice of crops to sustain crop production.
Highlights
Salinity and sodicity have been extensively reported among the major problems of irrigated agriculture across the world [1,2,3,4,5]
The methods that are commonly used as indices of salinity and/or sodicity in soils include electrical conductivity, osmotic potential, Sodium Adsorption Ratio [SAR) and Exchangeable Sodium Percentage (ESP) [3,9,12,14]
The SAR value was more than four times as much in borehole water (2.16) than in river irrigation water (0.47), there were not much difference in pH values for the borehole and river water which were slightly acidic to neutral (6.8 and 6.7 respectively)
Summary
Salinity and sodicity have been extensively reported among the major problems of irrigated agriculture across the world [1,2,3,4,5]. Soils are classified as saline or sodic based on the soluble salts in saturated extracts (EC) and the proportion of Na to Ca and Mg in saturated extracts (SAR = Na [Ca+Mg/2]1⁄2) or the proportion of exchangeable Na to the cation exchange capacity (ESP = Na/CEC/100) [4]. Saline soils have an EC of more than 4 dS∙m−1 and either a SAR of less than 13 or an ESP of less than 9% [9,10,11]. Sodic soils have an EC of less than 4 dSm−1 and either a SAR of more than 13 or an ESP of more than. The methods that are commonly used as indices of salinity and/or sodicity in soils include electrical conductivity, osmotic potential, Sodium Adsorption Ratio [SAR) and Exchangeable Sodium Percentage (ESP) [3,9,12,14]. SAR measures the proportion of sodium (Na+) to calcium (Ca2+) and magnesium (Mg2+) and can be used for assessing quality of the water used for irrigation and the status of the soils that receive the irrigation water [1,6]
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