Assessing the potential soil acidification risk under dryland agriculture in the Mlondozi district in the Mpumalanga Province of South Africa

Abstract

The farming community in the Mlondozi district in the Mpumalanga Province of South Africa has been part of a government liming intervention with the objective to ameliorate the serious soil acidity problem in the district. The current study was undertaken in order to evaluate the impact of the liming intervention and the risk of re-acidification of the soil due to natural and agricultural activities. Acid production in the 0-250 mm depth varied from a measured 0.21 to 10.31 (mean 3.70 kmol H⁺ ha^{- 1} year^{- 1}) in crop production sites. Approximately 190 kg lime ha^{- 1} yr^{- 1} is required to maintain current soil pH levels under crop production. The rate of pH decline for the top 0-250 mm depth was between 0.051 and 0.918 (mean 0.237) pH units year^{- 1}. In the absence of remedial lime applications, pH(H₂O) values in most of the area are projected to decrease to the critical value of 5.68 or lower within 4 years. The upper and lower critical pH(H₂O) were found to be between ca. 5.73 and 5.68. Below the lower critical value a reduction in crop production can be expected and above the upper critical value, accelerated acidification takes place. Soils with an extractable Al and acidity of <0.180 and <0.253 cmol (+) kg⁻¹ soil, respectively, a clay content of 26%, and an ECEC value of 3.29 cmol (+) kg soil⁻¹, or high initial soil pH values, are more at risk to accelerated acidification than soils with lower extractable Al and acidity, higher clay contents and higher ECEC values.