Abstract:
An imperative of the South African government is to increase agricultural production in rural areas. In support of this, a project was initiated in the Mlondozi district of Mpumalanga Province under the National LandCare programme. The goal was to assess land management practices contributing to sustainable and profitable agricultural production. Medium-term liming experiments were sampled to a range of lime treatments in a Hutton and Oakleaf soil. Critical thresholds where a reduction in relative grain yield was found were at a pH (H2O), extractable acidity, Al and acid saturation of 5.49, 0.277 cmolc kg soil-1, 0.145 cmolc kg soil-1 and 13%, respectively. Critical soil fertility threshold levels were established at 50 mg K kg-1, 228-345 mg Ca kg-1, 78-105 mg Mg kg-1 and 1.68-2.83 mg Cu kg-1. Nutrient vector analysis showed a toxic build-up of Fe, followed by Al and to a lesser extent Mn, which depressed the uptake of Ca, Mg and B in the Hutton soil. In the Oakleaf soil, Al toxicity, followed by high concentrations of Mn and Fe, markedly reduced the uptake of Ca, Mg and K by maize. Net rates of acid production in the soil profile varied between 1.61 and 2.44 kmol H+ ha-1 year-1 for the Hutton soil and between 4.59 and 8.82 kmol H+ ha-1 year-1 in the Oakleaf soil due to liming. A decline of 0.046 pH unit year-1 for an initial pH(H2O) value of 5.33, and 0.140 pH unit year-1 for an intial pH(H2O) of 6.47, respectively, in the Hutton was recorded. For the Oakleaf these declines were 0.044 and 0.110 pH unit year-1, from pH(H2O) 4.54 and 5.15. Maintenance liming amounts at different pH values for the Hutton soil were equivalent to 0.2, 0.3 and 1.4 tonnes CaCO3 ha-1 annually, while 0, 0.8 and 0.8 tonne CaCO3 ha-1 annually were recorded for the Oakleaf soil. The study was extended to 80 random topsoil samples in the district. Relationships of soil BC over limited pH ranges showed that at soil BC (pH<4.5) the main buffering mechanism was extractable Al > organic C > clay. At soil BC (pH4.5-6.5) the buffering mechanism was extractable Al > clay > CBD-Al > organic C > CBD-Fe. The main buffering mechanism between pH 6.5-8.5 was clay > CBD-Fe, organic C > CBD-Al. Acid production for 30 crop production sites varied from a measured 0.21 to 10.31 (mean 3.70) kmol H+ ha-1 year-1 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. Soil with a pH (H2O) value of >5.73, extractable Al and acidity of <0.18 and <0.25 cmolc kg-1 soil, respectively, clay content of ≤26%, and a ECEC value of ≤3.29 cmolc kg soil-1, are at greater risk of acidification as gradual acceleration in soil acidification takes place at the above-mentioned critical thresholds.
