Organic matter, pH and nutrient distribution in soil layers of a savanna Thornveld subjected to different burning frequencies at Alice in the Eastern Cape

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Abstract
Fire is an important tool for the management of grazed savanna grasslands in the Eastern Cape, South Africa. The aim of this study was to quantify the influence of veld burning frequency on soil properties especially organic matter, pH, exchangeable Ca, Mg, K, Na and extractable P. Soil samples were collected from different depths in plots of a 17-year old experiment where burning frequencies were: no burning (B0), annual (B1), triennial (B3) and sexennial (B6). A land under continuous grazing and not burned (G) and strips between the plots (R) were also sampled for comparison. Burning significantly (p < 0.05) increased the pH and concentration of exchangeable Ca, Mg, Na and K but reduced the organic C and extractable P of the soil. The soil properties differed significantly between surface (0–10 cm) and lower (10–40 cm) layers: Ca, Mg, Na and pH increased with depth while K, organic C and extractable P decreased down the profile. Soil organic C was in the order B0>G>B3>B6>B1>R. The trend in the distribution of extractable phosphorus in the soil profiles mirrored that of organic C suggesting that most of the P was organic in nature. It is concluded that fire can make a significant contribution to the distribution of nutrients in a veld soil by rapidly mineralising the above ground biomass into ash. However, concern about the sustainability of a fire-managed system is raised in view of the high losses of carbon, nitrogen and sulphur from the biomass during burning. Considering the importance of organic matter in maintaining soil fertility and productivity, the use of other methods of nutrient cycling and redistribution should be encouraged.