Evaluation of Brush Packing Methods on different Semi-Arid Savanna Soil Types and Properties

Abstract

In Africa, savannas are important biomes used for raising livestock. Encroachment by woody species (so-called “bush encroachment”) disrupts the natural equilibrium between the herbaceous layer and the tree and shrub densities in savanna biomes, thus suppressing palatable grasses and decreasing the carrying capacity for livestock and other herbivores. Bush encroachment threatens sustainable agriculture and initiates land degradation which is threatened by aspects such as overgrazing, climate change and incorrect implementation of fire as management tool. This study forms part of a larger investigation conducted by North-West University (NWU) on brush packing as a restoration technique to increase species diversity and grass biomass production for livestock keeping in semi-arid savanna areas. Bush encroachment can alter soil properties (or so-called “soil quality indicators”), such as the soil moisture and soil nutrient contents influencing the microclimate. Soil properties will also be altered by the brush packing restoration technique (the technique used to restore bare areas caused by bush encroachment), which will have an influence on the processes leading to land degradation. Sufficient knowledge of brush packing techniques on various soil types and soil properties is vital to combat rangeland degradation and increase soil quality (capacity of soils to maintain biological activity, sustain environmental quality and promote animal and plant productivity). The study sites chosen for this project are situated in the D’Nyala Nature Reserve (conservation land-use type) and Shongoane communally-managed area in the Limpopo province, South Africa. Pattern Sampling was used to collect soil samples of the topsoil from each site for chemical and physical soil analysis, while a hand auger was used to drill holes to determine the soil moisture at different depths using a neutron probe. Methods of soil investigation included: 1) a neutron probe for measuring soil volumetric moisture at 10-centimetre depth intervals; 2) Kopecky rings for measuring soil bulk density; 3) loss-on-ignition testing for determining soil organic matter; 4) a single ring infiltration test for measuring soil infiltration rates; 5) sieve and hydrometer analyses for the estimation of particle size distribution; 6) soil classification by qualitative and quantitative methods (colour, texture and structure); 7) ammonium acetate testing to determine cation exchange capacity and exchangeable cations (Ca2+, K+, Mg2+ and Na2+); and 8) extractable phosphorous (P) using the Bray 1 extraction method. Data processing consisted of various statistical methods, and data visualisation using figures and graphs. The research presented in this project will provide evidence that restoration treatments containing brush packing improves the physical and chemical properties (soil quality indicators) of bush encroached arid and semi-arid soils regarding: 1) soil infiltration rate; 2) soil moisture; 3) soil bulk density; 4) pH; 5) electrical conductivity (EC); 6) soil organic matter; and 7) soil nutrient status. Six treatment methods or so-called “restoration treatments” were implemented, which

incorporated a mixture of soil disturbance, clearing of bush, re-seeding and brush packing. The project lastly substantiated which of the restoration treatments applied after bush clearing will best restore soils of bush encroachment savanna areas. The D’Nyala study area, containing medium-grained sandy soils, indicated greater success in improving overall soil quality after the application of the restoration treatments containing brush packing combined with re-seeding. The Shongoane site, consisting of fine-textured, calcareous, shallow soils, indicated little success in the improvement of soil quality, although the treatments containing re-seeding and brush packing did slow the decline in soil quality indicators (soil bulk density, soil moisture, pH, EC and soil organic matter). Evidence from this study concluded that brush packing, and brush packing combined with re-seeding, increases soil quality when applied and managed correctly. The success of these restoration treatments is dependent on the soil type and co-operation of the human population managing the rangeland area. In areas with sub-optimal soil types, poor treatment application or poor rangeland management, brush packing and brush packing combined with re-seeding reduced soil quality decline. This study recommends further research on the topic with a wider variety of soil types and frequent maintenance of the study areas to lessen the effects of brush packing removal by local inhabitants.