Effects of tillage practices on some key soil parameters : A case study in the Kwazulu-Natal Midlands, South Africa

Thesis (MSc)--Stellenbosch University, 2017.ENGLISH ABSTRAT: Soil organic carbon in its different forms play an important role in the biological, chemical and physical quality of the soil and need to be better understood and managed to farm in a sustainable manner. Four different farming systems were evaluated in this study and the results were compared to grasslands that were used as a reference value. The four farming systems were: Conventional tillage maize, reduced tillage maize without legume rotation, reduced tillage maize with legume rotation and conservation agriculture maize (no-till). The experimental study site is situated in the Kwazulu Natal Midlands close to Greytown South Africa. Thirty five individual sites were sampled and studied; 8 conventional tillage sites, 7 reduced tillage without legume rotation sites, 5 reduced tillage with legume rotation sites, 9 conservation agriculture sites and 6 natural grasslands. Samples were taken in triplicate using 5 cm steel cores at depths of 2.5, 7.5, 12.5, 17.5, 30, 40, 50, 75 and 100 cm (unless restricted by rock) for bulk density and SOC determination, total microbial biomass, aggregate stability and other important soil parameters. The objective of the study was to determine the influence of different long term tillage systems have on the soil organic carbon stocks and other soil parameters up to 1 m depth that are key to overall soil health. . The total Soil Organic Carbon (SOC) stocks declined in the following order CA (231,1 Mg/ha) > RT + legumes (217.3 Mg/ha) > CT (192.8 Mg/ha) > Grasslands (180.1 Mg/ha) > RT – legumes (177.5 Mg/ha). The reduced tillage without legume rotation treatment yielded the highest average C: N value over the 1 m depth, where the reduced tillage with legume rotation treatment yielded the lowest average from 5 cm – 20 cm depth. %. Significant differences in average soil porosity (α = 0.005) were found between CT and grasslands (P = 0.0357) as well as between RT with legume rotation and grasslands (P = 0.0175). Conservation agriculture produced significantly higher Total Microbial Biomass (TMB) values as well as Water Stable Aggregates (WSA) compared to all the other farming systems including grasslands, with values ranging from 7.34 g/kg of soil in the top layer to 3.67 g/kg of soil at 50 cm for TMB. The results for TMB showed that there were significant differences (α = 0.05) between CA and CT (P = 0.0267) as well as between CA and grasslands (P = 0.0445). Water stable aggregates were clearly affected by tillage treatments according to these results. Strong significant differences (α = 0.05) were also found in the results between CA and CT (P = 0.0096), CA and grasslands (P = 0.0158) as well as between CA and RT (P = 0.0456). These results show that practicing long term conservation agriculture approximates the soil carbon distribution pattern to a natural exponential decline function and improves some important soil parameters that play a key role in overall soil health and sustainability.AFRKAANSE OPSOMMING: Geen opsomming beskikbaa