The impact of different land use types on selected soil quality parameters in northern KwaZulu-Natal.
Soil is a natural resource that must be sustainably managed for the future of humankind. Maintaining soil resources in the long term is of primary necessity since sustainable agriculture is a global issue that has received special attention by the scientific community, policymakers and the agricultural community over the past few years. Agriculture should involve the successful management of the soil resource to satisfy changing human needs while maintaining or enhancing its quality. Therefore, land use change that might impact on soil quality parameters continues to be a focus for researchers. Land use changes such as conversion of native grasslands to cultivated mixed croplands or pasture are known to result in changes to soil physical, chemical and biological properties. However, the direction and magnitude of these changes vary with soil type, land cover and management. The objective of this study was to investigate the effects of land use change from undisturbed grassland to pasture and crop farming on selected properties of four contrasting soils. The study was conducted at Owen Sitole College of Agriculture (OSCA), which is located at Kwesaka-Mthethwa, KwaZulu-Natal Province, South Africa, about 12.5 km north of Empangeni and 163 km north of Durban. Nine different soil forms are found on the College land of which Inhoek and Mayo are the most common. Shortlands and Westleigh each cover approximately 6% of the area and Oakleaf is common on the low-lying areas along the Cwaka and Enseleni Rivers. The land uses studied were undisturbed native grassland, irrigated pasture, and cropland on Shortlands, Westleigh, Inhoek and Oakleaf soil forms. Due to the absence of any areas of undisturbed Oakleaf in the study site, only pasture and arable land use systems were compared on this soil form. The impact of these different land use types were evaluated through their effects on some soil chemical and physical characteristics. Samples were collected from each of the soil forms under each of the available land use types by digging mini-pits (90 cm wide, 120 cm long and 30 cm deep). Four bulk samples were systematically collected from each soil form at depths of 0-1, 1-5, 5-10, 10-20 and 20-30 cm using a spade. For soil aggregate stability determinations, samples were air dried and sieved to collect sufficient aggregates between 2.8 and 5 mm. The aggregate size distribution under the different land use management systems was assessed by three different treatments namely WT: water treatment, ET: ethanol treatment and SCWET: slow capillary wetting ethanol treatment. The rest of the bulk samples were crushed with a pestle and mortar and passed through a 2 mm sieve for the analysis of soil organic carbon (SOC), particulate organic matter (POM), pH (H2O and KCl). Samples for effective cation exchange capacity (ECEC), available phosphorus (P), base cations and particle size distribution were bulked to three depths of 0-10, 10-20 and 20-30 cm prior to analysis. The results revealed that some soil properties significantly (p < 0.05) changed following the conversion of undisturbed grassland to pasture and arable land use systems. Soil pH (H2O and KCl) was significantly affected by land use in the Shortlands soil. The interaction between land use and soil depth was only significant in the Inhoek soil form. The mean pH (H2O) values were 6.31 for the soils under the arable land use and 6.50 under pasture and undisturbed systems. In KCl the mean pH values were 5.10 for arable and 5.43 for both pasture and undisturbed soils. The concentration of SOC significantly followed the order: pasture > undisturbed > arable with mean values across all depths and for all soils. Significant interactions between land use and soil type were observed for POM in the Westleigh and Inhoek soil forms. Inhoek soils under pasture had significantly higher POM than the other land uses in the 0-1, 1-5 and 5-10 cm depths, while only 0-1 and 1-5 cm depths had higher POM in the Westleigh soil under pasture. The Oakleaf form under pasture had significantly higher POM than the arable soil at all depths. Mean POM values were in the order of pasture (0.608%) > undisturbed (0.223%) > arable (0.118%). Pasture soils had lower available P in the 0-10 and 10-20 cm depths than the other land uses in the Shortlands, while in the Westleigh pasture and undisturbed land uses were similar and higher than the arable soil. In the Oakleaf, the arable soil had higher P (18.6 mg kg-1) than pasture (7.0 mg kg-1) in the 0-10 cm depth, while both land uses were similar at the other depths. There were no significant effects of land use on ECEC in the Shortlands, Westleigh and Inhoek. The overall ECEC mean values (cmolc kg-1) for the Shortlands, Westleigh and Inhoek soils were in the order of arable (24.12) > pasture (19.29) > undisturbed (16.11). However, in the Oakleaf, mean ECEC values (cmolc kg-1) were in the order of pasture (14.9) > arable (7.40). Arable soils had higher amounts of Ca in the 20-30 cm depth of both Shortlands and Westleigh soils and in the 0-20 cm depth of the Inhoek soil form. The overall mean values of exchangeable Ca were 23.6 cmolc kg-1 (arable), 19.0 cmolc kg-1 (pasture) and 18.9 cmolc kg-1 (undisturbed grassland). The overall mean values for exchangeable Mg were 10.0 cmolc kg-1 (arable), 9.29 cmolc kg-1 (undisturbed grassland) and 8.21 cmolc kg-1 (pasture). In the Oakleaf, the mean values (cmolc kg-1) of Ca were 19.1 (arable) and 9.58 (pasture) while those for Mg were 2.76 (arable) and 5.89 (pasture). The mean K values (cmolc kg-1) were 3.33 under arable, 2.55 for pasture and 1.95 for the undisturbed land use system. The soils under undisturbed and pasture land uses showed significantly higher mean weight diameter (MWD) values than under the arable land use system in all the treatments. The mean values of MWD for the undisturbed, pasture and arable land use systems were 2.99 mm, and 2.95 mm and 1.92 mm, respectively. The 38 years of continuous cultivation of undisturbed grassland has led to changes of the measured physical and chemical properties of the soils at OSCA. The pasture soils showed similar trends in the measured parameters to those of the undisturbed soils and values were higher than those found in the arable soils. The soil characteristics negatively affected by cultivation practises were SOC, POM, pH and aggregate stability. These results showed the need to improve agricultural practices at OSCA to limit degradation of some vital soil properties. This can be achieved by long term monitoring of soils at the College so as to measure changes in soil attributes and link them to a land management plan.