Tillage and tractor traffic effects on soil compaction in horticultural fields used for peri-urban agriculture in a semi-arid environment of the North West Province, South Africa

Urban and peri-urban agriculture is becoming increasingly important as a source of income and food for the urban population in South Africa. This study was prompted by concerns of city authorities that soil compaction in horticultural fields that used conventional tillage practices with tractors was, not only limiting the performance of fruit and vegetable crops, but also affecting the quality of water resources. The extent of soil compaction in selected fields used for peri-urban agriculture within the city of Mafikeng was quantified by measuring soil penetrometer resistance and bulk density in the profiles of four adjacent fields (vegetable, orchard, vineyard and unploughed control) but with different tillage traffic systems over 14 years. Soil organic carbon and water content of the profiles were also measured. Tractor traffic was used for seedbed preparation and weeding respectively in the vegetable and orchard fields while the vineyard was not heavily trafficked. A field that remained unploughed over the same period was used as control. Higher soil compaction was more evident in the orchard and vegetable fields compared with the vineyard. The location of compacted layers within the profiles was related to the nature of tractor traffic applied. There was subsoil compaction in the orchard and vegetable fields while shallow compaction was observed in the vineyard. The maximum penetrometer resistance and depths were: unploughed land (1500kPa at 42cm), vineyard (1587kPa at 5.0cm), vegetable field (2315kPa at 30.0cm) and orchard (2550kPa at 20cm). The unploughed and untrafficked soil profile had the largest soil water content and the least penetrometer resistance. The trafficked fields had significantly greater (P =0.05) bulk density than the unploughed land. Apart from tractor traffic the small organic matter content and the hard-setting behavior of the soil could have exacerbated the compaction. It was concluded that tillage practices are diminishing the quality of soils used for horticulture production in peri-urban areas of the study area. The compacted soil profiles were considered to have the potential of contributing to increased runoff and soil erosion and subsequent sediment concentration in surface water courses around the city. The study recommends that controlled tillage and other management strategies, especially those that increase the organic matter content of the soils, be adopted to not only ameliorate the existing compaction, but also sustain the productivity of the soils used for urban agriculture and the water resources in this semi-arid environment.