Effect of surface applied pressure by vehicles fitted with pneumatic tyres on properties of a virgin soil

Soil compaction is often studied by comparing virgin sites to commercial fields with a long-term cultivation history. The establishment of a new research farm in a virgin area of the Mpumalanga province in South Africa provided an opportunity to induce compaction to quantify the buffer capacity of a virgin soil against degradation of its soil physical properties. The soil of the trial site is a shallow (±0.4 m) Cambisol with uniform texture (±44% clay), and 3.8% organic matter in the A-horizon, overlying weathered basalt. Row spacings were 1.5 m and 1.8 m, and only haulage vehicles with matching wheel spacings were allowed in the field. Compaction treatments, starting after harvest of the plant crop in 2003, consisted of pressure applied with commercial 30 ton capacity loaded haulage vehicles fitted with either radial tyres or high flotation tyres. After harvest the soil was kept dry in one part of the field and watered elsewhere to create differential water contents at the time of treatment application. Results showed reduced water infiltration rates, increased soil bulk density, increased penetration resistance and reduced root distribution in all the compaction treatments. The treatments with a higher water content were more susceptible to degradation. It is concluded that even a virgin soil in good physical condition will be degraded over the period of a few years to the physical threshold limits. The buffer capacity of this soil against compaction (and therefore of most other soils) degraded over a period of only three years. Permanent traffic lanes should therefore be considered essential in most agricultural systems to protect the productive areas of fields. Proper spacing of interrows and the use of low pressure high flotation tyres on all axles should be considered as additional measures to achieve this goal. (Résumé d'auteur