Dr Neil Miles

Soil compaction can be a serious yield-limiting factor in crop production, and indications are that this problem is more widespread than is generally perceived. Since compaction is below ground and, thus, not visible, its role in yield decline often goes unnoticed. Soil types Soils vary in their susceptibility to compaction. Research indicates that soils with higher sand and silt contents are more vulnerable to being compacted than loam and clay soils. In addition, the higher levels of organic matter in humic soils reduce their susceptibility to compaction. Causes Management practices that promote compaction include the following: Intensive tillage. Tillage may temporarily loosen the soil; however, in the long-term regular tillage increases the bulk density of soils through the weakening of soil structure and the depletion of soil organic matter. In addition, implements such as the mouldboard plough and disk harrow compact the soil beneath their working depth. Repeated use of these implements can result in the development of plough pans (compacted zones immediately below the ploughed layer (Figure 1, left). Wheel traffic. The wheels of vehicles used to apply fertilizers, lime and other products, harvest crops and pull implements, result in decreases in soil porosity and yield-limiting compaction (Figure 1, right). In the production of annual crops, 60% or more of the soil surface can be trafficked through cultivating, planting, fertilization, herbicide and other chemical applications (Mitchell & Berry, 2001). The in-field traffic associated with maize silage operations and the production of sugarcane frequently results in severe compaction problems (Figures 2 and 3) and associated yield losses. Animal hooves. The role of animals, particularly in intensive grazing systems, in promoting soil compaction is often overlooked. However, although animals are not as heavy as machinery, their weight is applied to the soil in a relatively small hoof-print, and so can cause significant compaction and damage to pastures, thereby decreasing forage yields and animal performance (Figure 4). Importantly, soils are most susceptible to being compacted when they are wet. Unfortunately, timing of farming operations, such as harvesting, fertilizing and rotating livestock, often makes it difficult to exclude machines and animals from soils when wet. Effects of compaction Detrimental impacts of compaction on soil health and plant growth are due largely to the decrease in the proportions of macro-pores in compacted soils. Effects associated with this include the following: Root penetration into compacted soil layers is restricted, and root and biological soil health compromised through water logging and anaerobic conditions in soils. Water infiltration as well as water-holding capacity of soils are reduced, and effective rainfall is diminished through increased runoff. Ponding on the soil surface after rain or irrigation is usually an indication of compaction. Nutrient uptake by roots is poor in compacted soils. This is due to, firstly, limited root development, and secondly, restrictions on the movement of nutrients to roots by mass flow. Yellowing of plants due to nitrogen deficiency is often associated with poor nitrogen recovery by roots in compacted soils. The picture in Figure 2 (left) provides clear…