State of the Environment Report 2007
Loss of soil health is classed as an emerging issue due to its potential to cause significant impacts to agricultural productivity, however there is currently inadequate information to report on this issue.
Soil health is essential to farmers for maintaining the viability of their business. Many farmers and pastoralists regularly test soils and plants to determine soil health and combine this information with satellite imagery to identify problem areas. Soil health encompasses several factors, including soil physical structure, chemical composition and biological content. Deterioration of one factor may not necessarily cause an immediate loss of soil health, but may contribute to a decline in other factors and a gradual reduction in soil health.
Loss of soil health may be associated with a drop in soil fertility, caused by a decrease in soil nutrients that are necessary for plant growth (e.g. nitrogen, phosphorus or potassium). Soil nutrient decline is caused by removal of nutrients in harvested crop products, hay, plantation timber, animal products, as well as through soil erosion, leaching, acidification, burning of crop residues, and other chemical processes. Nutrient losses must be replaced by inputs over time (such as compost or fertiliser addition, or the growing of nitrogen fixing plants) if the soil is to remain productive. Native vegetation is not affected as much as it has evolved to the naturally low nutrient levels typical of Western Australian soils.
Loss of soil health may also be associated with a decline in soil structure, which tends to be a less visible or recognised form of soil degradation. A decline in soil structure can occur when wet soil is exposed to stresses of machinery, tillage activities or stock trampling. Similarly, exposure of bare soil to heavy rainfall can cause dispersion, slaking and crusting of the soil surface, which can lead to increased runoff and erosion. A decline in soil structure may also be due to poor soil drainage and subsequent waterlogging and soil dispersion (characterised by high sodium content). Soil dispersion leads to reduced cohesion between soil particles, and results in increased surface runoff and erosion, and reduced water infiltration. Although it occurs naturally, high soil sodium content can be exacerbated by land salinisation (see 'Land salinisation').
Lack of organic or biological material (material from plants or animals) affects nutrient content, soil structure and the presence of soil organisms. Soil organic matter content typically decreases following clearing and cultivation of land, with regular tillage contributing to significant losses. Tillage physically breaks soil into small aggregates and reduces binding by soil organisms (e.g. fungi). Initial years of farming may result in the loss of one-third to one-half of soil organic matter. Loss of soil organic matter and biological activity will result in increased erosion potential, decreased nutrient storage and availability; and reduced water retention, infiltration and water-holding capacity.
Measuring soil health is difficult because soils are dynamic and often change in response to land uses, management practices, soil treatments, weather conditions and climate change. However, a good understanding of the many factors that contribute to soil health can avert deterioration in soil condition. A loss of soil health results in declining agricultural, pastoral or forestry productivity, causing significant economic impacts. A desire for short-term economic gain and ever-increasing levels of production from land must not compromise long-term soil productivity and health. This will only serve to place an additional burden on land managers who have to undertake expensive soil treatments and rehabilitation activities. Other environmental implications arising from a decline in soil health include exacerbation of soil erosion and associated inland water sedimentation, contamination and eutrophication.
