Soil Physical Attributes
Soil Groups
Australia's agricultural landscapes support a great range of soils. Most are ancient, strongly weathered and infertile by world standards. Those on floodplains or derived from more-recent volcanism are younger and more fertile. The natural limitations of many soils, human impacts and their interactions with climate have made it difficult to develop sustainable agricultural systems. Some forms of degradation (e.g. nutrient deficiencies) can be corrected, others are either irreversible (e.g. past soil erosion) or difficult to remedy (e.g. subsoil acidity).
Water, changing temperatures and chemical action decompose rock to form the basic soil materials - sand, silt and clay. In many Australian soils, materials deposited by wind or water have built up over thousands of years, and have subsequently been changed further by wind and weathering. Weathering may be physical (i.e. cracking caused by heating and cooling), chemical (leaching weak acid formed by dissolved CO2) or biological (action of roots, etc.). The proportion of sand (large particles), silt (medium-sized particles) and clay (very small particles) determines your soil texture and affects drainage and nutrient availability, which in turn affect plant growth.
Soil Stability
Soil structural decline can reduce crop emergence, and restrict growth due to poor infiltration of rain, leading to less biomass and return of organic material to the soil, while waterlogging and lack of aeration can reduce the activity of soil organisms. It can also result in increasing run-off and erosion. Sediments accumulate in waterways and the phosphorus exported can contribute to eutrophication.
Soil Compaction
The economic basis for ameliorating soil compaction is strongest where the employment of management practices that support the natural repair of soil structure is possible, such as stubble retention to increase organic matter or incorporating deep rooted crop species in rotation. One of the factors against the adoption of management practices that support natural repair methods such as zero-tillage, stubble cover, controlled traffic on soil compaction is the length of time sometimes required for responses to be observed.
Soil Erosion
Soil erosion is a natural process caused by two main agents – wind and water. Soil erosion is often accelerated under agriculture, especially on cropped land. The susceptibility of an area to wind erosion depends on climate, soil texture and vegetative cover. Wind erosion is most common on loose, coarse, dry surface soils (most typically sandy soils) in landscape positions exposed to strong winds. Erosion by water is largely episodic and is normally associated with intense rainfall events combined with inappropriate tillage practice, poor infiltration, steep slopes and a lack of surface water flow control measures.
Available Water
Soil is made up of solid particles (clay, silt and sand), with both air and water occupying the spaces in between – rather like a sponge. The amount of water stored is the balance between water entry into and losses from the soil profile and represents the water holding capacity (WHC) of the soil. Water entry or infiltration into Australian soil is adversely affected by a number of factors including water repellence which is associated with waxy organic residues that coat the surface of soil particles decreasing water infiltration, and the condition of the soil surface.
Soil Permeability
Poorly drained or waterlogged soils lack the interconnected air spaces and large pores that are essential for the movement of water and gases, and the growth of plant roots. The economics of ameliorating soil drainage depend on the individual situation, soil type, severity of compaction, land use and the management options available. A balance is required between increased drainage, and the wish to retain as much water as possible within the profile for use by crops or pastures. The general aim should be to maximise infiltration of rain (or irrigation), to achieve a well-structured profile to store as much water as possible where it is available to plant roots, but to also ensure that excess water can drain gradually through the soil.