Our Soil Vulnerabilities

The following key vulnerabilities to the soil resources of the NAR have been identified:

Dryland Salinisation

Most dryland salinity is caused by an altered water balance, which, at some time after clearing (decades to centuries) will reach a new equilibrium and the area of land affected by salinity will cease to expand. Within the NAR region, a recent assessment by the Department of Agriculture and Food WA suggests that land on the Dandaragan Plateau and the east Binnu sandplain are at greatest risk of the expansion of dryland salinity. This is largely due to increasing groundwater levels in these regions and medium to long term time frames for these areas to reach equilibrium. The coastal areas of the NAR are considered to be at low risk of expansion of dryland salinity, whilst the risk in the Yarra Yarra catchment is moderate, where the expansion of salinity has slowed (DAFWA, 2013).

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Fertility Decline

Most soils of Western Australia have low fertility as they are ancient and highly weathered. They have low organic matter (frequently less than 1%) and are inherently low in all nutrients, with phosphorus, nitrogen and trace elements such as copper and zinc being historically of most significance. However, continual removal of potassium and sulphur in agricultural produce, and the use of fertilisers low in these nutrients, has resulted in wider problems. Losses from leaching or surface water run-off and soils that have a low capacity to retain anions can cause substantial losses of nutrients, especially nitrate. Loss of nutrient availability can also occur for some nutrients by fixation into organic matter and reactions with soil minerals.

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Wind Erosion

With frequent high winds, wind erosion threatens most unprotected land in the NAR. Almost half of all the soils in the region have a high to extreme risk of being affected by wind erosion, with 10% of these soils classified as having a very high to extreme risk. Wind erosion occurs on all soils, but the potential risk is highest in the Greenough and West Midlands sub-regions where extensive sandy soils occur and crop stubbles are often light. Risk factors including the type of soil, the degree of disturbance (eg trampled or cultivated), amount of ground cover (>50% ground cover significantly reduces risk) and landscape position. The climate is also an important influence – reduced rainfall may expose whole landscapes to erosion. The severity of wind erosion will depend on the soil type. Even heavier-textured hard-setting soils to the east and south of the region will blow if they are over-cultivated and left exposed for long periods. Significant erosion will be minimised are protected by >50% groundcover.

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Water Erosion

Water erosion is caused by heavy rainfall (such as summer thunderstorms in this region) impacting on unprotected land where there is insufficient drainage or cover to absorb or channel the water appropriately. Salinity and waterlogging exacerbate the problem. Gullies, sedimentation of waterways, flooding and increased salinity, are all consequences of water erosion. Since erosion is often associated with natural drainage systems, the many tons of soil that are detached and transported into the waterways goes largely unnoticed. In the NAR, water erosion is not a widespread problem, but in some areas, such as the Nangetty area near Mingenew, the Moresby Range near Geraldton, and steeper sloping land to the south of the region it is more significant. Localised erosion also occurs throughout the region, particularly in conventionally-cultivated hilly areas, or on land where heavy summer grazing or animal tracks concentrate water. Critical periods for water erosion risk are break of season and summer storms when land cover is likely to be lowest.

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Structure Decline

A decline in soil structure occurs when the relationships between soil particles, organic matter and soil pores change. Pores decrease in size and the movement of air, water and nutrients is limited. The main causes are excessive cultivation, large raindrops, stock movement on wet soils,  the loss of organic matter and burning to reduce stubble. In addition, in medium and fine textured surface soils (clay 10-35%), the wetting and drying of clay may cause soil structure decline in the formation of hard crusts. In the NAR, a third of all soils are susceptible to developing soil structure decline. The most extensive areas of susceptible soils are in the Yarra Yarra sub-region due to the high clay and sodium content, but in the Greenough sub-region there are also soils with naturally poor soil structure that are exacerbated by cultivation practices or stock movement.

Soil Compaction

Soil compaction is due to compression from a vertical force such as cultivating machinery or livestock and by shearing and smearing from horizontal forces such as spinning and slipping of machinery wheels. This causes soil aggregates to break and smaller silt and clay particles to be carried through the profile until they are trapped creating an impermeable hard pan (DAWA, 2004). The typical ‘traffic hard-pan’ caused by heavy machinery is more common on sandy soils and usually 10-40 cm below the soil surface. It has been estimated that over a third of soils in the region are susceptible to soil compaction, particularly in the Greenough and Yarra Yarra sub-regions where sandy soils and uniform coarse textured soils dominate.

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Soil Acidity

Soil acidity is a widespread problem in the NAR, particularly on the sandy soils. It is a natural process that occurs during weathering, such as the acidic wodjil soils in the east of the NAR, but it can be accelerated by some farming practices. These include the addition of acidic fertilisers, alkali product removal and the leaching of nitrate. Soil acidity is most prevalent in sandy soils with a low capacity to buffer pH change. Addition of lime is required to maintain soil pH at optimum levels.

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Non Wetting Soils

Non wetting or water repellence is widespread on soils with low clay content or high organic matter. Hydrophobic materials including fungal hyphae, waxes and other organic matter, consist of long hydrocarbon chains which are chemically water repellent. In sandy soils these hydrophobic materials coat soil particles and soil aggregates making the soil water-repellent, reducing infiltration, (especially early in the growing season) and increasing runoff. This problem appears to be worsening due to minimum tillage practices, early sowing and reduced break of season rainfall (DAFWA 2013). Over 40% of soils susceptible to non-wetting in the NAR are found in the West Midlands sub-region, because of the predominance of sandy duplex and deep sand profiles.

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Waterlogging is most significant in areas of low relief that receive more than 400 mm of annual rainfall and in areas with soil profiles like sand or loam over clay, that have a permeable A horizon, that overlie an impermeable or slowly permeable B horizon. Conditions are accelerated when the clay subsoil is closer to the surface. Land with a high to very high waterlogging risk is minimal in the NAR. Land with a moderate or high risk of waterlogging is more pronounced in the Moore and Yarra Yarra sub-regions and some areas of the Greenough sub region, due to broad valley floors and susceptible soils in these areas. However, many soils are affected by subsoil waterlogging which is not readily visible and so the true extent of waterlogging is often underestimated. Satellite imagery can be used to assess the extent of waterlogging and DAFWA has undertaken risk mapping of this issue.

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