Red, Brown and Grey Sodosols (Sandy Clay Loams/Sandy Loams over Heavy Clays)

General Description

These soils are located on steeper slopes (2-3%) as compared to similar soils on the Gently Undulating Plains unit. The soil erosion hazard is high on this soil type because of the poor surface soil structure, the presence of non-wetting sands in some places, slope and slope length. Poor management of these soils will decrease productivity locally but also have off-site impacts on other soils located in other units. Particular issues of concern include water run-off leading to erosion and flooding of soil positioned lower in the landscape. Therefore, considered management is required particularly for cropping i.e. reduced tillage, stubble retention and good crop husbandry.

Surface soil:	Shallow, generally less than 20 cm deep, sandy clay loams and sandy loams. Prone to surface crusting, structure often hardsetting with pH varying from slightly acid to slightly alkaline.
Subsoil:	Heavy clays that are sodic, strongly alkaline and generally increasing in salinity with depth. Some subsoils show high shrink/swell (vertic) properties. Abrupt texture change between the surface and subsoils.

Major Limitations

Soil Problem	Result
Surface soil crusting.	Erosion, poor plant emergence.
Hardsetting.	Erosion, poor root growth.
Sodic subsoil.	Poor root growth, water movement and aeration.
Saline subsoil.	Reduced plant growth.

Management Suggestions

Cropping on these soils presents higher risks in some years due to soil structure problems resulting in surface water runoff leading to soil erosion and waterlogging on the adjacent Grey Clay Plains and Gently Undulating Plains units. Surface water runoff also reduces the amount of water available for crop and pasture growth on this unit, therefore, leading up to a reduction in yield potential in some years.

Soil Problem	Management Suggestions
Surface soil crusting.	Stubble retention, reduced tillage, gypsum*.
Hardsetting.	Reduced tillage, gypsum*, good crop and pasture husbandry.
Sodic subsoil.	Gypsum**, deep rooted crops and pastures.
Saline subsoil.	Good crop and pasture husbandry to maximise plant water use.
Water erosion.	Maintain year round groundcover (minimum 30%), attention to the above suggestions.

* Soil tests or test strips are recommended prior to investing in gypsum.
** Requires incorporation to depth, normally not economically viable.

Soil Characteristics

Water Infiltration and Permeability
The fine sandy clay loam texture and near sodic nature of the surface soils provide the soils with the propensity to slake and disperse and the tendency to be hardsetting and form surface crusts. This results in poor water and air infiltration, waterlogging on flat country and surface water run-off on sloping country. Management practices of extensive cultivation and stubble burning have exacerbated the problem of crusting and hardsetting. Water infiltration is only moderate in surface soils but can vary depending on the degree of soil compaction.

Retaining stubble to protect the surface from raindrop impact, reducing cultivation and promoting optimal plant growth through good crop and pasture husbandry will assist in minimising surface crusting and hardsetting.

Water Holding Capacity (Soil water storage capacity)
The water holding capacity of the surface layer is moderate due to lighter soil texture and poor soil structure. The subsoil clays have a higher soil water holding capacity, however poor soil structure and chemical conditions (high pH and salinity) reduce the depth to which plant roots are able to penetrate.

Soil Salinity
Salt contents are usually low to moderate in the surface soils and high (greater than 1.0 dS/m) in the subsoils. If these soils frequently become waterlogged, salinity levels may increase at shallow depths. The subsoil salinity is likely to restrict growth of salt sensitive species (legumes) from 50 cm down the profile. Soils located lower in the landscape are likely to be at a greater risk of salinity due to rising groundwater. Appropriate catchment management is required to decrease the risk of salinity.

Soil Erosion
Water erosion is a hazard on these soils due to the rising nature of the country, long slopes and unstable surface soils. Where non-wetting sands occur, summer and autumn rains can induce significant water erosion. Stubble retention, reduced tillage, gypsum (if responsive) and maybe contour banking will reduce the frequency of soil erosion events. Deep eroded gullies are a common result of highly unstable deep subsoils.

Wind erosion is also a hazard following the removal of vegetative cover, over cultivation or over stocking when soils are in a dry state. Maintaining vegetative cover at levels greater than 30 percent year round and reducing tillage and stock trampling will reduce the hazard.

Soil pH
Surface soil pH can vary from acid to alkaline. Lower soil pH levels are associated with lighter textured soils and/or higher rainfall. Subsoil pH levels are commonly above 9.0 and this may reduce the rooting depth of plants due to chemical toxicity's, however no economic management option is available.

Fertility Status
The sandy clay surface soil layers have a moderate ability to hold nutrients and leaching is less severe. Low levels of phosphorus, calcium, potassium, sulphur, nitrogen and trace elements of copper and molybdenum have been recorded.

Surface soils are the most fertile and have the highest levels of organic matter. Soil erosion by either wind or water removes the most fertile layer and significantly reduces the site productivity potential. Reduced cultivation and stubble retention techniques will assist in preventing the loss of surface soil and nutrients through wind and water erosion. Productivity is greatly reduced with the loss of surface soil; sedimentation of dams, waterways and low lying areas may be another cause for concern.

Soil Performance Under Management

Response to Cultivation (Seedbed preparation)
In general, these soils are considered difficult to manage due to waterlogging. Repeated cultivations will destroy soil structure, promote dispersion and often results in a compacted hardsetting cultivation layer upon drying. Techniques that reduce the need for soil disturbance are best suited to these soils and the results are most noticeable following high rainfall periods. The use of chemical weed control and narrow tynes or discs is advisable.

Plant Establishment (Germination, Emergence)
The soil structure problems result in establishment problems for crops in some years. Surface crusting reduces the emergence of plants in drier years while waterlogging reduces the effective establishment of plants in wetter years.

Root Development (Limits to depth)
The sodic clay subsoils offer high resistance to root penetration and as the surface soil depth is generally shallow (less than 20 cm), severe limitations to plant root growth of annual crops and pastures occurs. It is common in these soils to find that root development is confined to the surface horizons and that the majority of roots extend along the surface of subsoil columns rather than penetrating into the deep subsoils. The shallow rooting depth of annual plants restricts the water and nutrients available to the plant thus reducing productivity. Perennial plants are more suited to these soils as root growth is not limited to one season. High pH levels and high salinity levels will restrict deep root development. In wetter years, rooting depth is often shallower than in drier years due to the easier access to moisture in the surface soil.

Chemical Residues
Agricultural chemicals such as herbicides and pesticides are made inert via four main processes; a) direct binding to soil particles particularly clays and organic matter, b) direct binding to surface organic matter such as stubbles, c) chemical hydrolysis (chemical breakdown) and d) microbial breakdown. Each of these processes is affected by soil type, soil moisture and temperature, soil pH, organic matter content and microbial activity.

Higher levels of organic matter will increase binding sites for chemicals in the soil and increase the overall microbial activity. Higher levels will also increase the breakdown of the chemicals in the soil.

Sulphanyl Urea herbicides will remain active longer in higher pH soils and will be leached with excessive rainfall. As breakdown of the chemical is by microbial activity and acid hydrolysis, little breakdown will occur by acid hydrolysis (due to the alkaline nature of the soil) and therefore microbial breakdown will become the principal form of herbicide degradation. Surface soils that have a neutral or acid pH will de-activate the chemical faster as a result of acid hydrolysis.

Imidazolinones are less soluble in alkaline soils and will not be leached to a great depth. Breakdown will be via microbial activity. In acid and neutral surface soils Imidazolinones will remain soluble and may be leached from the surface soil. The quantity of rainfall will affect plant back periods.

Triazines will be fixed onto the alkaline clays and thus more subject to microbial breakdown.

Trifluralin is fixed to the surface soil and is degraded by microbial activity. Lontril is fixed onto, and persists on, stubble material.

Management

These soils are best suited to permanent perennial grasses supported by subclover or medics. Successful rotational cropping can be achieved using reduced tillage, stubble retention and other good crop husbandry practices.

Implications for Revegetation
The surface soils are prone to hard setting and/or crusting, particularly following excessive cultivation. Direct seeding for revegetation will not benefit from the preparation of a fine seedbed. Ripping and a cultivation would suffice, and the more trash left as mulch will reduce the physical effect of raindrops breaking down the surface soil structure.

Winter and Spring waterlogging may be of concern to some species. The timing of these operations will depend on the species chosen:

• at early break (late autumn) for plants that will withstand being very wet;
• late winter/early spring for less tolerable species. This has to be weighed against the potential for surviving the long dry summer.

Current Practices
Many of these soils are currently over cultivated and are suffering from soil structure decline. Water erosion and sedimentation of low lying areas can be a major problem as well as loss of surface soil and nutrients from wind erosion.

Soils are cropped with wheat, oats, barley, field peas, chickpeas, faba beans and occasionally safflower. Winter waterlogging often reduces cropping options to wheat and oats.

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