Red Sodosol (Duplex sandy clay loam over clay)

General Description

These soils are positioned higher in the landscape and on slopes of up to 2% as compared to the Uniform Grey and Brown Clays of the Gently Undulating Plains unit. The management of these soils will have a strong influence on soils positioned lower in the landscape within this unit (i.e. the Grey and Brown Clays) and on the Grey Clay Plains. Issues of concern are surface water runoff resulting in flooding of lower areas, associated soil erosion and increases in groundwater as a result of poor plant water use.

Soil Characteristics

Water Infiltration and Permeability

The fine sandy clay loam texture and near sodic nature of the surface soils provide the soil with the propensity to slake and disperse and thus tend 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 that include extensive cultivation and stubble burning exacerbate this problem. Water infiltration is only moderate in surface soils but can vary depending on the degree of soil compaction.
The poor structure of the subsoil as a result of sodicity reduces vertical water movement from the surface soil into the subsoil. On sloping country, lateral flow of water along the surface of the subsoil may occur reducing the volume of water to be utilised by plants growing on this soil type. The lateral flow of water along the surface of the subsoil increases the risk of tunnel erosion.

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 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 undulating nature of the country, long slopes and unstable surface soils. 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 subsoils that allow for the lateral flow of water along the surface of the subsoil.

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% yearly and reducing tillage and stock trampling will reduce the hazard.

Soil pH

Surface soil pH can vary from acid to alkaline. Soils with lower pH levels are associated with lighter textured soils and/or higher rainfall areas. Subsoil pH levels are commonly above 9.0. Acid surface soils are unlikely to affect crop and pasture productivity in the short term.

Fertility Status

The sandy clay surface soil layers have a moderate ability to hold nutrients. Low levels of phosphorus, calcium, potassium, sulphur, nitrogen and trace elements 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 following erosion. Sedimentation of dams, waterways and low lying areas also occurs as a result of erosion.

Soil Performance Under Management

Response to Cultivation (Seedbed preparation)

These soils are generally considered difficult to manage due to soil structure problems which induce waterlogging. Repeat cultivations will destroy soil structure and promote dispersion, resulting 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 ease of access to moisture in the surface soils.

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 (check the label). 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. Winter waterlogging and fragile soil structure make cropping difficult and increase the risk of erosion. Soils with deeper profiles to clay (greater than 60 cm) can be cropped occasionally or, in some cases, sown to lucerne.

Implications for Revegetation

Surface soils are prone to hardsetting and/or crusting, particularly following excessive cultivation. Direct seeding for revegetation will not benefit from the preparation of a fine seed bed. Ripping and a cultivation would suffice and the 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 species that are less tolerant of wet conditions. This has to be weighed against the potential for surviving the long dry summer.

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 the 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 for wheat and oat cropping.