Brown Sodosol (Duplex Sandy Loam and Sandy Clay loam over Clay)

Site: LS11A, LS24.

General Description

These soils are located higher in the landscape than the Uniform Grey and Brown Clays and often on long slopes up to 2 per cent. The management of these soils will have a strong influence on soils located lower in the landscape within this unit (i.e. the Grey and Brown Clays) and on the Grey Clay Plains unit. 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.

Surface soil:	Shallow sandy loam and sandy clay loam, poorly structured, slight to moderately alkaline, low salinity and non-sodic. Surface soils may be non-wetting (see water repellent).
Subsoil:	Deep poorly structured (sodic) medium to heavy clay, strongly alkaline and increasing salinity with depth. Abrupt texture change between the surface and subsoils.

Major Limitations

Soil Problem	Symptom
Surface soil crusting.	Erosion, waterlogging, poor plant emergence.
Hardsetting soil surface.	Erosion, waterlogging, poor root growth.
Non-wetting.	Water run-off, erosion.
Sodic subsoil.	Poor root growth, water movement and aeration.

Management Suggestions

Cropping on these soils presents a higher risk in some years due to soil structure problems as compared to the Grey and Brown Clays and enterprises such as pasture production. Cropping needs to be undertaken with due consideration of the affects of surface water movement to areas lower in the landscape. Reduced tillage, stubble retention and other contemporary crop husbandry practices are required to minimise impacts on areas lower in the landscape.

Soil Problem	Management Suggestion
Surface soil crusting.	Stubble retention, reduced tillage, gypsum*.
Hardsetting soil surface.	Reduced tillage, gypsum*, good crop and pasture husbandry.
Non-wetting.	Stubble retention, reduced tillage, addition of clay**.
Sodic subsoil.	Gypsum***, good crop and pasture husbandry.

* Soil tests are recommended prior to investing in gypsum.
** Addition of clay to non-wetting sands is a relatively new practice and although is likely to be beneficial, the practice may not be economically viable.
*** For best results it requires incorporation to depth, not normally economically viable.

Soil Characteristics

Water Infiltration and Permeability
Water infiltration is moderate but can vary depending on the degree of soil structure decline or the presence of non-wetting sands (see water repellent). The sodicity of the subsoil reduces the permeability of water and air and the growth of plant roots which induces waterlogging and poor plant performance. Retaining stubble and reducing cultivation will assist in increasing infiltration and minimising the affects of water erosion, particularly on sloping sites.

Water Holding Capacity (Soil water storage capacity)
As compared to the subsoil, the light textured surface soil has a moderate water holding capacity. Water holding capacity of the subsoil is good although the availability to crops and pastures is moderate due to soil structure and chemical conditions (salinity and pH) reducing the depth to which plants roots will grow.

Soil Salinity
Salt content is usually low to moderate in surface soils and high (greater than 1.0 dS/m) in 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 below the soil surface.

Soil Erosion
The degree and length of slope and the water infiltration characteristics of the surface soils makes these soils prone to water erosion, particularly when vegetation cover is less than 30 per cent. As a consequence, surface soil and subsoil erosion is common on these soils. In some areas, deeply eroded gullies have resulted. Where non-wetting sands (see water repellent) occur, the risk of erosion is high during rainfall events on dry soil. Modified management techniques such as reduced tillage, stubble retention and in some areas contour banking will assist in reducing the affect of water erosion.

Where these soils are located on land of gentler relief, the poor water infiltration characteristics and sodic subsoils increases the risk of waterlogging.

Soil pH
Surface soil pH varies from acid to alkaline (generally acid in the higher rainfall areas and/or lighter textured soils). subsoils are alkaline, commonly rising above pH 9.0. Surface soil acidity is unlikely to affect crop and pasture performance in the short term.

Fertility Status
The sandy clay surface soils have a moderate ability to hold nutrients. Low levels of phosphorus, calcium, potassium, sulphur, nitrogen, copper and molybdenum have been recorded.

Surface soils have the highest fertility. 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 following the loss of even small quantities of surface soil following erosion. Sedimentation of dams, waterways and low lying areas may be another cause for concern following erosion.

Soil Performance Under Management

Response to Cultivation (Seedbed preparation)
In general, these soils are considered difficult to manage due to poor structure resulting in erosion and waterlogging. Over the years repeated cultivations have resulted in soil structure problems such as surface crusts and hardsetting surface soils. Techniques that reduce the need for soil disturbance are best suited, with results most noticeable following periods of high rainfall. The use of chemical weed control, reduced tillage and narrow tynes or discs is advisable.

Plant Establishment (Germination, Emergence)
Crops can establish poorly on these soils due to the hardsetting condition of the surface layer. This would be particularly noticeable when dry periods follow sowing. Wet periods following sowing can result in waterlogging and a reduction in establishment. The presence of non-wetting sands reduce the infiltration of water into the soil early in the season potentially delaying sowing or seed germination.

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 top 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.

The subsoil below 50 cm depth is generally inhospitable to plant roots with high pH levels (commonly 9.0 or greater), high salinity levels (commonly greater than 1.2 dS/m) and tight swelling clays (Exchangeable Sodium Percentage greater than 15). Exploitation of these layers by plant roots is often very low.

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 are 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, fragile soil structure and non-wetting sands make cropping more risky especially in rainfall zones higher than 450 mm per year. Soils with deeper surface soils over the clay subsoil (greater than 60 cm) can be cropped occasionally, or in some cases, sown to lucerne. This type of management will minimise the affect of waterlogging and surface water flows across long gradual slopes and reduce the impact of flooding on lower areas.

Implications for Revegetation
The surface soils are prone to hardsetting and/or surface crusting, particularly following excessive cultivation. Direct seeding for revegetation will not benefit from the preparation of a fine seed bed. Ripping with one cultivation would suffice, with the trash left as mulch. Waterlogging will reduce the establishment of some species during wet years.

The timing of native vegetation 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 less tolerant of wet conditions. This has to be weighed against the potential for surviving the long dry summer.

Current Practices
Many of these soils have been over cultivated and now suffer from poor soil structure. 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 wheat and oat cropping options.