Grey, Red and Brown Vertosols (Uniform Grey and Brown Cracking Clays)

Site: LS16, LS11, LS13, LS7

General Description

Within the Grey Undulating Plains unit, the Grey and Brown Vertosols are positioned low in the landscape, the Brown Vertosols being generally higher in the landscape than the Grey Vertosols. Both the Grey and Brown Vertosols are influenced by the management of the surrounding Sodosols and can suffer from surface water flows (flooding) and increases in groundwater as a result.

Surface soil:	Shallow light medium clays and medium clays, moderate to well structured, some self mulching, strongly alkaline pH, low salinity and low sodicity. Cracks when dry and expands when wet (vertic properties).
Subsoil:	Deep medium to heavy clay, moderate to well structured closer to surface, dispersive and poorly structured at depth (sodic), strongly alkaline showing increasing salinity with depth. Cracks when dry and expands when wet (vertic properties). Diffuse texture change between the surface and subsoils.

Major Limitations

Soil Problem	Result
Surface soil structure.	Decreased water permeability, poor crop/pasture performance.
Subsoil alkaline pH.	Reduced root depth.
Position in landscape.	Flooding.

Management Suggestions

Like the soils on the Grey Clay Plains unit, these are some of the best cropping soils in the area. They are however at risk from flooding and salinity as a result of management of surrounding areas. Although these are generally good cropping soils, attention needs to be made to minimising cultivation, maintaining groundcover levels and promoting good crop growth through contemporary crop husbandry practices.

Soil Problem	Management Suggestion
Surface soil structure.	Stubble retention, reduced tillage, gypsum*.
Subsoil alkaline pH.	No economic management options.
Flooding.	Suitable management of other soil units (see recommendations associated with each).

*Soil tests are recommended prior to investing in gypsum.

Soil Characteristics

Water infiltration and Permeability
These soils have a high shrink/swell capacity (vertic properties) which alters the structure of the soil between the dry and wet phases. When dry, these friable cracking clay soils are usually well structured having good aeration and high permeability to water. Upon wetting, these clays swell reducing the total porosity which reduces aeration and water infiltration. Increased runoff from upper slopes will place greater pressure on these soils and waterlogging may occur in higher than average rainfall years. The Grey Vertosols are generally better structured than the Brown Vertosols as the latter tends to have surface soil aggregates that slake which may result in a surface crust following excessive tillage. A surface crust will reduce water and air infiltration into the soil and inhibit plant establishment.

Water Holding Capacity (Soil water storage capacity)
These clay soils have a high moisture holding capacity that enables crops to finish later in an average year as compared to a sandy soil. However, because of the heavy texture of the clay soils, increased early season rainfall is required to instigate crop and pasture growth. This becomes a problem in dry years with increased risk of crop failure or low yield.

The good structure of the soil allows roots to grow to reasonable depths and therefore exploit this moisture. The depth to which plant roots will grow in this soil will be dictated by seasonal conditions. In wetter years, root depth will be shallower than in drier years due to excess moisture. In average years, it is not uncommon to find reasonable amounts of unused available moisture at depth because roots are unable to adequately exploit lower horizons because of physical or chemical constraints such as high pH or soil salinity levels.

Soil Salinity
Most sites show increasing salinity with soil depth. The salt levels may create a chemical barrier to root exploitation. The location of these soils low in the landscape increases the risk of soil salinity through discharging saline groundwater. In addition, the low lying nature of these soils in the landscape increases the risk of flooding via overland flow from areas higher in the landscape. Catchment based actions to increase plant water use via trees and the growing of high water use crops need to be implemented to decrease this risk.

Soil Erosion
Grey Clays - Potential for water erosion is low due to the low relief. The potential for wind erosion is moderate when soil is dry, has less than 30 per cent vegetative cover, is heavily stocked or over cultivated. Maintaining greater than 30 per cent vegetation cover will significantly reduce the risk of erosion.

Brown Clays - In comparison to the Grey Clays, the potential for erosion by water of the Brown Clays is moderate due to poorer soil structure, gently sloping relief and long slopes. Poor soil structure reduces water infiltration and in combination with sloping country and long slopes, causes runoff which has the ability to erode soil materials. Water running off the Brown Clays will flow to the Grey Clays exacerbating flooding on the Grey Clays.

Wind erosion is common on dry fine surface soils and is exacerbated by over stocking, cultivation and removal of groundcover. Reduced tillage and the retention of stubble or pasture at levels greater than 30 per cent will minimise the effects of wind and water erosion.

Soil pH
Soil is alkaline throughout and increases with depth. Although productive agriculture has an acidifying effect on soil, the high soil pH will provide a long term buffer to the effects of acidification.

Fertility Status
Grey Clays - These soils have moderate to high fertility and have a high capacity to retain nutrients. However, nutrient replacement through fertiliser is required to maintain productivity. Reduced cultivation and increased vegetation cover will enhance fertility through the build-up of organic matter. Recorded low levels of phosphorus, nitrogen and zinc (trace elements) need constant monitoring particularly under cropping. The high pH may restrict the availability of some elements to plants, particularly iron, zinc and copper.

Brown Clays - Moderate fertility and a good capacity to retain nutrients against leaching is a characteristic of these soils. Reduced cultivation and increased vegetation cover will enhance fertility through the build up of organic matter. Recorded low levels of phosphorus, nitrogen and trace element zinc need constant monitoring. High amounts of lime in some soils may restrict availability of some elements to plants, especially zinc, iron and copper.

Soil Performance Under Management

Response to Cultivation (Seed preparation)
Grey Clays - In the dry to moist stage, most of these clays are self mulching therefore being friable and easy to work. They become very sticky when wet. Non-friable clays occur in some depressions and low lying areas.

These soils have been extensively cropped over the years and have under gone significant cultivation. Soil structure has declined as a result. Reduced tillage techniques will improve soil structure and enhance root development, particularly early in the growing season which is important in optimising yield. Improved soil structure will also enable higher water infiltration and therefore reduce runoff.

Brown Clays - The surface soils can vary from friable (crumbly) to non-friable (hard) when dry. Both surfaces should not be cultivated when wet (very sticky) as the surface sets hard and often produces large clods that are hard to break down. Reduced tillage techniques are preferred to maintain soil structure.

Plant Establishment (Germination, Emergence)
Crop establishment is dependent on whether the surface soil is friable (crumbly) or non-friable (hardsetting). Non-friable or hardsetting soils have suffered soil structure decline because of excessive cultivation or over stocking. Soil structure decline has reduced water infiltration, germination and emergence. Retention of stubble (greater than 30 per cent) will protect the soil surface aggregates from raindrop impact and therefore decrease slaking and dispersion and the formation of a surface crust.

Root Development (Limits to depth)
On these clays, deep root exploitation is often dependent on climatic conditions and the friability of subsoils. The sodic nature of many of these subsoils results in poor structure which restricts root growth as well as reducing moisture and air movement. High pH levels and salinity levels will restrict deep root development. In wetter years, rooting depth is often shallower than during drier years due also to the easier access to moisture by plant roots in the surface horizons.

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 alkaline 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.

Imidazolinones are less soluble in alkaline soils and will not be leached to a great depth. Breakdown will be via microbial activity. 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
Grey Clays - In average rainfall years, these are generally good cropping soils. The surface soil is self-mulching (friable) and subsoils are not excessively saline. Due to the location in the landscape, the risk of crop loss through waterlogging or flooding in years of above average rainfall is increased. Management of other areas in the catchment that minimises water run-off needs to be considered for longer term production from these soils.

Soil structure is improved by maintaining vegetation cover and increasing organic matter. The fragile structure of these soils can be destroyed by excessive cultivation or poor timing of operations. Cultivation of these soils when either too wet or too dry will destroy structure, increasing the soils tendency to disperse and crust. Cultivation also speeds up the degradation process of organic matter in the soil.

Brown Clays - These soils often suffer the pressures of continuous cropping. Severe soil structure decline occurs, restricting production and increasing the risks associated with water erosion. These soils would benefit from a longer pasture phase and modifying cultivation techniques to allow soil structure to be maintained or improved. To improve soil structure, the same methods as recommended for the Grey Clays are utilised.

Implications for Revegetation
Grey Clays - The good structure of the surface soils will allow good seedling emergence. They are usually free draining and are not likely to be prone to waterlogging. Revegetation, either by direct seeding or transplanting container plants, will achieve best results after the autumn break. This will ensure good root development over the following spring and an increased chance of survival during summer.

Brown Clays - Good moisture holding capacity of these soils will allow autumn and spring establishment. Soils with shallow surface soil (less than 10 cm) will be prone to waterlogging and would be best established in spring. Direct seeding activities should be drilled into an existing mulch cover to prevent crusting of the soil surface.

Current Practices
Grey Clays - These soils are intensively cropped in rotations, often fallowed and sown to wheat, followed by barley or sometimes oats. Legume crops such as field peas, faba beans and chickpeas are used as a disease break and to supplement nitrogen. Other crops such as canola, safflower and sunflowers are also grown.

Where livestock (mainly sheep) are included as a farming enterprise, legume crops may be replaced by medic clovers for grazing. Lucerne can be grown instead of medic clovers providing higher and more regular production and the benefit that lucerne will often responds to summer and autumn rainfall which falls outside the growing season for medics.

Brown Clays - These soils are often continuously cropped with wheat, barley, oats, canola, faba beans, chickpeas and field peas. They are not suitable for lupins, sunflower or safflower.