Red Sodosol (Sandy Surface over Clay)

General Description

These soils are located on steeper slopes (2-3%) as compared to similar soils on the Gently Undulating Plains unit. The water erosion hazard is high on this soil type because of the presence of non-wetting sands, slope and slope length. The wind erosion hazard is also high because on the structureless sandy surface soil. 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:	Poorly structured loamy sand and sand, slightly alkaline, low fertility, low salinity.
Subsoil:	Poorly structured, dispersive (sodic) medium clay, strongly alkaline increasing in salinity with depth. Abrupt change in soil texture between the surface and subsoils.

Major Limitations

Soil Problem	Result
Weakly structured surface soil.	Wind erosion.
Sandy texture.	Low water holding capacity but responsive to light rains.
Sandy texture.	Low nutrient holding capacity.
Non-wetting sand.	Poor infiltration of water on dry soils.
Depth of surface soil.	Reduces water and nutrient holding capacity.

Management Suggestions

Cropping on these soils presents higher risks in some years due to soil structure problems resulting in surface water runoff resulting in soil erosion, 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 and therefore a reduction in yield potential in some years.

Soil Problem	Management Suggestions
Weakly structured surface soil.	Continuous groundcover, i.e. stubble & pasture retention, reduced tillage.
Water holding capacity.	Improve organic matter via above suggestion.
Nutrient holding capacity.	Improve organic matter via above suggestion.
Non-wetting.	Groundcover, addition of clay*.
Depth of surface soil.	Prevent erosion via above suggestions.

* 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.

Soil Characteristics

Water Infiltration and Permeability
Infiltration on these soils is generally high but can be restricted by the non-wetting sands, that reduce water infiltration into dry soils. Water run-off causing rill erosion may result. When wet, the sandy textured surface soils are highly permeable but deeper infiltration can be restricted by hardpans and the poorly structured sodic subsoils. Lateral flow of water along the surface of the subsoil on sloping soils will reduce the volume of water able to be utilised by crops and pastures on this soil. This can lead to adverse impacts due to waterlogging further down slope.

Water Holding Capacity (Soil water storage capacity)
Surface soils have a low clay content and, as a result, the capacity to hold and store moisture is low compared to a clay soil. This property allows the soil to release moisture to crops and pastures easily following light rains. That is, the surface soil holds as much water, however less water is required to instigate growth. As with most of the other soils, subsoil clays have moderate water storage capacity. However, because of the clay texture and structure of the clay much of the water remains unavailable to plants.

Retaining vegetation cover and minimising soil disturbance will assist in improving soil structure and increasing the water holding ability of the surface and subsoils. Deep rooted crops and pasture will assist in opening up the clay subsoil.

Soil Salinity
Soil salt levels are usually very low in the sandy surface soil because of leaching but are higher in the subsoils. Salinity levels at 40cm are relatively low (the deepest recorded), but it is presumed that salinity continues to increase with depth. These soils are located high in the landscape and are unlikely to be at risk of salinisation, however unused water in this part of the landscape is likely to contribute to salinity problems lower in the landscape. Increasing water use through good crop and pasture husbandry, the use of deep rooted pastures or re-establishing native vegetation will assist in reducing the risk of salinity in lower areas.

Soil Erosion
The weakly structured or friable nature of these sands, together with the position higher in the landscape, considerably increases the risk of wind and water erosion. Any soil lost through wind or water erosion removes nutrients and decreases the depth of soil from which plants can effectively access nutrients and water. The risk of erosion is increased by the absence of adequate groundcover (minimum of 30 percent cover year round) due to fire, cultivation, or more commonly, over-grazing. Careful management of these soils, considering the influence of landform and the impact of management practices further down the catchment, is important in these high risk areas.

Soil pH
Surface-soils are neutral to slightly alkaline but show increasing alkalinity with depth. Some nutrients (e.g. phosphorus, iron, manganese and zinc) may not be fully available to plants. No extra management options are required to correct pH.

Fertility Status
The sandy loam layers have a poor ability to hold nutrients, as compared to a clay and leaching by water can be severe. Increasing organic matter in the soil assists the surfacesoil nutrient holding capacity. Low levels of phosphorus, calcium, potassium, sulphur, nitrogen and trace elements, copper, zinc, molybdenum and cobalt have been recorded. Retaining vegetation cover and reducing cultivation will assist in maintaining and building on existing levels of organic matter.

Soil Performance Under Management

Response to Cultivation (Seed preparation)
Cultivation is not recommended as these soils are prone to extreme wind erosion. The friable nature of the surface soils makes them well suited to direct drilling, although rolling may be required to provide a firm seedbed and conserve moisture whereby improving germination and establishment rates. These soils are easy to work and offer very low draft resistance to machinery. The surface soil remains soft after wetting-drying cycles and are non-sticky when wet. These soils are prone to erosion and techniques that reduce the need for soil disturbances, such as direct drilling are best suited.

Plant establishment (Germination, Emergence)
The sandy friable condition of the surface soil assists in crop establishment. High moisture loss is possible with cultivation (including the sowing operation) and compaction may be needed to provide a firm seedbed.

Root Development (Limits to depth)
The sandy soils allow for good root development in the surface soil. The poor fertility may reduce plant size. Roots will penetrate to depth seeking moisture and nutrients, however root systems are often impaired due to poorly structured clay subsoils. The depth to subsoil clay varies between 10 cm and 30 cm and shallower surface soils will affect overall plant performance. Preventing soil erosion is critical to maintaining surface soil depth.

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

These soils are best suited to plants that provide vegetation cover throughout the whole year as wind and water erosion can be severe. Permanent perennial grasses supported by subclovers and/or lucerne provide good groundcover and extend the grazing period because of the plants deep root system. The re-establishment of native vegetation for stock and crop wind protection (shelter belts) is also recommended on very light soils or ridges.

Soils are capable of being cropped with wheat, oats, barley or lupins with extreme care utilising stubble retention and minimum tillage or preferable direct drilling. Currently, these soils support many low grade clover stands with annual grasses due to the high cost involved in establishment and maintaining high pasture production. The incorporation of clay into these light sandy surface soils has increased the pasture establishment options for many landowners and reduced the non-wetting nature of the surface soils.

Implications for Revegetation
Revegetation activities should be undertaken in the autumn to capitalise on the moisture available. Spring plantings are prone to the summer dry periods and may not enable adequate establishment. Most soils of this nature are free draining, therefore water-logging will not be a problem. Areas with shallow sands over clay will be affected by water-logging. Suitable species will need to be utilised.

Adequate soil cover should be retained to reduce moisture loss and prevent erosion.

Current Practices
These soils are currently sown to perennial pasture such as sub-clover and lucerne, and are occasionally cropped with oats, wheat, barley and lupins. Prohibitively high pasture establishment costs limit the extensive establishment of improved pasture on this soil type.