Grey and Yellow Sodosols (sandy surfaces)

Sites LS21

General Description

Within the Undulating Low Hills unit, these soils are positioned lower in the landscape than Deep Duplex Sands over Clays (Grey Sodosol) and occur part way down a ridge. 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 of 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.

Surface soil:	Shallow (less than 20 cm) loamy sands and sands, weakly structured, slightly acid.
Subsoil:	Clay, sodic and dispersive, increasing salinity with depth, strongly alkaline. Abrupt change in soil texture between the surface and subsoils (duplex).

Major Limitations

Soil Problem	Result
Weakly structured surface soil.	Wind erosion.
Sandy texture.	1) Low water holding capacity but responsive to light rains.
	2) 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

The low water and nutrient holding capacity and risk of wind erosion makes this soil type undesirable for cropping. The cost of inputs to grow a crop compared to the returns is high as is the risk of achieving a satisfactory crop. Permanent perennial pastures or native vegetation that provide wind protection to other soils is recommended.

Soil Problem	Management Suggestions.
Weakly structured surface soil.	Continuous groundcover i.e. stubble and 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.

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 as a result may result in rill erosion. When wet, the sandy textured surface soils are highly permeable but deeper infiltration can be restricted by hardpans and sodic subsoils. Site LS21 has a very dispersive band of clay over the subsoil which will greatly restrict root and moisture penetration.

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 low clay content allows the crops and pastures access to moisture following light rains. That is, the surface soil does hold as much water however less water is required to instigate growth. As with most of the other soils, subsoil clays have moderate to high field moisture capacity. However, because of the clay texture and structure 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
Salt contents are usually very low in the sandy surface soil because of leaching but are higher in the subsoils. 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 their position higher in the landscape, considerably raises the risk to 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 extra nutrients and water. The risk of erosion is increased by the absence of adequate groundcover (minimum of 30% 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 increasingly alkaline with depth. Some nutrients (e.g. phosphorous, 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 surface soil nutrient holding capacity. Low levels of phosphorus, calcium, potassium, sulphur, nitrogen and trace elements of 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
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 thereby improving germination and establishment rates. These soils are easy to work and offer very low draft resistance to machinery. There are no compaction problems. 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 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 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 in addition to increasing the growing period for pastures. The re-establishment of native vegetation for stock and crop wind protection (Shelter belts) is recommended for very light textured soils and exposed areas. Although soils are capable of being cropped with wheat, oats, barley or lupins utilising stubble retention and minimum tillage or preferable direct drilling. Cropping is not recommended due to the high risk of crop failure and soil erosion. 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 planting’s are prone to the summer dry periods and may not enable adequate establishment. Most soils of this nature are free draining and 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 wheat, barley and lupins.