Cropping soils of central Queensland

On this page:

• Subsurface soil properties
• Subsoil properties
• Soil fertility
• Water-holding capacity
• Soil types
• Further information

Key points

• Soils need to be able to store 120 mm of plant available moisture in the rooting zone for reliable raingrown cropping.
• Cracking soils have reduced soil water infiltration once the cracks close up.
• Self-mulching topsoils make seedbed preparation easier but can reduce establishment by poorer seed/soil contact.
• Interaction between the structural and nutritional properties of soils can influence their management.
• Cracking clays are the most common raingrown cropping soils.
• Non-cracking clay soils and non-sodic duplex soils are less commonly used for raingrown cropping.

The suitability of soils for cropping in central Queensland is determined by their surface and sub-soil properties, fertility and water holding capacity. Not all of the wide range of soils present can be cropped.

Surface soil properties

Surface soil properties affect infiltration rate, soil workability, plant emergence and establishment.

Cracking soils

These soils have a high proportion of montmorillonite clay that swells and shrinks as the soil wets and dries. When dry, cracks form in the soil and usually extend to the surface. Open downs soils are a good example of a strongly cracking clay soil.

Cracks in soil initially allow the soil to accept high rainfall rates, but as the soil wets from the bottom up, the cracks close. The initial infiltration rate when the cracks are open is greater than 25 mm/hour, however after the cracks close the infiltration rate falls to 1-2 mm/hour. Soil water storage is maximised if the cracks are undisturbed and left open. Evaporation losses from an open cracked soil are negligible.

Hardsetting and crusting soils

Raindrop impact causes the surface structure of these soils to break down, resulting in surface sealing and very low infiltration rates. Soils that are hardsetting form a crust when dry which is difficult to cultivate, produces cloddy seedbeds and restricts seedling emergence.

Self-mulching soils

Cracking clay soils and some non-cracking clay soils have what is referred to as a self-mulching topsoil. In such a soil the natural aggregates of the surface soil reform as the topsoil dries. When completely dry, the aggregates are loosely packed together and fall apart when disturbed. There is a friable surface, and little if any cultivation is required for seedbed preparation. These soils are ideally suited to zero tillage.

The size of the aggregates varies from coarse to fine self-mulch. Open downs soils have a coarser mulch than scrub soils. In coarse self-mulching soils, there are larger air spaces between the aggregates and the topsoil can dry rapidly, especially in summer. Presswheels are essential to ensure good crop establishment, particularly for those with small seeds.

Subsoil properties

Particular subsoil properties can affect plant root growth and available plant water. The two major subsoil properties that can do this are salinity and sodicity.

Salinity

Salinity causes reduced root growth due to osmotic pressure and toxicity and is largely a result of natural deep weathering. However, when water moves salt to other locations more severe problems such as dry land salting occur, often rendering land unsuitable for agriculture. A common example is the salinisation within the Emerald Irrigation Area.

There are three forms of salinity in central Queensland:

1. Seepage salting, where salting occurs when groundwater seeps at the ground surface. This is most commonly found in basaltic (open downs) areas. The occurrence of Black Tea Tree is a useful guide to seepage areas (both fresh and salt water);
2. Water-table salting, where salts are concentrated on the surface from evaporation of water from a shallow water-table. This form of salting is less commonly found in the area and is associated with alluvial soils; and
3. Soil salinity, where soils have formed a high salt concentration at depth due to weathering. This reduces water availability and root depth for dry-land agriculture. This form of salinity is common in scrub soils and is often associated with high sodicity. It is uncommon on open downs soils.

Sodicity

Sodicity refers to the amount of sodium in soils. It develops through a process whereby sodium ions build up in preference to other soil cations (particularly calcium) on the exchange complex of the soil. Increases in soil pH and deceases in calcium and magnesium usually accompany this process. Soils with these features are known as sodic soils and are relatively common in cropping country in central Queensland. A measure of soil sodicity is known as the Exchangeable Sodium Percentage or ESP.

Sodicity in soils has a strong influence on the soil structure. A high proportion of sodium within the soil can result in dispersion. Dispersion occurs when the clay particles swell strongly and separate from each other on wetting. On drying, the soil becomes dense, cloddy and without structure. This dense layer is often impermeable to water and plant roots.

Sodicity is generally dependent on the parent material from which a soil is formed and is found on older soils where there has been sufficient weathering of clay minerals to cause a dominance of sodium. Within central Queensland, sodicity occurs on older alluvial and scrub soils. Vegetation is a good indicator of the location of these soils. Brigalow, Blackbutt, False Sandalwood and Poplar Box are the main trees found on sodic soils. Most of the grazing soils in central Queensland are sodic at depth.

Soil fertility

Soil fertility refers to the ability of a soil to supply the crop with adequate nutrients for a desired productivity. This is related to both soil structural and nutritional factors. Nutritional factors that could limit plant growth include soil salinity and sodicity (at depth in Brigalow soils), organic matter content (affects soil structure and nutrient-holding ability) and nutrient deficiency (fertilisers required).

The interaction between structural and nutritional factors is often important. For example, soils at risk of waterlogging (such as clay soils) will lose nitrogen faster through gaseous loss (denitrification) whereas soils at risk of leaching (irrigated sandier soils) will lose nitrogen faster through the profile. Compacted soils restrict root growth and may reduce phosphorus uptake.

See also "A guide to crop nutrition".

Water holding capacity

Soils must be able to store at least 120 mm of plant available water within their rooting depth for reliable cropping. Rooting depth is the depth of soil to an impervious barrier such as rock or to a zone in the subsoil with a high concentration of soluble salts (greater than 0.8 dS/m) (Figure 1). and or sodic conditions (greater than 15% Exchangeable Sodium Percentage) (Figure 2). These zones may be referred to as the salt bulge or sodic bulge. Crops are unable to use the majority of water present in these conditions.

There are three groups of soils that can store at least 120 mm of available water in their rooting depth:

1. Cracking clay soils,either derived from basalt with a rooting depth of at least 60 cm, or derived from other parent materials with a rooting depth of at least 80 cm;
2. Non-cracking clay soils with a rooting depth of at least 80-100 cm;
3. Non-sodic duplex soils with a rooting depth of at least 100-120 cm.

Cracking clay soils

Cracking clay soils are the most important and widespread soils used for raingrown cropping in central Queensland. They occur on a variety of landscapes and parent materials and are suitable for cultivation provided they are deep enough, do not have well developed melonholes (gilgai) and are not prone to flooding. The shrinking and swelling nature of cracking clay soils also assist with compaction recovery.

The predominant land types are:

• open downs, brigalow or gidgee scrub; with black, brown and red cracking clay soils developed on basalt;
• brigalow and/or softwood scrub; with black, brown or grey cracking clay soils developed on other parent materials;
• coolibah or blue gum; with black cracking clay soils on river and creek flats.

Basalt Cracking Clay Soils

When compared with other clay soils, the clay soils developed on basalt have high clay contents (usually greater than 70%) and hold significantly more water with a low minimum rooting depth of 60 cm. These soils support two main vegetation types, open downs and brigalow and/or gidgee scrub.

Open downs soils

The open downs soils are the most extensive and cover large areas of land from Rolleston through to north of Clermont. Their features include a very high water holding capacity and a moderate fertility with low to medium nitrogen and phosphorous contents. They are strongly cracking and coarse self-mulching topsoils.

There are minor areas of open downs soils that occur on parent materials other than basalt, such as shales. As they have similar properties, they are included with the basalt open downs soils. The main difference is that shale soils contain significant soluble salt at depth.

Brigalow and/or Gidgee Scrub Soils
The brigalow and gidgee scrub clay soils developed on basalt are less extensive but have a very high water holding capacity. They have the same minimum rooting depth as the open downs soils, good fertility with medium to high contents of nitrogen and phosphorous and a fine self-mulching topsoil with a high organic matter content.

Brigalow grey clays
Those with strongly developed melonholes occur over large areas of central Queensland. These soils are generally not suitable for cropping because they have a shallow rooting depth due to high concentrations of soluble salts in the upper 30-50 cm of the subsoil and they are highly sodic (i.e. high in sodium). The melonholes also restrict machinery operations.

Brigalow and/or Softwood Scrub Cracking Clay Soils

The other brigalow, gidgee and softwood scrub clay soils (that are not formed on basalt) occur throughout all areas of central Queensland and are probably the most extensive cropping soils. These soils have a rooting depth defined by the depth to a sodic and/or salt bulge, usually around 80-100 cm (Figures 1 and 2), have high clay contents of 50-60%, and a high water holding capacity. They were originally fertile soils with medium concentrations of nitrogen and medium to high phosphorous. They generally have fine self-mulching topsoils.

Figure 1: Salt profile of a brigalow scrub cracking clay soil

Figure 2: Sodic conditions reduce PAWC in a brigalow scrub cracking clay in the Emerald district.

Coolibah or Blue Gum Black Clay Soils

The coolibah or blue gum black cracking clay soils are found on the floodplains of the major river systems and their tributaries in the region. Areas of flooded brigalow clay soils also occur on these floodplains. Usually these soils have high clay contents (50- 65%), high water holding capacities (sufficient for raingrown cropping) and variable rooting depth according to the location of the salt bulge. They also have high to very high phosphorous contents with low nitrogen levels and variable surface conditions (from fine self-mulching to hardsetting surfaces). Some river flats are prone to flooding which prevents or restricts cropping.

Non-cracking clay soils

These soils occupy a small area when compared with the cracking clay soils and are only of minor importance for cropping. They consist of well-structured red to brown soils with brigalow or softwood scrub vegetation and deep black, well-structured alluvial soils with blue gum vegetation. They have well drained profiles with low soluble salt contents and a deep rooting depth (with clay contents around 45-55%, a rooting depth of at least 80-100 cm is needed to store sufficient moisture for raingrown crops). They have low to medium nitrogen and medium to high phosphorous contents, depending on which landscape they occur in. They are usually firm and finely structured with self-mulching to hardsetting topsoils.

Non-sodic duplex soils

The duplex soils are those with a light textured topsoil (i.e. sand to clay loam) overlying a clay subsoil. Duplex soils suitable for cropping are also very limited in area. These soils occur mainly to the west of Moura and on some river and creek floodplains. Vegetation varies from brigalow-Dawson gum (blackbutt) scrub to eucalypt woodlands.

The features of these soils are sandy loam to clay loam surfaces overlying well structured red, brown or black subsoils which are non-sodic. They have a moderate water holding capacity requiring a rooting depth of 100-120 cm to store sufficient water for raingrown cropping. Nutritionally, they have low to medium nitrogen and medium to high phosphorous contents in the brigalow and alluvial soils in this group. They have low nitrogen and phosphorous concentrations in eucalypt woodlands with duplex soils along with hardsetting, poorly structured surface soils that are difficult to work.

Thin surfaced (less than 10 cm thick) duplex soils often develop into a cracking clay soil after a period of cultivation, as the topsoil becomes mixed with the subsoil.

Further information

• Gillespie, R.L. et al. 1991. Land Management Manual, Dawson/Callide Districts, DPI&F.
• Jacobsen, C. et al. 1992. Understanding soil ecosystem relationship (USER) kit, DPI&F CON92005.
• Dalgliesh, N. and Foale M. 1998. Soil Matters - Monitoring soil water and nutrients in dryland farming, APSRU, Toowoomba.
• Thwaites, R.N. and Maher J.M. (eds.) 1993. Understanding and Managing Soils within the Central Highlands, DPI&F Training Series QE93002.

Information compiled by Stuart Buck, DPI&F, Peter Muller, formerly DPI&F and Scott Irvine, NRMW.

The DPI&F web site (www.dpi.qld.gov.au) provides easy access to DPI&F information, products and services.

The Customer Service Centre (phone 13 25 23) provides generalist information and specialist referral services for the cost of a local call from anywhere in Queensland. The Customer Service Centre is open Monday to Friday 8 a.m. to 6 p.m. (excluding public holidays). E-mail callweb@dpi.qld.gov.au

Last reviewed April 2006.