IYS Soil of the Month - August
‘Sand over clay’ soil (sandy Red–Brown Chromosol)
During the 2015 International Year of Soils, each month we ask a soil researcher to select his or her favourite soil. James Hall is the Vice President of the South Australian branch of Soil Science Australia and a Director of Juliet Creek Consulting (email@example.com) .
If you had to choose a special Australian soil type what would it be?
There are many special soils that are special for a range of different reasons. The one I have chosen to write about here is commonly called a ‘sand over clay’.
The term ‘sand over clay‘ can cover a range of soil types, that mostly differ owing to the thickness and nature of the sandy topsoil, the nature of the subsoil ‘clay’, and the amounts of and depth to soft and hard carbonate (where present).
My focus here is the sand over clay soils of the Murray Mallee farming region in South Australia – and those in particular that have a clay loamy to light clayey subsoil texture. In South Australia, sand over clay soils are categorised into five soil types (Hall et al. 2009), with those having clay loamy to light clayey subsoils forming types G1 and G2. The latter possesses a bleached subsurface layer.
Sand over clay soils (Soil Group G) are the fifth most extensive of the fifteen Soil Groups defined for non-arid South Australia – occupying 1.4 million ha or almost 9% of land area. In the Murray Mallee Biophysical Region they cover 240,000 ha or just over 8% of land area. Within Soil Group G, G1 soils cover 79,000 ha of the Murray Mallee Biophysical Region (2.7%), while G2 soils cover 57,000 ha (2%) – but a much higher proportion of actual farmed land. (See Hall et al. 2009).
What are the main properties of this soil type?
These soils – which are mostly of aeolian origin – occur in a range of landscapes and landscape positions. In the Murray Mallee they are associated with dune–swale landscapes, and mainly occur on low to moderate height sand dunes, as well as in swales and flats. They are mostly a sub-dominant soil in the Murray Mallee, in association with deep siliceous sands, calcareous loams, shallow soil on calcrete, and other sand over clay soils. They are, nonetheless, usually the most productive soils in these moderate to low rainfall farming areas – and as such are highly valued.
Interestingly, the unbleached G1 soil type forms an unusually consistent linear feature on Murray River valley slopes – occurring along its entire length in South Australia and on both sides of the river. Also of interest is the presence of clay lamellae (thin bands of heavier texture within a sandy matrix) in the topsoils of bleached G2 types. These are likely relict from discrete ‘dust storm’ events over past millennia – and indicate how many subsoils may have formed.
These texture-contrast profiles typically comprise thick to very thick sandy topsoils overlying sandy clay loam to sandy light clay subsoils. The subsurface layer of the topsoil is often bleached in the moderate rainfall areas of the Murray Mallee, but unbleached in lower rainfall areas. Soil pH usually ranges from slightly acidic to alkaline in the topsoil, and alkaline to strongly alkaline in the subsoil. An accumulation of fine and sometimes hard carbonate is usually present in the middle to lower subsoil. Mid to lower subsoils are commonly dispersive and sodic, however, upper subsoils are generally not.
Why do you find this soil type particularly interesting?
The sandy topsoils have limited capacity to store and retain nutrients, so consequently nutrient deficiencies are common. Subsoils, however, have a greater capacity to retain nutrients – but levels are typically low.
Drainage within the topsoil is rapid, but is then slowed by the more clayey subsoil. Nevertheless, seasonal waterlogging within the profile is rare, given the moderate drainage potential of the subsoil. In contrast, sand over clay soils with heavier textured subsoils are typically affected by internal seasonal waterlogging owing to low subsoil permeability. The additional drainage capacity of the soils in question also means that toxic accumulations of substances (e.g. boron and sodium) predominantly occur below 1 m deep. Strong alkalinity (pHwater > 9.2) also usually occurs below 1 m for similar reasons. Therefore, there are usually no significant chemical or physical barriers to root growth within the top 1 m (which is predominantly not the case with sand over clay soils that have heavier textured subsoils).
Do the properties of this soil type have consequences for its management, e.g. in terms of land use, soil quality, conservation, …
The number one priority when farming these soils is to maintain sufficient vegetative cover to prevent wind erosion. Minimum and no-till farming – made possible by modern machinery and herbicides – mean that many areas no longer experience significant wind erosion.
Fertility management is also important, with management of the major elements phosphorus, nitrogen and the minor elements zinc, manganese and copper particularly important. Sulfur and potassium can also be deficient; while even boron can be deficient within sandy topsoils.
Many of these soils are now continuously cropped, with sheep no longer part of the farming system (unless as short term lodgers on stubble paddocks). Predominant crops are wheat and barley, with lupins and canola also grown. Low rainfall limits the rotation options that dryland farmers have. Field peas are generally not grown as their weak and poorly anchored stubbles present a soil erosion risk after harvest. Where pastures are utilised, volunteer grasses and medics are grown.
These soils are favourable to farming in this environment because of their specific profile features. Most of the light and showery rainfall that is common in these districts enters the sandy topsoil and is available for extraction by crop and pasture plants – crop utilisation of incident seasonal rainfall is therefore high. Also, water loss as drainage below the rootzone is limited by the presence of ‘clayey’ subsoils. Nutrient leaching below the rootzone is limited for similar reasons.
The practice of ‘clay spreading’ on these soils is now relatively common – which improves waterholding and nutrient retention capacity of the topsoil, together with potential to increase organic carbon levels. Subsoil clays are often too deep for ‘spading’ or ‘delving’ techniques, however.
Can you tell us your most memorable story concerning this soil type?
Sand over clay soils have facilitated productive dryland farming in the Murray Mallee for over a century. The northern Murray Mallee is one of the lowest rainfall but successful cropping regions in the world (250–300 mm mean annual rainfall) – largely owing to the properties of sand over clay soils. Crops yields on these soils have increased markedly over recent decades; in contrast, yields on adjacent deep infertile dune sands have remained largely unchanged for forty years.
In recent years, numerous low-lying areas of sand over clay soil in the Murray Mallee have become too wet to crop. What were highly productive flats and swales only a decade ago, are now semi-arable to non-arable owing to year-round soil saturation – a phenomenon known as ‘mallee dune seepage’. This is quite remarkable in the driest state in the driest habitable continent on earth, and in one of the lowest rainfall cropping regions there is! It is thought that continuous cropping along with control of summer weeds on sand dunes – thanks to modern herbicides – has led to an excess of localised sub-catchment waters (the regional groundwater is below 50 m deep). Investigations into this phenomenon by Natural Resources Murray-Darling Basin South Australia are underway. The recent manifestation of mallee dune seepage areas highlights how finely balanced many agricultural land use systems truly are.
Hall, JAS, Maschmedt DJ, Billing NB (2009). The Soils of Southern South Australia. The South Australian Land and Soil Book Series, Vol 1. Geological Survey of South Australia, Bulletin 56, Vol 1. Soil and Land Program, Government of South Australia.