Soil horizons in a podzol. Photo: USGA
Soil horizons
We’ve all seen soils in cross-section – either in a trench we’ve dug or in a road cutting. The most notable feature of the soil in cross-section is the bands or layers of different colours or textures. These layers are termed soil horizons.
In this first of a series of articles on the origin of soils, we explore the formation and meaning of soil horizons, which have a huge impact on what we can do with a soil. Understanding a bit about soil horizons gives us the tools to understand the climate of the region, the way the soil behaves at different times, and what limits it will impose on us.
Layer upon layer
When we examine a road cutting or dig out a soil core with an auger, the most obvious feature is changes in soil texture and colour with depth. The photograph shows what we mean.
In a typical soil formed on sedimentary shale in eastern NSW, for example, beneath the cover of plant litter we might find a dark loamy soil to a depth of about 20 cm. Beneath this would be a paler loam. Beneath that we might find a more clayey and brightly coloured layer. As we continue to go down, the bright colour fades until fragments of rock start to appear, and finally we hit solid rock.
Fundamentally, most soils, including this one, can be divided into up to five major horizons, in descending order:
O: Organic matter. Sometimes not present in degraded agricultural soils and arid-zone soils. Very pronounced in swamp soils and peaty soils.
A: Topsoil. Usually shows an accumulation of decayed organic matter (humus), which can diminish below the surface. Usually chemically favourable to plant growth.
B: Subsoil. Often shows the brightest colours and highest organic matter content. Usually chemically hostile to plant growth.
C: Often clay. Contains decomposed rock and shows less structure and development than the B above it. Usually chemically and physically hostile to plant growth.
R: Regolith, or the parent material (rock) the soil forms on and from.
These horizons can be further subdivided according to physical or chemical differences, and are labelled A1, A2, B1, B2 and so on.
Horizons as clues to soil history, uses and problems
The colour, texture and chemistry of the horizons all provide clues to the origin of the soil, a knowledge of which is vital to an understanding of the soil’s suitability for different uses.
In our example of a soil formed on shale, as the shale weathers as a result of millions of cycles of wet and dry, heat and cold, it loosens, exposing its iron to oxygen. As the iron oxidises (rusts), it changes colour from grey to reddish. Deep down, where oxygen is limited, the iron remains grey or white. A bit further up, especially in cracks and pores, the oxygen penetrates well enough to oxidise the iron, giving the deep subsoil a mottled appearance.
In the upper layers, organic matter and nutrients accumulate. The organic matter, being acidic, slowly breaks down clay minerals, aiding their downward transport during rainfall, and leaving more sand and silt in the topsoil than clay. During heavy rain or wet periods, the water can sit on top of the deeper clay, and the soil becomes anaerobic, dissolving iron and manganese. As the soil dries out again, the iron and manganese form spherical nodules: these are a sure sign of periodic waterlogging and erratic rainfall, and are an important clue to both the climate and the potential uses of the soil.
These same wetting and drying cycles cause the clayey B horizon to shrink and swell, resulting in the formation of blocks of soil called peds. Enough shrinkage and swelling cause the peds to polish each other, giving them a shiny appearance. This offers yet another clue to the climate in the area and the possible effects on plant roots.
So a basic knowledge of the different layers of the soil can offer valuable information for the farmer or developer. In future articles we will take apart all of these soil-forming factors to see how a soil is a function of its age, its position in the landscape, its parent material, the climate and living things.
Further reading
Terminology and soil classification
Northcote KH. 1979. A Factual Key to the Recognition of Australian Soils. Rellim, Adelaide.
Isbell RF. 1996. The Australian Soil Classification. CSIRO Publishing, Melbourne.
Stace HCT et al. 1972. A Handbook of Australian Soils. Rellim, Adelaide.
Soil types in Australia and the Sydney area
Northcote KH et al. 1975. A Description of Australian Soils. Wilke and Co, Melbourne.
McKenzie N et al. 2004. Australian Soils and Landscapes. CSIRO Publishing, Melbourne.
Bannerman SM, Hazelton PA. 1989. Soil Landscapes of the Penrith 1:100,000 Sheet. Soil Conservation Service of NSW, Sydney.
Hazelton PA, Tille PJ. 1990. Soil Landscapes of the Wollongong–Port Hacking 1:100,000 Sheet. Soil Conservation Service of NSW, Sydney.