A few weeks ago you may have gone out and grabbed some soil to have a play with. There’s a good chance that the properties of your particular sample — including its colour, texture, mineral content, structure, and chemistry — will be quite different to one a kilometre or more away. You may even have very different soils from one end of a property to another.
Soil properties do vary enough from region to region such that vast soil maps can be created. And just as soils vary significantly in their properties horizontally across the land, so too do their properties vary significantly vertically.
Dig down, down and down, and you’ll discover that soils are made up of horizons, or layers, that extend vertically right to the bedrock. (Please note that horizons are evident in soils which are undisturbed and in their natural state. Horizons, especially the upper ones, are destroyed in cultivated soil.)
There are five main soil horizons as shown in the following diagram: O, A, B, C and R. Together they make the soil profile.
The O (organic) horizon is the topmost layer. It comprises organic matter at different stages of decomposition from leaf fragments to highly decomposed humus. Its depth can range from non-existent in deserts to 50 mm or more in rainforests.
The next layer down is the A horizon, or the surface soil. ‘Topsoil’ is another word for this layer and may be no more than 100-200 mm thick. This is the most fertile horizon as it contains the highest amount of organic matter and microbial and animal life. It is usually the darkest horizon because of that organic matter, and plant roots are numerous here.
The B horizon is the subsoil, and could range from tens of centimetres to several metres thick. It isn’t as fertile or as rich in soil life as the A horizon but it is still recognisable as soil with a structure, though different to that of the A horizon above it. Plant roots extend well into the subsoil. The B horizon has much less organic matter and is typically lighter in colour than the A horizon as a result. Its colour is often red to brown owing to the iron oxides present. This horizon is actually where iron oxides and other minerals accumulate if transported from the surface by water, and this tendency to concentrate in the B horizon defines the colour enough that this layer is readily distinguishable from the others.
The C horizon is the substratum, comprised mostly of fragmented bedrock with clay minerals. Very little soil formation occurs here. (What Is Soil? briefly covers how soil is formed.) It can range from quite shallow to quite deep within the soil profile.
The R horizon is the hard bedrock — it would be called an outcrop if it were exposed. While the C horizon contains fragmented bedrock that can be removed by hand (perhaps with effort), the bedrock in the R horizon is a continuous solid mass that would require a lot of mechanical force to break up and remove.
This is a very general overview of the soil horizons within a profile. Some soils don’t even have all five horizons. In many soils the horizons don’t stack in neat cake-like layers, but rather have ‘tongues’ of one protruding deep into another. The thickness and composition of individual horizons along a piece of land will often be influenced by the slope of the land. The top of a hill may be exposed bedrock, for example, due to water runoff and erosion, with a very thick A and B horizon collected at the bottom because of that water runoff.
In many soils the horizons (A and B especially) are categorised further into sub-horizons. Still other soil profiles have other horizons not mentioned here, like the P (peat) and D horizon classifications in Australia and the E and I (ice) classifications used elsewhere. Some horizons aren’t distinctly one or another but have the combined properties of two and are called ‘transitional’. For example, an AB horizon (where the characteristics of an A horizon dominate) or a BA horizon (where the characteristics of a B horizon dominate).
But these finer details are of more use to soil scientists and civil engineers. For us as growers of trees, it’s enough to be aware that an undisturbed soil profile will more often than not consist of O, A, B, C and R horizons, and to plan accordingly. Now (autumn) is a great time to get out and dig holes in readiness for tree arrivals. If you’re unfortunate to strike a shallow C horizon on a slope, consider relocating the hole further down (but not where water is likely to pool) — even a short distance away things can be very different. Or perhaps with this knowledge of horizons, you’re eyeing an existing well-cultivated garden bed instead, knowing that what would otherwise have been distinct O, A and B horizons in it were long ago churned into a deep, fertile, open blend of all three, with the C and R layers too deep down to know or care about.
In the next post we’ll zoom in a little more and look at the different types of soil ped: the individual, natural soil aggregates found in the soil horizons (or that garden bed churn!).
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