Soil is quite probably the resource that is most taken for granted by modern society. How many people who aren’t farmers or gardeners really think about soil and how important it is to the human race? Soil provides the nutrients for crops to grow, it’s the building block of forests, it provides a home for countless animals and micro organisms. It even has a part to play in regulating the climate. Soil is an invaluable resource that provides invaluable services and if we want to preserve it, we need to know where it comes from and how to make more of it.
When I was an undergraduate environmental scientist, I was taught the following equation as a way to visualise the many things that contribute to the formation of soil and determines what type of soil you get. I like the simplicity of this little function, so have a look at it and I’ll introduce all of its terms.
Soil is a function of:
- Parent Material
Of these, I will begin with Parent Material. The parent material is the surface from which the soil will be formed. Soil doesn’t come out of nowhere and all soil has a ‘mineral component’ even if this is very small in the case of peat (will come back to these later). Parent materials are usually rocks, though in a few cases they could be animal or man made structures such as shells or concrete. The type of parent material is highly important in determining what the soil is going to be like. For example soil that forms from a red sandstone is likely to be red and gritty whereas soil that forms from a mudstone is likely to be dark and clammy. The sandstone soil probably drains better than the mudstone soil but bear in mind that the soil you get is a function of all of the contributing factors not just one or two.
So how much soil do you get for your parent material and how do you get it? Climate is an important control on the rate of the soil formation, Soil from rock is usually formed by weathering. Weathering can be physical (e.g. cracking from water seeping into cracks and freezing, or abrasion from water movement dragging pebbles over the parent material) or chemical (e.g. chemical reactions with air, water or some other chemical causing the parent material to break down into smaller components) or biological (e.g. ‘digestion’ of rock by lichens or pressure from plant roots). You can probably imagine how these weathering examples are variable according to the climate.
The topography of the land also controls the rate of soil formation and what type of soil you’re going to get. Imagine three different landscapes: a mountain range, a hilly heathland and a flat plain. The mountain range has steep slopes with very few places for soil to collect. The hilly heathland has a range of hills where the soil could be shallow and valleys where soil can accumulate. The flat plain is more uniform in soil depth.
The biota that are present in the soil forming environment is also key. Biota such as earthworms play an important part in moving established soil around and in aerating it (allowing oxygen to pass through it). Lichens have the remarkable ability to turn a rock into a soil and survive on bare rock alone. They are a symbiosis of two organisms; a plant that can photosynthesise and a fungus that clings to the rock and stores moisture. You can see soil making in action on old dry stone walls, where lichens first create a thin layer of soil and then mosses colonise this and as they eventually die away, through their decay become soil themselves. This, of course, is another major contribution of biota. Not only do they help to make soil from rock, they also become soil themselves when they die. Peats are soils that have a very high percentage of ‘organic matter’ as opposed to minerals. Peats usually form in waterlogged environments where the usual process of decay cannot occur as frequently due to a lack of oxygen. Therefore decay is very slow and soil accumulates over the years. The type of plant that grows on a soil also controls the soil type. For example, an oak woodland soil will have a rich ‘humus’ (organic) layer on the surface that is contributed to every autumn. A pine woodland will have a shallow organic layer that is highly acidic (because pine needles are acidic and because the trees don’t shed their leaves at a particular time of year).
Finally, time. The influence of time as a soil forming factor cannot be ignored. The amount of time that it takes to form a soil will depend on all the factors I have already discussed. What is the parent material? Does it break down quickly? Is the weather likely to help it break down? Are lichens likely to colonise it? What else grows nearby? Is it on a slope or in a hollow? Peat can accumulate fairly quickly by soil’s standards, it can take about 10 years to form a centimetre whereas from a bare rock, it could take between 200 and 500 years to accumulate the same amount.
When we think about the rate of soil erosion in the areas most prone, we should be concerned. Soil is lost much faster than it is replenished, so soil conservation is of major importance worldwide. In my next article, I will continue to discuss soil and will write some more about the influence of plants on soil type. I will use heathlands as an example to describe how drastically plants can change the soil and the effect that this can have on land use (this was the topic of my undergraduate dissertation).