|Ron Allen's soil and water pages
|Ron Allen's 'Rough Guide to Soils'
Stand in the middle of a wood and look up and you see half a habitat. Look down, and you can’t see the other half, it lies buried and unseen, mysterious and unknown.
But dig a hole in that same woodland and (if you can read the language), you will have displayed in front of you evidence of how that piece of land has developed over the past 10 000 if not 50 000 years, from the ice ages to the present day.
Soil is a living and dynamic material containing a vast range of living breathing interdependent creatures from bacteria, algae and fungi to earthworms and arthropods and even including mammals such as moles, rabbits and badgers. These myriad creatures, together with the roots of plants, not only live in the soil but are a fundamental part of it. They form a complex web of life within the pores and fissures of layers of mineral and organic materials that together have developed and changed over many thousands of years. Near the surface, or deep in the soil, all of these creatures are feeding, respiring, decomposing and recycling materials in an environment alien to all those we are used to seeing above ground.
So important is the soil to our wildlife that it is amazing that so few of us know anything about it.
Soil not only supports plants and creatures, it positively heaves with life, as roots lengthen and expand, fauna reproduce, moles chase earthworms in their tunnel systems and all within an environment of mineral and organic particles, pores and fissures and biochemical reactions quite unlike any that in any other terrestrial habitat.
Soil thus occupies perhaps 50% of our terrestrial wildlife habitats. In some habitats it might be nearer 90% and elsewhere perhaps only 10%. Soil also affects the properties of freshwater aquatic habitats and to a lesser extent our marine habitats. But just what is soil?
If I were starting to study soil today, I would probably give up at the first text book. Understanding soil from scratch would be more difficult than understanding how to identify and classify beetles for instance. Soil science is difficult, abstruse and highly technical, but soil itself is almost everywhere.
So to set you on your way, here is my rough guide to soil, a compilation of soil-speak based on thirty years of looking in the ground. You might watch birds, study plants or manage habitats. Alternatively, you could peer into the ground and ponder the meaning of it all.
What is soil?
I have on many occasions asked groups of adults (naturalists, gardeners and others) just what is soil. Most adults cannot see the wood for the trees, or rather the soil for the stones. They dissect the soil and talk about sand and nutrients, clay and water, or organic matter. In contrast, children tell you that soil is that stuff in which plants grow, and how right they are. Engineers have a different view again and so do archaeologists, but children have it about right.
If you can grow plants in it, it is probably soil. The key thing is that the soil provides moisture, nutrients and support for plants as well as a whole range of interdependent, and often microscopic, subterranean creatures.
As ecologists we know about different habitats, but different habitats all have their own soil systems that are fundamental to them and without which they could not exist.
How deep is the soil?
If soil is that material in which plants root, then the soil is as deep as the roots go. In upland rocky areas, this may be a few millimetres. In the lowlands, this depth may be perhaps, five metres.
British soils can be man-made, disturbed, cultivated or semi-natural. Nature reserves are important because they are often our sole reserve of undisturbed semi-natural soils. Without semi-natural soils, semi-natural habitats could not occur. Heavily disturbed soils are sometimes called ‘earths’.
Soils come in many types, but can generally be divided into organic (peaty) soils made up of decomposed plant remains or, mineral soils made up of sand, silt and clay particles. To confuse matters, many soils often have an upper organic layer and a lower mineral layer. This means that soil also comes in ‘layers’ technically known as soil horizons.
To see these layers, dig a hole in your garden or, (with permission) in your nearest nature reserve. The deeper you dig, the more layers you may find until the layers become less distinct or you hit hard rock. These layers have formed over many thousands of years, mostly since the end of the ice ages but some soils have been affected by frost and ice action during the ice ages and some have been formed during the warm interglacial periods in totally different climates to those of today.
The uppermost layer, often loose and made up of partly decomposed plant remains is the litter layer. If the plant remains are decomposed they either form a layer of humus or, deeper layers of peat. These are organic layers. The top mineral layer is usually dark and rich in decomposed organic matter and known, not surprising, as ‘topsoil’. The next group of layers are usually paler and known as ‘subsoil’. Subsoil layers accept materials washed down from the topsoil and pass them on in an ordered way down the soil profile. Thus some subsoil layers are depleted in some soil materials such as clay or aluminium compounds, and others are enhanced in them. Below the subsoil is the substrate, also known as the parent material, and from which the soils have been formed by a large number of ‘soil forming processes’. At greater depth is the bedrock, unaltered by soil forming processes and more the domain of the geologist.
Some soils have many layers, the classic type is the podzol, always described in text books because of their bright colours and easily seen layers. In fact, they are restricted to heathlands and their origins and formation are the subject of continuing research. These beautiful soils generally have a thick litter layer derived from heather and other heathland plants over dark topsoils with a thick litter layer. Their subsoils are divided into an upper pale layer (leached of nutrients), a middle black layer (enhanced in carbon) and a darker bright orange layer (rich in iron and aluminium). These soils have usually formed by prolonged leaching below heathland plant communities
Soil water regimes
We are now beginning to get rather technical. Plant roots can go deep into the soil to find water. Generally the drier the soils the deeper down the roots go. But not always, horsetail (Equisetum spp) roots can penetrate deep into waterlogged soils.
Soil water regimes are generally variations on two themes and understanding these can be fundamental to the management of many nature reserves. Soils can be permeable, like those composed of loose sands for instance, or they can be slowly permeable like those composed of clays. Permeable soils can be well drained, that is they are seldom or never waterlogged, or they can be seasonally or permanently waterlogged to different depths. Groundwater can rise into such soils and permeable waterlogged soils are groundwater affected soils. Soils on river floodplains are often like this. In contrast, clayey soils have impeded drainage. Groundwater cannot rise into them and rain water cannot pass down into them. This means that the surface layers are wet in winter and such soils are called surface water affected soils.
Many soil layers are quite colourful and soil colour is often the clue to identifying different soil types.
Reddish subsoils for instance, have generally formed in hotter climates and may give evidence that they first developed during warm interglacial periods between the ice ages. These soils are called ‘palaeosols’.
Brown colours generally indicate that iron in the soil is in an oxidised state implying that the soil is well aerated and well drained. These soils are called ‘brown soils’.
Grey or bluish soils indicate that the iron in the soil is in a reduced state and such soil layers remain waterlogged for most of the year. Soils with many grey colours are called ‘gley soils’.
Mottled soils with grey and orange colours (you may need good eyesight or a lens to see these) are generally seasonally waterlogged and the more grey mottles present, the longer the duration of waterlogging.
Black or very dark brown soil layers are generally rich in decomposed organic matter. Thin well-drained organic layers are generally made up of decomposed leaves known as humus. In waterlogged conditions, organic matter accumulates as peat. Peaty soils can be made of Sphagnum bog mosses (in acidic conditions) or sedges and grasses (in more neutral or slightly alkaline conditions).
Take a lump of very moist soil in your hands and rub it between the fingers and you may be able to distinguish large particles (greater than 2mm across) known as stones.
Feel the material between the stones, known as the ‘matrix’ and you may find that it is made of sand partlcles (that flow between the fingers if pure and wet or that can be heard grating together if rubbed close to the ear if impure).
You may find that the soil is more solid and smears between the fingers indicating the presence of clay. If the soil feels silky, a little like wet talcum powder, it is probably made of silt particles.
Soil materials made up of mixtures of these particle types are known as ‘loams’ and according to their proportions of sand, silt or clay can be described as sandy, silty or clay loams. Clays can similarly be sandy or silty clays. Technically, the different ratios of soil particles are called soil particle classes such as sandy clay or silty clay loam.
With skill you can divide soil materials into the following particle size classes: clay, silty clay, sandy clay, clay loam, sandy loam, silt loam, loamy sand and sand. If you really want to be clever, you can divide the sand particles into those that are coarse, medium or fine in size and so you can have fine sand or coarse sandy loam for instance.
The structure of soil is fundamental to how soil behaves. Take a lump of loamy soil and carefully break it apart. You will find that the soil breaks into naturally occurring fragments known as ‘peds’. In the soil, peds are divided by ‘fissures’. In loamy or clayey soils, these peds shrink and swell summer and winter as they become drier and then wetter. Most activity occurs on the edges of the peds or within the fissures. Soil can get washed down in the fissures, roots tend to follow the surfaces of peds and badgers take advantage of this blocky nature of soil to make digging easier.
One of the reasons that plants need soil is because soil contains the nutrients required for a plant to grow. Topsoils tend to be richer in phosphorus and nitrogen as well as smaller amounts of iron, copper and manganese for instance. This is the reason why most plant roots are in the topsoil and why good topsoils are so important for agriculture. Soils rich in calcium carbonate are generally alkaline (they fizz in 10% hydrochloric acid). Those devoid of calcium carbonate and rich in aluminium are generally acidic.
When it comes to soil acidity, we find some confusion. To the chemist, acidity means that the soil pH is less than neutral and alkalinity means that the pH is greater than neutral. Neutral is taken for highly technical reasons to be a pH value of 7. Plants are not like that. Neutral plant communities generally occur on slightly acidic soils and acidic plant communities generally occur on strongly acidic soils. This means that neutral grassland may not occur on neutral soils. You need to be very careful how you use these terms.
Habitats and soils
Different habitats are characterised by different soils. Limestone grassland commonly occurs on thin loamy soils rich in calcium carbonate and with chalk or limestone within about 20-30cm depth.
Acid grasslands usually occur on well-drained sandy or light loamy soils lacking in the nutrients required by other grassland communities.
Heathlands typically occur on podzols, formed by the strong leaching of soils over many centuries. Dry heathlands occur on well-drained podzols (typical of the Surrey heathlands). Humid heathlands are usually on podzols with impeded drainage and wet heathlands are on permeable soils with prolonged seasonal waterlogging.
Valley mires are typically on permanently waterlogged acidic peaty soils made mostly from Sphagnum bog-mosses.
Most ‘neutral’ (or mesotrophic) grasslands are developed on seasonally waterlogged loamy soils, either on floodplains or on seasonally waterlogged soils with impeded drainage.
Soil maps can be purchased or obtained through libraries. These maps come in different scales in different areas and allow you to look up the kind of soil occurring in your area. Large scale soil maps show areas of individual soils (usually called ‘soil series’) and often on a field-by-field basis. Small scale soil maps show areas of land occupied by regularly occurring groups of different soils. These groupings of soils are known as ‘soil associations’ and such maps give an indication of the range of soils likely to be found in any one area, rather than the particular soil type. If you want to know the soil you generally need to dig your own hole.
If the maps are in colour, you can often see at a glance what the different soils are in any one area. Blue colours on the map indicated groundwater affected soils, green colours indicate seasonally waterlogged soils with impeded drainage, brown colours indicate well drained brown earths, pinks and reds indicate podzols and purples indicate peat soils.
Most soil maps are accompanied by (rather expensive) descriptive books. These books tell you about the characteristics of the different soils, how they occur and usually provide lots of background information on local climate, geology and land-use. The regional bulletins of the Soil Survey of England and Wales also provide keys to the identification of the soil in your particular hole using the various characteristics I have described above such as texture and colour.
How can I find out more?
Email Ron Allen or telephone 01730 231019.
|The Environmental Project Consulting Group
44A Winchester Road, Petersfield, Hampshire GU32 3PG
email: Ron Allen, tel: 01730 231019,
Copyright April 2005 Ron Allen
Geologist, Soil Scientist, Applied Ecologist, Hydro-ecologist, Chartered Environmentalist