I hope the previous summary of what habitat is and what makes a natural home for plants and animals has convinced you of the importance of soil. To understand habitat better a basic understanding of soil is helpful.What is soil?Soil consists of ground up fragments of rock; the smallest of these might be microscopic, larger fragments might be as large as pieces of sand. Soil also contains decaying organic matter (humus), water and air. It is “home” for roots, seeds, spores, insects, fungi, bacteria, worms and other organisms, even mammals - the mole.Soil has delicately balanced food chains which are probably more important than the plant and animal food chains that depend on it for their existence. The soil food chains are easily damaged, especially by machinery or through water logging.Soil Composition:Soils are classified into types according to the proportions of different sized particles in them because it is the mix of ground up rock particles that mostly determines how the soil behaves.For convenience and ease of communication, soils are often referred to as being sandy or clayey or silty. Different countries use different standards to define what size a sand particle or a silt particle is, but a fairly popular basis is: Course or medium sand - particles from 2mm to 0.2mm in diameterFine sand - particles from 0.2mm to 0.06mm in diameterSilt - particles from 0.06mm to .002mm in diameterClay - particles less than .002mm in diameterSoil will be a mixture of grains of various sizes and the mix of these grains, that is the percentage of each of these four sizes, will determine the structure and nature of the soil. Bear in mind that although you will think of a grain of coarse sand as being small, it is at least a thousand times bigger than a grain of clay! By measuring the proportions of these particle sizes in a soil sample, the soil can be classified into a particular type. Clay soils, for example, are those which have more than 40% of their particles within the less than 0.002 size grade. As usual, it isn’t that simple in practice. Sometimes the grains get stuck together, making them into larger multi-particles.Other Influences: In addition to the fine particles of rock granules, soil will often contain much larger pieces of rock which, of course, we know as stones or pebbles. In some habitats (a shingle beach for example) there will be no fine grains at all; the substrate will be formed solely of pebbles of varying sizes or possibly even by bare rock (mainly in mountainous habitats). The nature of the ground up rock will also affect the chemistry of the soil that it forms. Some rock (chalk and limestone for example) tend towards being alkaline or calcareous whereas some sandstones can be acidic. Other soils can be neutral and not inclined to be alkaline or acidic. Field guides to plants will often state if a certain plant has a preference for one of these soil chemistry types. The measurement of this chemistry provides what is known as a pH value for a sample. In general, British soils range from about pH 4 to pH 8. (pH stands for “potential Hydrogen ions”). It is worth remembering that as well as affecting soil chemistry lime is the fourth most important plant food.Many plants are very sensitive to soil pH. Heathers, for example, will not tolerate lime in the soil while traveller’s joy (a wild clematis) will only be found on lime soils.Soil does not always reflect the qualities of the bedrock below it. Soil can be an accumulation of rock types that have been brought together by wind, glaciation, rivers and flooding.Soil is a very variable substance and its composition in any one place will affect how it retains moisture and air and that will govern the nature of the habitat that will occur on that soil type.Sand, silt and clay:Having established that soil is formed of ground up pieces of rock we should consider how the mix of the various grain sizes affects the soil and, as a result, affects the vegetation that can grow. Rarely does a sample of soil consist of just one classification of grain size, there will usually be a mix but it is quite common for a single size to form the majority of grains present. This is a complex area and leads into the science of soil analysis and testing that is used to influence agricultural practices and that is not something I want to delve in to! For understanding habitat is it really only necessary to consider what happens when one of the three grain types is dominant.Sand Dominant:Where the grain sizes are mainly large one gets a sandy soil. In sandy soil the individual grains do not fit closely together and so there is a lot of air between them allowing plant roots to find their way easily between them. That may seem ideal at first but there are two downsides to this from a plant's perspective. Firstly, water will permeate easily through it, little moisture is retained and it can dry out very quickly and plants need moisture. Secondly, sandy soil will be formed of loose grains that readily move and so many plants will find it difficult to get a firm 'foothold'. In general sandy soil is a pretty inhospitable environment and only plants that form deep, complex root systems can thrive happily in such conditions.Clay Dominant:The other end of the spectrum to sandy soil is clay. Here the fine grains cause them to be very close together, often they will actually bind together and this presents the opposite problem for plants. Water finds it difficult to permeate through and the soil can quickly water log in rain which does not suit many plants. Plants also find it difficult to penetrate clay with their roots and so plants with shallow root systems will benefit in such conditions.Silt Dominant: The ideal soil type for many plants is where the granules are mainly silt sized. Gardeners call this soil loam and it is the favoured soil for good crops. A lot of effort can go into trying to form a loam soil in gardens! Loam will retain moisture but allow surplus to drain through and plant roots can easily penetrate and form a firm hold. It is not all good news for plants though! Being ideal growing conditions the competition amongst plants for space is immense and will favour the strongest, most vigorous of plants. Grain mix, pH level and humus content all go to define a soil type and create an environment where a specific array of dominant vegetation can grow and these forms the various common habitats that one will encounter.