Soil Composition: Understanding Soil
The mixture of mineral and organic material that makes up the soil is subject to infinite variation in texture, colour and chemistry. The key factor is the parent rock itself — which may be granite, chalk, sandstone or beds of gravel or clay.
Any gardener who has wrestled with intractable clay, hungry structureless sand or the thin, sour soil of heathland is acutely aware of soil type, and of the wisdom of choosing plants with appropriate requirements — rhododendrons on acid, for example, or roses on clay. Beyond the garden gate such soil diversity is reflected in our natural flora — orchids and the pasque flower on lime-rich chalk downland, for instance, and heather and bilberry on acid heath.
What is soil?
The fundamental components of soil are rock fragments, clay and broken-down organic matter. The rock fragments are the result of physical erosion by frost, heat, wind and water, all of which crack and splinter the parent rock into smaller and smaller pieces. These may be carried away and deposited elsewhere as gravel, sand or silt, or remain to be broken down further where they lie. A cross-section of undisturbed soil overlying a friable parent rock can show every stage of this fragmentation process, from the fissured rock of the deepest layers to the fine sand of the topsoil.
In addition to the physical weathering, rock is also subject to chemical erosion by rainwater which, in passing through the atmosphere, combines with carbon dioxide to form very weak carbonic acid. This dissolves alkaline minerals such as calcium, and in time completely destroys the structure of calcareous rocks. Acid minerals such as silica are unaffected, and consequently granite, which contains a high proportion of silica, is relatively resistant. Nevertheless, the alkaline components are attacked, and the rock is eventually broken down. The acid silica is liberated as sand, the alkaline elements are dissolved, and the residue is converted, roughly speaking, into clay.
Clay has particular properties which set it apart from other soil minerals. Its particle size is extremely small, and consequently the total particle surface area is comparatively large. Given the tendency for soil particles to retain a surface layer of water, it can be seen that the smaller the particle size, the greater is the potential for water retention. Furthermore, each clay particle can take up water internally, unlike particles formed by physical erosion. This capacity is accompanied by a tendency to flocculate, or cohere, into larger units.
Mixed with other soil material, clay has the effect of binding or clogging the assorted particles into granules, the size of which tends to increase with the proportion of clay. Each of these granules is capable of retaining water and dissolved minerals essential for plant growth, and it is largely the clay fraction which distinguishes a true soil from a heap of inert mineral fragments.
The other main component of soil is organic matter. On the surface of undisturbed soil this is present as recognisable plant and animal remains, but under normal circumstances these are subject to continual attack by decomposing agents — soil bacteria, fungi and invertebrates. One consequence of this is oxidisation, caused by certain types of bacteria which favour light, well-ventilated conditions.
Where the soil is sufficiently moist or compact to impede oxidisation, the organic remains may be converted into humus, a dark spongy material generally found in the upper layers of soil. Humus shares with clay the properties of water retention and flocculation, and is therefore important in soil formation. Furthermore, containing as it does the nutrients (such as potassium, phosphorus and calcium) taken up by the plants, it is a rich source of plant food.
The chemical nature of the soil is modified by the climate. Rain is acidic — and is becoming more so, through atmospheric pollution — so it dissolves alkaline material. As it filters down through the soil it carries the alkaline elements with it — for example potassium, phosphorus and calcium. Acid material such as silica is unaffected, but is of no nutritional value. Consequently acidity, or the absence of alkaline material, may be equated with nutrient deficiency.
This does not mean that alkalinity implies richness. A high concentration of calcium, for example, is not necessarily matched by equal amounts of the other nutrients, and shallow chalk soils are often significantly poor in major plant foods. Such soils are characterised by an absence of clay and silica, for chalk, being almost pure calcium carbonate, is completely dissolved by chemical erosion. There is consequently no mineral subsoil, and the topsoil is composed of organic material mixed with eroding chalk fragments. This type of soil is called a rendzina.
Well-balanced brown earth
The dissolution of nutrients by rainwater does not render them inaccessible to plants. Acidic conditions only develop if the alkaline solution drains away. In our temperate lowlands rainfall is balanced by evaporation: if the soil is sufficiently full-bodied to prevent rapid drainage, the nutrients tend to crystallise out of the solution as fast as they are dissolved, and the status quo is maintained. Nevertheless, there is normally some downward flow of alkaline material ; it is replaced from the humus in the topsoil. This type of soil is known, appropriately, as a brown earth, for the even distribution of alkaline nutrients is indicated by the brown tint given by dissolved iron oxides, extending well down into the subsoil.
Where, for some reason, the alkaline solutions drain away too fast for the evaporation process to salvage the nutrients, the upper layers of soil become completely denuded and acid, assuming a bleached, ash-like appearance. The alkaline material collects below this region, in a layer stained dark red-brown by the iron compounds. These may form a hard ‘pan’ which arrests further drainage. This type of soil is called a podzol, a Russian word meaning ‘ash-soil’. It is common on free-draining gravels and sands, and in areas where a cool, wet upland climate, in which rainfall exceeds evaporation by a wide margin, acts upon soils derived from acid siliceous rock.
The opposite effect — of drainage impeded to the point of stagnation — occurs in very heavy soils eroded out of an impermeable substrate of sedimentary clay. Such soils remain waterlogged in patches for much of the year, and exhibit a characteristic colour mottling of brown and blue-grey. This effect, known as gleying, is caused by the action, respectively, of aerobic and anaerobic microorganisms on the iron compounds. The proportion of blue-grey material indicates the degree of saturation with stagnant water; this soil is called a stagnogley.
A similar pattern, an alluvial gley, may be induced by the welling up of ground water from below, as occurs in low-lying areas influenced by seasonal flooding. Below the permanent water table the soil is uniformly waterlogged and anaerobic. In the fluctuation zone it is mottled with grey anaerobic patches. Above this the soil is unaffected, and may resemble that of a brown earth. Because the nutrients are not leached out by drainage, gley soils are potentially very fertile, but their agricultural value is limited by the standing water and by their tendency to become compacted and unworkable under the weight of farm machinery.
Coping with nutrient deficiency
The ecological implications of nutrient leaching and anaerobism are profound. In a brown earth soil the plant foods are made available by micro-organisms which break down organic material into humus and convert the nutrients into a form readily absorbed by plant roots. The majority of these micro-organisms are unable to function under acid or anaerobic conditions; consequently podzols, and soils that are permanently waterlogged, develop a substantial surface layer of unmodified plant remains — or peat. Raw peat is of little value to plants; if they are to survive they require either a new source of nourishment or a way of extracting it from peat.
In alkaline but waterlogged conditions water-tolerant plants can extract the nutrients dissolved in the water and a flourishing wetland ecosystem, or fen, develops. In acid conditions, however, free nutrients are in short supply. Sphagnum moss, characteristic of wet acid areas, acquires them by absorbing vast quantities of water and replacing the nutrient atoms with hydrogen. This acidifies the water; consequently, if sphagnum mosses become established in fenland, it is converted into an acid-rich habitat.
Other solutions to the problem of overcoming nutrient deficiency are shown by the carnivorous plants that trap and digest insects, and by the saprophytic fungi that derive nourishment directly from dead organic materials.
Fundamentally soil is composed of broken down eroded bedrock, clay, humus enriched topsoil and organic debris.
Brown earth soil profile (see image above right)
This soil usually has a loamy texture and is very fertile. The even spread of alkaline nutrients is shown by the brown tint (dissolved iron oxides) that extends well down into the subsoil. The horizon between topsoil and subsoil is indefinite.
Rendzina soil profile (see image above right)
Shallow chalk soils are recognisable by their lack of clay and silica. Since chalk is almost pure calcium carbonate, it is completely dissolved by chemical erosion. There is therefore no mineral subsoil at all, and the topsoil is made up of organic material mixed with eroding chalk or limestone fragments. Chalk soils are often significantly poor in major plant foods — alkalinity does not necessarily imply richness.
Humus, iron and nutrients drain through the sandy soil and collect in a hard iron ‘pan’ which arrests further drainage. This type of soil is common on free-draining gravels and sands and in areas where a cool, wet upland climate, in which rainfall far exceeds evaporation, acts upon soils derived from acid, siliceous rock. Below the iron pan the soil is much healthier than the leached sandy soil above.
In a stagnogley drainage of water is impeded to the point of stagnation by an impermeable substrate of heavy clay.
Such soils, waterlogged for much of the year, exhibit a characteristic colour mottling (left) of brown and blue-grey; this is caused by the action of aerobic and anaerobic micro-organisms on the iron compounds. A similar pattern, an alluvial gley, may be induced by the welling up of ground water from below.