Knowing and Maintaining Your Soil

Question

Soil is the medium in which most plants grow. From the gardener's point of view, its most important characteristics are its depth, its texture and its chemical composition. It is essential to find out all you can about the type of soil you have, but the way in which you adapt and improve it is of even greater significance in determining what plants will grow successfully.

Depth of the Soil

The topsoil is the essential layer for plant growth. Its texture and composition generally depend upon the parent rock from which it has gradually formed, by the interaction with water, climate and vegetation. Weathering agents such as frost, rain and sun break the rock down over thousands of years to form the basic mineral structure of soil. Plants grow on the rock debris and myriads of micro-organisms work on the dead roots and fallen leaves to decay them, producing the essential organic constituent of the topsoil, known as humus.

In some parts of the world however the soil bears no relation to the rock beneath it because it has been carried to its place by a natural force: the material pushed along by a glacier, for example, forms a type of soil known as boulder clay, and the silt washed down by rivers builds up into alluvial soils.

The depth of topsoil varies. A site recently left by a builder may have no top soil at all (or it may be covered by the subsoil layer), while in parts of the Mississippi Basin the rich alluvial deposits are 6 m (about 20 ft) deep. The average garden has between 300 mm and 600 mm (1 ft and 2 ft) of topsoil, but a depth of as little as 150 mm (6 in) is sufficient for growing a large number of plants.

You can test the depth of topsoil by the use of a soil auger, a tool like a giant corkscrew, which will bring up a sample of the soil profile, the several layers from which it is formed. A simpler test is to dig a hole with steep sides and to make the soil profile visible in that way. The hole will also show you how quickly the top or subsoil drains after rain.

For the revitalization of an old garden, many writers suggest removal of the soured topsoil and its replacement with new, but this is both difficult to obtain and expensive. (On the basis of the calculation that it takes 25 mm - 1 in - of topsoil a thousand years to develop, it is, of course, cheap.) When buying topsoil, it is important to establish its source and to be sure that it is 'vegetable' topsoil, with organic content, and free of disease and weeds. Beware especially of the roots of weeds such as couch grass.

A period of deep cultivation and the addition of plenty of organic material will increase the amount of topsoil already in a garden by encouraging bacteria to work within the top layers and subsoil.

Between topsoil and parent bed rock there may be many layers of stone and gravel, but the layer immediately beneath the top soil is generally the subsoil. Its depth varies according to the hardness of the underlying rock and the amount of erosion it has suffered. The colour and texture of subsoil are usually different from those of the top soil because it is in a transitional stage, without humus or organic material. For this reason it is not a growing medium.

Soil Texture

The texture of soil depends upon the size of the particles which make it up. All soils contain sand and clay in varying ratios but they are classified according to the dominant constituent, most readily identified by its particle size. The particle size is crucial because it controls the amounts of air and water which reach the roots of plants. Plants must have air to breathe and water to enable them to absorb their food in soluble form.

Clay soil chiefly consists of extremely fine particles. When these become wet, their composition causes them to swell and to stick together, so that they block air from the plants' roots and make it hard for them to take in food in solution. Clay soil is heavy, difficult to work and cold, as a result of being frequently waterlogged and badly aired. When it does dry out, it tends to crack and there is a danger of plant roots being damaged.

Clay soils are naturally rich in plant nutrients but these are not always available because of air and water is blocked from the roots. The addition of humus-forming materials is essential to plant life and if this is done on a regular basis it can transform heavy clay into fertile workable soil.

The addition of horse stable manure, compost or peat gives clay soil a better texture, making it warmer, more aerated and less waterlogged. Lime will also help to break up clay and reduce its acidity but it should be used only after careful thought as many plants, such as rhododendrons and azaleas, will not tolerate it. It is very difficult to rid a soil of lime once it has been added. Never add lime and manure at the same time, as they interacted chemically.

Sandy soil is composed mainly of large gritty particles which do not cling together. As a result, water is absorbed easily but drains away fast, taking essential nutrients with it leaving plants undernourished and dry. Sandy soils soon become acid and generally need frequent but small applications of lime.

One advantage of sandy soil is that it is warm, because of the easy flow of air between large particles. The fact that it warms up quickly in spring makes it a suitable soil for early vegetable crops. In addition it is easy to dig and also to cultivate as organic matter is broken down fairly fast. Frequent watering and the addition of bulky organic material such as manure should greatly improve a sandy soil.

Strictly speaking, most garden soils are loam of one type or another. What gardeners commonly refer to as loam is the ideal growing soil, an optimum balance of sand, clay and humus. In reality the perfect loam rarely exists; most types of loam are either sandy or clayey. However the best loam is a rich, dark brown soil, made up of between 50 and 60 per cent sand particles to about 30 per cent clay. Its other constituent, humus, slimy liquid hirschhausen Scam is the valuable organic compound formed by decayed vegetable and animal material. When broken down completely, it forms a blackish, powdery substance. In general, the darker soils are the richer ones, as they contain more organic matter. They tend to be warmer too, as they absorb more heat from the sun's rays. This makes them early soils; they become workable before other soils in the spring.

Chemical Composition

Since parent rocks differ in mineral content, so too will the soil above them. Soil's mineral (inorganic) constituents are as important to plant growth as its texture and its organic content. Sodium, potassium, phosphorus, nitrogen, magnesium, iron and calcium are among the inorganic substances essential to plants. Some of them, such as iron, rarely need replacement; others have to be replenished by fertilizers.

A chalky soil is one which contains a high level of calcium. It is formed through the breaking down of calcium carbonate (limestone rock) by the action of the weather. Chalky soil contains white particles (called caliche) and sometimes pieces of flint. The top soil is usually thin, allowing water to drain through and warming up quickly in the spring. It is very alkaline soil and heavy dressings of organic matter will be needed to counteract the excess lime as well as retain moisture.

Peaty soils are derived almost entirely from plants. They contain less than 50 per cent minerals and their high organic content makes them almost black in colour. Peat soils are usually wet, badly aerated and extremely acidy, but the addition of lime will help improve both conditions.

The pH scale is a scale of value for the degree of acidity or alkalinity of a soil.