Soils and soil management

Soils are very complex and most have developed over a very long period of time. They provide a suitable medium for plants to obtain water, nutrients and oxygen for growth and development. Most soils also have enough depth to allow plant roots to provide a firm anchorage. Mineral soils are formed initially by the weathering of parent rock, often accompanied by deposition of material by ice, water and/or wind. Organic material is added by the growth and decay of living organisms. If a farmer is to provide the best possible conditions for crop growth it is necessary for him to understand what soils are, how they were formed and how they should be managed.

The topsoil is a layer up to 30 cm deep which may be taken as the greatest depth which a farmer can plough or cultivate and in which most of the plant roots are found. The subsoil, which lies underneath, is an intermediate stage in the formation of soil from the rock below. Some deep-rooting plants such as cereals and oilseed rape can grow in the subsoil down to depths of 1.5–2 m.

A soil profile is a section taken vertically through the soil. In some cases this may consist only of a shallow surface soil of 10–15 cm on top of rock such as chalk or limestone. In deeper well-developed soils there are usually three or more definite layers (or horizons) which vary in colour, texture and structure (Fig. 2.1 ). The soil profile can be examined by digging a trench or by taking out cores of soil from various depths using a soil auger. A careful examination of the layers can be useful in deciding how the soil was formed and the cropping potential. The colour of the soil in the various horizons will indicate whether the soil is well or poorly drained.

There are a number of ways of classifying soil for crop production. Soils have been grouped into soil associations. Each association consists of a number of soil series each of which has distinct characteristics, both of parent material, soil profile and topsoil. The soil series is usually named after the place where the soil was first described. The same soil series can occur in different regions. Soil characteristics together with relief and climate and cropping potential have also been used to classify land for farming (see Appendix 8).

2.1 Soil formation

There are very many different types of topsoils and subsoils. The differences are partly due to the kind of material from which they are formed. However, other factors such as climate, topography, plant and animal life, the age of the developing soil material and farming operations affect the type of soil which develops.

2.1.1 The more important rock formations

• Igneous rocks, e.g. granite (coarse crystals) and basalt (fine crystals), were formed from the very hot molten material. The minerals (chemical compounds) in these rocks are mostly in the form of crystals. Igneous rocks are very hard and usually weather very slowly.

• Sedimentary or transported rocks have been formed from weathered material (e.g. clay, silt and sand) carried and deposited by water and wind. The sediments later became compressed by more material on top and cemented to form new rocks such as sandstone and shale.

• The chalks and limestones were formed from the shells and skeletons of sea animals of various sizes. These rocks are mainly calcium carbonate but in some cases also contain magnesium carbonate. The calcareous soils are formed from them (pages 43–4).

• Metamorphic rocks, e.g. marble (from limestone) and slate (from shale), are rocks which have been changed in various ways such as by heat or pressure.

2.1.2 Some other deposits

2.1.2.1 Organic soils

Deep deposits of raw organic matter or peat are found in places where waterlogged soil conditions did not allow the breakdown of dead plant material by micro-organisms and oxidation (due to the anaerobic conditions). There are different sorts of peat depending on the type of vegetation and area where it developed.

2.1.2.2 Glacial drift

Many soils in northern Europe are not derived from the rocks underneath but from material deposited by glaciers, often known as boulder clays. This makes the study of such soils very complicated.

2.1.2.3 Alluvium

This is material which has been deposited recently, for example, by river flooding. It has a very variable composition. The texture depends on the speed of river flow (e.g. fast rivers–stones and sand, slow rivers–silt and clay).

2.1.3 Weathering of rocks

The breakdown of rocks is mainly caused by physical and chemical action.

Physical weathering, due to frost action, causes the mineral crystals in rocks to expand and contract by different amounts, resulting in the occurrence of cracking and shattering. Water can cause pieces of rock surfaces to split off when it freezes and expands in cracks and crevices. The pieces of rock broken off are usually sharp-edged, but if they are carried and knocked about by glaciers, rivers or wind, they become more rounded in shape, e.g. sand and stones in a river bed.

Chemical weathering is the breakdown of the mineral matter in a developing soil brought about by the action of water, oxygen, carbon dioxide and nitric acid from the atmosphere, and by carbonic and organic acids from the biological activity in the soil. The soil water, which is a weak acid, dissolves some minerals and allows chemical reactions to take place.

Clay is produced by chemical weathering of some primary minerals. In the case of rocks such as granite, when the clay-producing parts are weathered away, the more resistant quartz crystals are left as sand or silt. In the later stages of chemical weathering the soil minerals are broken down to release plant nutrients. This is a continuing process in most soils.

In poorly drained soils, which become waterlogged from time to time, various complex chemical reactions (including a reduction process) occur, referred to as gleying. This process, which is very important in the formation of some soils, results in ferrous iron, manganese and some other trace elements moving around more freely and producing colour changes in the soil. Gleyed soils are generally greyish in colour (but may also be greenish or blueish). Rusty-coloured deposits of ferric iron (oxidised iron) also occur in root and other channels, and along the boundaries between the waterlogged and aerated soil, so producing a mottled appearance. Glazing or coating of the soil structure units with fine clay is also associated with gleying.

2.2 Other factors in soil formation

2.2.1 Climate

The rate of weathering partly depends on the climate. For example, the wide variations of temperature and the high rainfall of the tropics make for much faster soil development than would be possible in the colder and drier climatic regions.

2.2.2 Topography

The slope of the ground can considerably affect the depth of soil. Weathered soil tends to erode from steep slopes and build up on the flatter land at the bottom. Level land is more likely to produce uniform weathering.

2.2.3 Biological activity

Plants, animals and micro-organisms, during their life cycles, leave many organic substances in the soil. Some of these substances may dissolve some components of the mineral material; dead material may partially decompose to give humus. The roots of plants may open up cracks in the soil. Vegetation such as mosses and lichens can attack and break down the surface of rocks. Holes made in the soil by burrowing animals such as earthworms, moles and rabbits help to break down soft and partly weathered rocks. Biological activity usually increases with higher temperatures and decreases under waterlogged and/or acid conditions.

2.2.4 Farming operations

Deep ploughing and cultivation, artificial drainage and liming can speed up the soil formation processes very considerably (Chapter 14).

2.3 The physical make-up of soil and its effect on plant growth

The farmer considers the soil from the point of view of its ability to grow crops. To produce good crops the soil must provide suitable conditions in which plant roots can grow. It should supply nutrients, water and air. The temperature must also be suitable for the growth of the crop.

The soil is composed of:

• Solids. Mineral matter (stones, gravel, sand, silt and clay) and organic matter (remains of plants and animals).

• Liquids. Soil water (a weak acid).

• Gases. Soil air which occupies a variable amount of the pore spaces.

• Living organisms. Micro-organisms (bacteria, fungi, small soil animals, earthworms, etc.).

2.3.1 Mineral matter and soil texture

The relative proportion of the mineral material, clay, silt and sand, in a soil is called the soil texture. There is a large variation in the texture of soils found on farms. Soil texture can be assessed...