Soil Types and Agriculture
The soil a farmer happens to have on his land dictates, to a great extent, the type of crops he is able to grow. Barley, for instance, does well on a light, calcareous soil, whereas wheat and beans are best suited to deep, heavy land.
For farmers the soil is not an ecological phenomenon but a piece of capital equipment ; their concern is with the potential of the soil as a means of producing profit. Whereas the naturalist may be attracted to unproductive land for its specialised flora, to the farmer it is damned by its incapacity to bear a good crop.
Texture and particle size
In the artificial conditions of an arable field, short-term fertility is a variable under the control of the farmer, but the essential texture of the soil is something which he can do little to influence. This is of fundamental importance: the capacity of the soil to hold water and dissolved plant nutrients —and consequently its seasonal behaviour, working characteristics and long-term fertility — is largely defined by the nature and size of the particles of which it is composed.
Soil consists of mineral material, in the form of stones, sand, silt and clay, mixed with the remains of plants and animals in varying stages of decomposition. Of the minerals, the stones, sand and silt are the result of physical weathering of the parent rock — impervious fragments which retain water and dissolved nutrients in a surface film. The smaller the fragments are, the greater is their total surface area. Therefore silt, which has a particle size of between 0.002mm and 0.02mm, retains very much more water, and correspondingly less air, than an equal volume of coarse sand (which has a particle size of 0.2-2mm).
Clay, which has an even smaller particle size than silt (less than 0.002mm) is formed by the chemical weathering of rocks by rainwater, which is weakly acid. It is distinguished by its ability to retain water within its porous particles, and by its tendency to cohere into large masses. Mixed with the other mineral material (the stones, sand and silt) in balanced proportions, clay binds the soil into granules, each containing soil water and plant nutrients, separated by air spaces into which plant roots may penetrate.
Land that is heavy invariably contains a large amount of clay. When wet it tends to form clods of tightly packed material, or even a solid compacted layer which impedes drainage and root development. On the farm such compaction, known as poaching, may be caused by the weight of grazing animals or agricultural machinery. Used at the wrong time the plough itself may convert a wet but well-fragmented heavy soil into a series of clods lying on an impermeable `plough pan’ of smeared clay. Cultivation, which requires much power, must on this type of soil be restricted to the dry months. The technique of overwintering a rough-ploughed field is frequently used to allow the frost a chance to shatter the heavy clods.
The disadvantages of heavy land are balanced by its naturally high fertility, which is due to the retention of soluble plant nutrients in the substantial clay fraction. Furthermore, the restricted aeration of the soil slows the rate of decomposition of organic remains, and encourages their conversion into a dark, spongy cohesive material called humus. This contains all the nutrients taken up by the plants during life, and acts as a kind of slow-release fertiliser.
Land that is light has a very friable structure for it contains little clay and may be composed largely of sand, which has no cohesive properties. Consequently drainage is very free. There is little risk of poaching, the soil may be cultivated without difficulty at any time of year, and it warms up quickly in the spring, promoting plant growth.
However, these advantages are offset by sandy soil’s poor resistance to drought and a tendency to infertility. In the well-ventilated conditions organic material is rapidly oxidised by aerobic bacteria to form carbon dioxide and water. Liberated alkaline plant nutrients, such as calcium, phosphorus and potassium, dissolve in the acidic rainwater and drain away leaving, in the worst cases, a barren, inhospitable soil composed mainly of acid silica.
Variety of soil types
Within the basic categories of light, medium and heavy land the farmer recognises a variety of soil types, defined by their most obvious features. In addition to the sandy soils, loans and clay soils already described, there are: stony soils, which are often fairly shallow and contain much unweathered mineral material from underlying rock or gravel beds; calcareous soils, which have developed from chalk or limestone bedrock and are often stony but normally fairly fertile; alluvial silts, in which the mineral particle size is intermediate between sand and clay and which, in consequence, behave rather like loans; and ‘peaty’ soils, which have a very high organic content of drained alkaline fen peat undergoing humification (humus formation). These peaty soils are extremely fertile but fugitive (quickly disappearing); the fen peat is a fossil resource under erosion from the combined effects of cropping and bacteriological oxidisation and is, in practice, unrenewable.
The wise farmer assesses his soil and crops it accordingly. Barley, for example, does well on a light calcareous soil. Wheat and beans, with their sturdy root systems and high nutrient requirement, are best suited to deep, heavy land, but where a high water table would inhibit root development, a shallow-rooted crop such as grass is a better investment. Oats and rye tolerate relatively poor acidic conditions, and for cash root crops, such as sugar beet and carrots, a light soil which falls away during harvesting is preferred. Rich fenland soils are particularly suitable for valuable, demanding vegetable crops.
Improving soils by drainage
Despite the range of crop options open to the farmer, modifications to the soil are often desirable in the interests of cropping flexibility. Although it is impractical to convert the soil directly from one type to another, various measures can be taken to influence its behaviour; these measures may make all the difference between ‘marginal’ land of little value and a useful addition to the farm capital.
Proper drainage is essential, even on light land, for crop roots will not penetrate waterlogged soil. Compacted layers of soil are broken up by means of a subsoiler, a simple tined (pronged) implement which penetrates well below plough depth and allows water to escape into the lower uncultivated levels. A build-up of standing water is prevented by installing a field drain network of earthenware or perforated plastic pipes set in gravel and leading to a main drainage channel or ditch. On heavy land the pipe drains may be fed by a secondary intersecting system of mole channels, formed by drawing a bullet-shaped ‘mole’ through the sticky soil.
Improving the drainage inevitably accelerates the rate of nutrient-loss from the upper levels of the soil, and in a light soil the leaching effects can be severe. The direct answer to this problem is to add a cohesive material to the land in an attempt to bind the loose particles into nutrient-conserving granules. In the 18th and 19th centuries it was common to dress light soil with a calcareous clay called marl which, as well as retaining dissolved nutrients, inhibited the oxidisation and consequent loss of organic matter. The lime, or calcium, content corrected the acidity of the leached soil and also enhanced the granule-forming properties of the clay. While marling is rarely carried out today because of high labour and transport costs, liming of all types of land has remained an important feature of the farming year. Irrespective of its effect on the soil, calcium is a major plant food.
Another less permanent means of improving soil structure is organic manuring. For centuries this has been employed as a way of restoring plant nutrients to regularly cropped land, but today, with the nutrients available in a more convenient granulated form, the importance of organic matter as a soil conditioner has become more apparent.
Converted into humus, manure leavens clay and gives body to light soil, provided the quantity is sufficient to balance the effects of oxidisation. Consequently, many farmers periodically top up the humus level in otherwise chemically fertilised land by ploughing in animal manure, weeds and crop remains, temporary pasture or a specially grown ‘green manure’ such as lupins or white mustard. Mature cereal stubble is of little value in this connection, for it is mostly cellulose and not easily broken down; the dead roots, however, can make a useful contribution and modern minimum cultivation systems exploit this by using herbicides and stubble burning to clear the vegetation, leaving the land unploughed. Preserved from the oxidising effects of exposure, the cereal roots are sufficient to maintain the humus level of a moderately well-balanced soil, allowing cereals to be grown year after year without interruption.
Humification (the formation of humus) will only take place if the soil is well-populated with decomposing agents such as soil bacteria, fungi, arthropods and earthworms. In their absence organic matter persists in a barely modified form as peat; plant nutrients remain inaccessible and the soil-conditioning value of the material is greatly diminished. Drainage, manuring and liming measures owe much of their efficacy to the soil life they encourage, which in turn improves the structure of the soil.
Soils and Crops
Not all crops grow equally well on all soils. The chart below is a general indication of which crops suit the different soils.