SOIL OF THE EARTH

Soil is the layer of minerals and organic matter, in thickness from centimetres to a metre or more, on the land surface. Its main components are mineral matter, organic matter, moisture, and air. Soils differ in the ratio of these components.

Modern soil concept

Soil is “the collection of natural bodies in the earth's [sic] surface, in places modified or even made by man of earthy materials, containing living matter and supporting or capable of supporting plants out-of-doors. Its upper limit is air or shallow water. At its margins it grades to deep water or to barren areas of rock or ice. Its lower limit to the not-soil beneath is perhaps the most difficult to define. Soil includes the horizons near the surface that differ from the underlying rock material as a result of interactions, through time, of climate, living organisms, parent materials, and relief. In the few places where it contains thin cemented horizons that are impermeable to roots, soil is as deep as the deepest horizon. More commonly soil grades at its lower margin to hard rock or to earthy materials virtually devoid of roots, animals, or marks of other biologic activity. The lower limit of soil, therefore, is normally the lower limit of biologic activity, which generally coincides with the common rooting depth of native perennial plants” (Soil Survey Staff, 1975).

The “natural bodies” of this definition include all genetically related parts of the soil. A given part, such as a cemented layer, may not contain living matter or be capable of supporting plants. It is, however, still a part of the soil if it is genetically related to the other parts and if the body as a unit contains living matter and is capable of supporting plants.

The definition includes as soil all natural bodies that contain living matter and are capable of supporting plants even though they do not have genetically differentiated parts. A fresh deposit of alluvium or earthy constructed fill is soil if it can support plants. To be soil, a natural body must contain living matter. This excludes former soils now buried below the effects of organisms. This is not to say that buried soils may not be characterized by reference to taxonomic classes. It merely means that they are not now members of the collection of natural bodies called soil; they are buried paleosols.

Not everything “capable of supporting plants out-of-doors” is soil. Bodies of water that support floating plants, such as algae, are not soil, but the sediment below shallow water is soil if it can support bottom-rooting plants such as cattails or reeds. The above-ground parts of plants are also not soil, although they may support parasitic plants. Rock that mainly supports lichens on the surface or plants only in widely spaced cracks is also excluded.

The time transition from not-soil to soil can be illustrated by recent lava flows in warm regions under heavy and very frequent rainfall. Plants become established very quickly in such climates on the basaltic lava, even through there is very little earthy material. The plants are supported by the porous rock filled with water containing plant nutrients. Organic matter soon accumulates; but, before it does, the dominantly porous broken lava in which plant roots grow is soil. (Soil Survey Staff, 1993)

Soil components and processes

Minerals in soil are obtained from a variety of sources, but the process which delivers the bulk is weathering of rocks. Weathering is the actions of wind, rain, ice, sunlight, and biological processes on rocks that break them down into smaller particles.

Weathering also releases ions such as K⁺ and Mg²⁺ into the soil solution. Some of these ions are taken up by plants, but the majority not left in solution are absorbed through ion exchange by clays such as montmorillonite. When the level of ions is low in the soil an equilibrium process forces ions back into solution, where they can be used by plants.

However if acid is introduced into soil, hydrogen ions bind in preference to clays, forcing ions out where they can be washed away during rain. Acidity also encourages the weathering of clays, releasing toxic aluminium ions (of which clays are composed) into the solution. To stop this occurring farmers apply alkalic materials such as slaked-lime.

Although there exist plenty of elements such as nitrogen, potassium and phosphorus necessary for plant growth in soil, very little of this is in a form which plants can use. In processes such as nitrification and mineralisation, bacteria and other organisms convert unusable forms (such as NH₄⁺) in to usable forms (such as NO₃^-). The raw products are initially present as gases in the atmosphere. Processes such as the nitrogen cycle and carbon cycle continually exchange nutrients between the soil and atmosphere.

The organic store in soil is made up of plant debris, animal excreta and other decomposing materials. A lot of the carbon compounds react to form humus, which is composed of very large molecules including esters of carboxylic acid, phenolic compounds and derivatives of benzene. Organic material in soil provides nutrients necessary for plant growth.

If oxygen enters the soil, e.g. due to lowering the ground water table, organic matter in the soil will be oxidised and this may lead to subsidence, also due to increased ground pressure.

References

Suggested addition from Household Cyclopedia of 1881

The following text is taken from the Household Cyclopedia of 1881, and should be updated and integrated with the above article:

Soils consist mostly of sand, lime, and clay, with certain saline and organic substances in smaller and varying proportions; but the examination of the ashes of plants shows that a fertile soil must of necessity contain an appreciable quantity of at least eleven different substances, which in most cases exist in greater or less relative abundance in the ash of cultivated plants; and of these the proportions are not by any means immaterial. In general, the soils which are made up of the most various materials are called alluvial; having been formed from the depositions of floods and rivers. Many of them are extremely fertile.

Soils consist of two parts; of an organic part, which can readily be burned away when the surface-soil is heated to redness; and of an inorganic part, which remains fixed in the fire, consisting of earthy and saline substances from which, if carbonic acid or any elastic gas be present, it may, however, be driven by the heat. The organic part of soils is derived chiefly from the remains of vegetables and animals which have lived and died in and upon the soil, which have been spread over it by rivers and rains, or which have been added by the industry of man for the purposes of increased fertility.

This organic part varies much in quantity, as well as quality, in different soils. In peaty soils it is very abundant, as well as in some rich, long cultivated lands. In general, it rarely amounts to one-fourth, or 25 per cent. even in our best arable lands. Good wheat soils contain often as little as eight parts in the hundred of organic animal or vegetable matter; oats and rye will grow in a soil containing only 1 1/2 per cent.; and barley when only two or three parts per cent. are present.

The inorganic portion of any given soil, again, is divisible into two portions; that part which is soluble in water, and thus easily taken up by plants, and a much more bulky portion which is insoluble.

Sir Humphry Davy found the following to be the composition of a good productive soil.

Thus the whole amount of organic matter in this instance is only 1 part in 200, or one-half of one per cent.; a fact which, in itself, would demonstrate the fallacy of supposing that decomposed animal and vegetable matter in the soil form the exclusive supply to growing plants.

Clayey soil

A clayey soil, though distinguished by the color which it bears, namely black, white, yellow and red, differs from all other soils, being tough, wet, and cold, and consequently requiring a good deal of labor from the husbandman before it can be sufficiently pulverized, or placed in a state for bearing artificial crops of corn or grass. Clay land is known by the following qualities, or properties.

It holds water like a cup, and once wetted does not soon dry. In like manner, when thoroughly dry, it is not soon wetted; if we except the varieties which have a thin surface, and are the worst of all to manage. In a dry summer, clay cracks and shows a surface full of small chinks, or openings. If ploughed in a wet state, it sticks to the plough like mortar, and in a dry summer, the plough turns it up in great clods, scarcely to be broken or separated by the heaviest roller.

Clayey soils, when sufficiently enriched with manures, are naturally well qualified for carrying crops of wheat, oats, beans, and clover; but are not fitted for barley, turnips, potatoes, etc., or even for being kept under for grass longer than one year.

Such soils ought to be regularly summer-fallowed once in six, or at least once in eight years, even when they are comparatively in a clean state, as they contract a sourness and adhesion from wet ploughing, only to be removed by exposure to the sun and wind during the dry months of summer. Soils of this kind receive little benefit from winter ploughing, unless so far as their surface is thereby presented to the frost, which mellows and reduces them in a manner infinitely superior to what could be accomplished by all the operations of man. Still they are not cleaned or made free of weeds by winter ploughing; and therefore this operation can only be considered as a good means for producing a seed-bed, in which the seeds of the future crop may be safely deposited. Hence the necessity of cleansing clay soils during the summer months, and of having always a large part of every clay farm under summer fallow. All clayey soils require great industry and care, as well as a considerable portion of knowledge in dressing or management to keep them in good condition; yet when their natural toughness is got the better of, they always yield the heaviest and most abundant crops.

One thing requisite for a clayey soil, is to keep it rich and full of manure; a poor clay being the most ungrateful of all soils, and hardly capable of repaying the expense of labor, after being worn out and exhausted. A clayey soil also receives, comparatively, smell benefit from grass; and when once allowed to get into a sterile condition, the most active endeavors will with difficulty restore fertility to it after the lapse of many years.

Upon light soils the case is very different. These flourish under the grass husbandry; and bare summer fallow is rarely required, because they may be cleaned and cropped in the same year with that valuable esculent, turnip. Upon light soils, however, wheat can seldom be extensively cultivated; nor can a crop be obtained of equal value, either in respect to quantity or quality, as on clay sand loams. The best method of procuring wheat on light lands, is to sow upon a clover stubble, when the soil has got an artificial solidity of body and is thereby rendered capable of sustaining the grain till it arrives at maturity. The same observation applies to soils of a gravelly nature; and upon both barley is generally found of as great benefit as wheat.

Thin clays and peat earths are more friendly to the growth of oats than of other grains, though in favorable seasons a heavy crop of wheat may be obtained from a thin clayey soil, when it has been completely summer-fallowed and enriched with dung. A first application of calcareous manure is generally accompanied with great advantage upon these soils; but when once the effect of this application is over, it can hardly be repeated a second time, unless the land has been very cautiously managed after the first dressing. Neither of these soils is friendly to grass, yet there is a necessity of exercising this husbandry with them, because they are incapable of standing the plough more than a year or two in the course of a rotation.

Wheat ought to be the predominant crop upon all the rich clays and strong loams, and light soils of every kind are well qualified for turnips, barley, etc. Upon the thin and moorish soils, oats must necessarily preserve a prominent rank, and grass seeds may be cultivated upon every one of them, though with different degrees of advantage, according to the natural and artificial richness of each soil, or to the qualities which it possesses for encouraging the growth of clover, in the first instance, and preserving the roots of the plant afterwards.

Modern soil concept

Soil components and processes

References

Suggested addition from Household Cyclopedia of 1881

Clayey soil

Suggested reading