How is the Soil Formed?

The following article will guide you about how the soil is formed.

The upper portion of the earth’s crust in which the roots of plants grow is known as soil and is formed by the weathering of various parent rocks.

The soil is disintegrated to smaller and smaller particles by weathering agencies such as climate, by the activity of micro-organisms such as bacteria, by earthworm and also actually by the plant roots themselves.

Rain falling on the earth’s surface takes the more soluble components from the upper layers and carries them below where they may again be redeposited. In the final form, the soil contains in addition to a fragment of the parent rock, much larger quantities of entirely new minerals.

A soil is thus a mosaic of rock particles, minerals, dead and living roots and micro-organisms and the spaces between these various par­ticles are either filled by air or water. Of the numerous components of soils, the two major ones essential for plant growth are the soil solutions and the oxygen carrying air.

A soil is generally composed of particles of wide range of sizes—from the macro­scopic, easily seen with naked eye to microscopic and even smaller, too minute to see even under a microscope.

A soil may be characterised in part by the proportion of particles in each size category which it contains and for convenience individual names have been applied to particles of particular size.

The very large particles are generally referred to as gravel, somewhat finer as sand and still smaller particles as silt and the minutest particles as clay. The distinction of particles among this wide range of sizes has been taken as the basis for the naming of the soil types.

Thus a soil in which the greater proportion of the particles are of very large size are referred to as gravel whereas one in which the minutest-sized particles predominate is known as clay.

To be capable of supporting good growth of plants a soil must contain a readily available supply of all the nutrient elements essential for plant growth in addition to large amounts of easily available water.

The predominant minerals of the soil, originally formed from rocks are generally oxides or other derivatives of silicon, which are scarcely utilised in plant nutrition.

The elements calcium, magnesium and potassium are also in the soil as components of the parts of rocks in small proportions but in those forms they are not available to the plants; they are only made available for plant growth by being adsorbed on the surface of clay particles from which they may pass into solution.

The element phosphorus is present in the soil in a variety of chemical forms, the majority of which are insoluble and hence are unable to provide the plants with the much sought for phosphate ions essentially required for plant nutrition.

So the water-soluble phosphate ions, essential for normal plant growth must be continually formed from less available, somewhat insoluble forms and the phosphate economy of soil is based on a balance between the ability to supply the plant and ability of the insoluble forms which resist washing out by rain.

Owing to this delicate balance, phosphorus deficiency is of wide­spread occurrence under agricultural conditions. The addition of phosphate fertiliser is thus a recognised agricultural practice.

Nitrogen, unlike the nutrients discussed above, is not contained in the primeval rocks from which soils are derived and all the nitrogen content of the soil can, in the last analysis, be traced to the molecular form in the atmosphere which is biologically fixed by micro-organisms in forms generally available to the higher plants.

The red or yellow colours of soil are due to insoluble iron salts, particularly to the more complex iron silicates.

Other nutrients, both the major and the trace elements may be present in the soil either in the soluble form or are precipitated in unavailable forms.

The constant removal of nutrients by plant from the soil by the roots and trans­location and elaboration of these nutrients to form various organic plant products constitute a continued drain on the mineral reserve of the soil.

The fact that mineral elements are removed from the soil by crop plants was first formulated more than hundred years ago by Liebig in 1840 who also suggested that the soil fertility can be re­established and maintained by the artificial addition of manure in the form of inorganic mineral nutrients.

In a vegetation where no crop is removed from the field, several essential elements are continuously cycled, that is they are taken up by the plants, utilised and elaborated inside the plant into various products and with the death of the plants are again returned to the soil ready for further absorption by other plants.

But in our modern agricultural practice, the crop once ready and removed for consumption, is not returned to the soil and thus if the soil has a small reserve of one nutritive element, it constantly requires replenishment.

The nutrients which are in most demand as replenishments in the soil, are nitrogen, phosphorus and potassium. The crop plants require them most for normal growth and development and as a result these are also taken up from the soil in large quantities and a deficiency of nitrogen, phosphorus and potassium frequently occurs in the soil.

Thus most of the modern artificial fertilisers which are added to the soil to maintain the fertility and ensure rich harvests from year to year contain the three elements in different proportions of N, P and K.

As a plant completes its life span and dies, all its tissues normally return to the soil either directly as plant remains or indirectly in the form of dead animal remains and excreta. Thus the soil is constantly kept supplied with organic material and all soils generally contain organic matter in addition to purely inorganic minerals.

These plant and animal remains do not long retain their original state but are rapidly attacked and disintegrated by the soil microflora consisting of many forms of bacteria and fungi.

A portion of the organic matter is utilised by the bacteria and fungi themselves as substrate for their metabolic activities particularly respiration but the bulk of the organic matter is liberated into the atmosphere as CO₂, the end product of microfloral respiration and this may again be available for photosynthesis of green plants.

In the process of decomposition of plant and animal remains, the simple sugar and amino acids are naturally preferentially attacked whereas more resistant constituents such as lignified woody parts are attacked much more slowly.

Thus as the decomposition of the organic material in the soil progresses, the lignified woody parts of the plant re­mains become more and more abundant in the soil.

The brownish or black colours of the soil which are rich in organic matter or humus consist mostly of lignified, woody plant remains and the relatively slowly decomposed organic materials.

The mineral and water resources of the soil are taken by the plants through a ramified system of roots and root hairs which bring the plant into intimate contact with the soil particles surrounding it.

The search for water of the soil by roots is achieved not only by repeated branching of the root as it goes downwards but also by production of unicellular outgrowths from the epidermal cells of roots—the root hairs—which develop in enormous number.

These hairs prodigiously increase the area over which the root is in actual contact with the soil and thus provide the predominant portion of the actual absorbing area of the root system.

The root hairs are certainly the main point of en­trance for water into the plant but the solutes and dissolved minerals are supposed to enter by the actual growing regions of roots themselves. No root hairs are found on the older portion of roots.

The diameters of the root hairs seldom exceed 10µ and in length generally less than 1 mm. Largest root hairs are found in some aquatics such as Trianea (8 mm), Elodea (4 mm) and among the land plants Tradescantia (3 mm), Avena (2.5 mm) and Pisum (2.5 mm) have quite long root hairs.

The extent to which the roots penetrate and ramify in the soil can be gained from the fact that a single wheat plant has been found to possess as much as 64-80 Km of roots if added together while a single rye plant growing in containers (12” X44″) pro­duced about 640 Km of total roots in 4 months of its existence!

In general, roots of her­baceous plants grow more rapidly and profusely than those of shrubs and trees. With most of the cultivated plants in agricultural lands, the roots are mainly concentrated in the upper 4-7 ft. of the soil.

The soil water reservoir may be considered to consist of a vast number of inter­connecting spaces, the spaces between the soil particles; some of these spaces are quite large as between larger particles and granules while others between individual clay particles are minute.

After a shower of rain the soil is at first saturated with water in the larger spaces, after which water drains out and goes downwards obeying the laws of gravity and the empty spaces draw in air from the atmosphere above the soil.

Some water is, however, retained by surface tension as films around individual particles and also in the smaller spaces or capillaries in which the capillary forces are large enough to counteract the forces of gravity.

The root hairs come into contact with the surrounding soil particles whose sur­faces are covered with thin films of water held by various forces including gravity. The plant absorbs from the soil far greater quantities of water than of any other single substance.

A determination of the total water content of a particular soil does not how­ever give us any information about the amount which is actually available for plant growth for most of that water may be retained by the soil particles with so much force that it is practically unavailable to the plants.

Soil air plays an important role in root respiration and maintenance of microbial activity. Roots are strongly aerobic and active respiration is required particularly for uptake of solutes as growth of the root system.

When the soil gets waterlogged, dissolved O₂ is the only source of root respiration. Such waterlogged soils gradually become a micro-aerophilic or almost anaerobic environment. While rice plants may thrive well ever under such conditions, this does not hold true for most other plants.

The main object of tilling the soil, is to make the soil porous and allow fresh air to enter the upper strata of the soil, permitting the accumulated CO₂ to escape. Development of anaero­bic conditions helps the growth of anaerobic micro-organisms some of which are harmful.

Denitrification resulting in the loss of N₂ is also favoured by anaerobic conditions. Microbial activity in the soil is important not only for the fertility of the soil but also to keep the soil porous due to activities resulting in gas exchange.