Soil Formation - Complete Guide - Factors and Process

Soil Formation

Soils are those friable portions of the Earth's surface which can be tilled and are capable of supporting vegetation. Soil forms a superficial layer of variable thickness. By digging down a certain depth unchanged rock material is invariably found. By disintegration and decomposition this rocky material can be converted into soil material. We may take it as certain that at one time in the world's history soils did not exist as we see them now. Their formation has been gradual. Water has been a powerful agent in all ages not only in directly forming soils, but also in the formation of those stratified rocks which when exposed to weathering influences yield by disintegration soil material.

Rain-water when it falls upon the earth either soaks into it or flows over its surface. It drains off the surface of land first in little streams. These join and form larger streams, which as they course onwards increase in volume by the inflow of their tributaries. Finally, they empty themselves into lakes or into the ocean. Running water has a very powerful effect in changing the earth's surface. Wherever it flows it furrows out for itself channels more or less large. In whatever direction it moves, it carries with it the material which it wears away from rocks or removes from existing soils. It is therefore called a denuding agent. Its power in this respect can be noticed by observing what goes on around us.

The denuding of flowing water

In India we have a rainy season- a season, when a considerable rainfall is experienced in a short period. On the Western Ghats of the Mumbai, often one hundred inches of rain fall in a few months. This represents approximately 10,000 tons of water on each acre of surface. This water as it drains rapidly down the slopes and steep precipices of these hills carries with it much denuded material. 

We may notice at the time that the water is obviously discolored and if we take the trouble to examine some of these hill-sides later on, say in the hot weather when there is no rain and very little drainage, we shall find the exposed face of the hard rocks a network of grooves and furrows which have been cut out by the wearing action of running water. In the rains most of our rivers are in flood and their waters are muddy. 

In the hot season, however, they are nearly dry. An exception to this rule is the Indus which begins to rise in the hot season, because it is fed by the snow of the Himalayas which begins to melt when the hot weather sets in. Where a river overflows its banks an appreciable deposit of fine mud will be found on the flooded land when the water recedes. Similar deposits may have been formed annually and we can well understand, how a soil layer of considerable thickness would accumulate in the course of years. Where the water runs slow, the mud is deposited in the greatest degree. 

Sluggish rivers flowing into an ocean form Deltas - the Ganges and Brahmaputra are examples. If the river flows quick the material is carried out to sea and deposited on the ocean floor: The Amazon is an example. If the material is deposited as a superlayer above existing soil, au alluvial soil is being added to ; if in the bed of a lake or in the ocean, stratified rocks are being formed.

Soils produced from three distinct classes of rocks through weathering influences.

The actual formation of stratified rocks pertains more to Geology than to Agriculture. It is sufficient to know that they have been formed by the action of water, and that by the latent heat of the earth they have been upheaved and now form part of the dry land. If, instead of being upheaved, they were depressed and subjected to the influences of the heat which we know exists at the center of the earth, they become somewhat modified, lose their stratified appearance, and become more or less crystalline in structure and are called metamorphic. 

These upheaved to the surface by volcanic action now form part of the earth's crust. We have therefore in the present age three distinct types of rocks forming the earth's surface which when subjected to certain weathering influences, that will he referred to presently, yield by disintegration soil material.

The three classes of rocks are :-

1. Primitive or igneous -  Granite.

2. Metamorphic - Marble.

S. Stratified; e.g. Limestone.

Sedentary and  transported soils

Soils may be of two kinds, sedentary and transported. A sedentary soil is formed from the underlying rock and resembles it in character and composition. Transported soil has been carried by the transporting power of water or wind and deposited on a rock which it need not necessarily resemble.

Minerals of agricultural importance

In whatever manner the soil has been formed its fertility depends greatly upon the mineral fragments which it contains ; therefore the composition (chemically) of the rock or rocks from which it has been formed is of prime importance. The minerals of agricultural importance found in rocks are: Felspar, hornblende, quartz, mica, tale, carbonates of lime and magnesia, and others of minor importance.

Natural forces which cause disintegration.

Rock disintegration and soil formation are attributable to certain forces of nature, which have been in operation during all ages and are actively at work now. These are :-

1. Water in its various forms.

2. The atmosphere.

3. Variations in temperature.

4. Vegetable and animal action

5. Volcanic action.

Denuding and solvent effect of water

Taking these in the order named, we have to notice the action of water in other respects than those already referred to, We have already seen how sluggish streams form deltas, and deposit alluvial beds along their flooded banks. All running water contains sediment in suspension not always observable by the naked eye. But nevertheless river water that appears clear is actually not so. The material held in suspension represents the effect of the denuding power of water. 

Water exercises also a solvent effect on rock and soil material. If common salt is mixed with Water the salt disappears and becomes dissolved in the water. Water exercises a similar effect on certain ingredients found in rocks and particularly so, if it contains oxygen or carbonic acid gas.

Effect of waves and tides

The waves of the ocean beating on a shore are powerful denuding agents. Angular pieces of rock may be broken off, and these when rolled about by an angry sea are smashed up into small fragments and rounded into water-worn pebbles, grayel and sand. The ebb and flow of the tides carry away and deposit this denuded material elsewhere, often in the bed of the ocean, but sometimes the dry land or shore is being added to.

The effect of freezing of water

Water is an exception to an almost universal law in nature, viz, when bodies are heated they expand, when cooled they contract. Water, however, contracts as it is cooled until it reaches the freezing point. In the act of freezing it suddenly expands about 1/10th of its bulk. Rain or water will soak into the substance of a rock, and fill up all cracks and fissures which there exist. In winter time in most countries~ the temperature is often below the freezing point, and the water which has found its way into the substance of a rock is turned into ice and suddenly expands with irresistible force. 

The particles of the rock are disrupted and the natural fissures forced further asunder. The full effect does not appear until the weather gets warmer and the ice melts. Then fragments, more or less bulky, fall away, and loosened particles of smaller size are, at the same time, conveyed away by the water which trickles from the rock.

The effect of icebergs

Icebergs are usually seen as vast blocks of ice floating in mid-ocean. They are nowadays, as they have been throughout long ages, carriers of soil material. As we see them in the ocean only about 1/3rd of their substance appears above water. They are each of them weighed down by a huge load of boulders, gravel, sand and earth. Each iceberg once formed a mass of ice attached to a shore in the Arctic or Antarctic regions, and was driven on to the beach by the action of the sea. 

In this way the boulders anu earth-material became incorporated with the ice. This would occur during winter. But a short summer would come once a year when the ice becoming partially melted would; by the ebb of the tide, be carried out to sea to be conveyed towards the Equator by ocean currents, and in a warmer temperature the iceberg would gradually melt and discharge its load.

The effect of Glaciers

Glaciers are ice rivers formed in mountainous regions of perpetual snow. These ice rivers travel slowly carrying with them much denuded material, scratching and polishing the rock surface over which they move. At some point in the valley below, the temperature is above the freezing point and there the ice is melted and the disintegrated rock-fragments are further transported by the stream which here has its origin. The Indus is fed by the melted snow and ice of the Himalayas. 

This river is in flood in the hot weather because the mean temperature is higher in the Himalayas at this season than at any other, and consequently the snow except in the higher peaks disappears. The hill streams carry down enormous quantities of denuded material of which more or less is conveyed finally by the Indus to be deposited upon the vast stretches of cultivated land which that river annually inundates, and also upon the lands which the Indus canals irrigate. The alluvium thus added to a pre-existing soil helps to enrich it.

Wind

Soils transported through the agency of wind are common on exposed sea-coast districts. The loose sand of the shore is blown inland to form a low ridge as seen on the flat coasts of Surat and Bharuch.

Effect of variations of temperature

Most bodies expand when heated and contract when cooled. But different substances contract and expand in different ratios when exposed to the same temperature. Thus the various minerals which compose the mass of a crystalline rock expand and contract in different proportions. The component particles are disturbed and disintegration follows.

Cracks in existing soils

The effect of extreme high temperature is exemplified by noting the cracks that form in existing soils in India as well as other countries. Some of our black and heavy clay soils are said to "plough themselves" through this agency. And we know that the subsoil is thereby exposed to the action of the atmosphere and other weathering influences so that further disintegration follows. 

Effect of difference between day and night temperatures

The mean difference between day and night temperature in India is considerable. Rocks of a homogeneous character like sandstone, limestone or trap, subjected to extreme variations of temperature, split and have their structure shaken. A man who has to quarry stones knows this fact and takes practical advantage of it. When blasting powder is not available he heats the rock by burning cheap fuel thereon. When sufficiently hot, water is poured on the heated surface. This occasions sudden contraction which causes the rock to split and a crow-bar let into the cracks will remove the loosened fragments .

Vegetable action

A primitive soil is only at first capable of supporting plant life of a very inferior description, such as mosses, lichens and the like. On the face of cliffs you will find these growing. But soils that have sufficient depth to hold moisture are capable of maintaining vegetation of a higher kind. The growth of vegetation particularly of large trees, exercises a disintegrating influence on soil or rock material, because the roots force themselves through the soil and subsoil, and even through fissures of underlying rock. As the roots grow the rocky material is forced asunder and broken up into fragments. 

The decay of vegetation adds to the fertility of a soil, because what is termed humus is being formed. This organic portion of the soil consists of the decayed and decaying roots and stems of plants along with the decomposed remains of insect and animal life which has existed in the soil. The fertility of the soil and the maintenance in it of moisture is intimately associated with the presence of a fair proportion of organic matter.

Peat soils

Some soils, however, are found in nature in which the percentage of organic matter is excessive. Such soils are known as peats. The percentage of organic matter in pure dry peat is often as high as 80. By reclamation such land may be made productive. But in its natural state it is surcharged with water. In fact, it probably owes its existence to some interruption of the natural drainage of the district. Clay often underlies peat. It was the pre-existing soil.

When the clay soil became water-logged, a condition favorable to the growth of the peat producing plants was induced. This family of plants are known as sphagnums. They have one peculiar characteristic. New shoots will grow while the lower extremities of the plants are dying. Thus spungy beds are formed often of great depth, but usually from four to eight feet deep. 

The reason why peat is found in beds of such depth is that its organic matter has never been brought under the influence of decay. Water fills the pores of peat, consequently the oxygen of the air cannot enter them and therefore the decay or oxidation of the organic matter cannot proceed. 

In the absence of oxygen other changes of the organic matter have taken place, and unreclaimed peat is in consequence soured and poisoned by an excess of organic acids detrimental to cultivated plants. Peats although common in the higher altitudes und cooler parts of Europe are not usually met with in India. True peats are, however, found in the Neilgherries and the bheel soils, or bottom tea soils of Assam are of a peaty character. 

The bheel soils probably cave their origin to the accumulation in the lower valleys of decaying forest growth which has been washed down from the uplands. Unimproved bheel soils are found in a water-logged condition. By drainage and tillage these soils become exposed to the action of the atmosphere when the high percentage of organic matter soon disappears through oxidation or natural decay.

Animal actions (earthworm)

Animal action is of some importance in soil formation and particularly in preparing soluble plant food from the mineral matter of an already existing soil. In most countries, earthworms in this respect perform an important function. Leaves, manure or other organic matter are carried below the surface by earthworms and become thus incorporated with the soil. Moreover, the food of earthworms consists of vegetable matter which with the assistance of grains of sand and earth particles, they masticate and digest in their intestines and when the nutriment has been extracted expel the refuse as "castings." 

These castings may be seen in small heaps at the mouths of the burrows of earthworms. Darwin estimated that 10 tons of fine earthy matter, per acre, annually was brought up to the surface of rich, grass lands 'of England by earthworms and left there as castings. The burrows of moles, earthworms, give free entrance to the subsoil of air and water so that disintegration of the substrata is accelerated.

Ants

In hot climates ants bring up to the surface immense quantities of fine earth to build their hills, and underground tunnels ramify in every direction therefrom. The nests of white ants found in black soil are constructed from murum carried up from the limestone bed below. In this way lime in the finest possible state of division becomes to a certain extent mingled with "black" soil.

Volcanic action

Active volcanoes throw out gases, ashes and lava. Lava soon cracks and pulverizes into a friable fertile soil. The vineyards and olive plantations on the slopes of Mount Vesuvius prove the fertility of recent lavas.