Soil Components and their effects on Vegetation

Earth is a spherical planet of the solar system. It is differentiated into three main layers—crust, mantle and core.

The core is the central portion of the earth containing nickel and iron in molten or vaporized state. This is about 3470 km from the centre.

The mantle is the middle layer of the earth which extends about 2900 km above the core. The mantle is in molten state. The crust is the outermost solid zone of the earth which is about 9 to 40 km thick.

The solid component of earth is called lithosphere which is not a uniform layer. It is composed of different kinds of rocks, which as a whole has a composition and density of granite rock. The layer lining the floor of continents is granite layer and the layer of rock lining the floor of ocean is basaltic. The crust is very complex in composition and its surface is covered with the soil which supports rich and highly diversified biotic communities (Fig. 2.9). It contains the continents and the oceans, the soil and the sediments, minerals and water.

The soil is one of the most important ecological factors called edaphic factors. It is the most characteristic feature of the terrestrial environment. It is the reservoir of biogenic salts and minerals which are essential for the living organisms. It is the outermost layer of earth’s crust and is being constantly formed through weathering of rocks. It is not only a factor of the environment but also a product of the organic activities causing the biological weathering of rock.

Soil is defined as the unconsolidated top or superficial layer of earth’s crust lying below any aerial vegetation and un-decomposed dead organic remains and extending down to the limits to which it affects the plants growing about its surface. Beneath the soil, lie the subsoil and un-weathered rocks.

Edaphic factors are those which are dependent on the soil as such—on soil constitution, soil water, soil air, soil organisms and so forth. Soils at different places vary considerably in their structure, components and properties. These differences in the soils are often largely responsible for differences in vegetation within the same climatic region, and consequently they are of great significance in the distribution of plant communities.

Soil is a stratified mixture of inorganic and organic materials. The inorganic or mineral constituents of soil are derived from some parent materials, the soil forming rocks by fragmentation or break-down or weathering and the organic constituents of the soil are formed either by decomposition of dead remains of plants and animals or through metabolic activities of living organisms present in the soil.

Weathering of rocks is the initial steps of soil forming process which is brought about by:

(i) Physical factors like temperature, water, ice, gravity and wind,

(ii) Chemical factors like solvent action, hydrolysis, oxidation, reduction, carbonation and hydration, and

(iii) Biological factors.

Soil which is formed from the products of weathering processes is called embryonic or primary soil.

It may mature into the following types of soils:

(i) Residual or sedimentary soil:

This is the mature soil lying immediately over the parent rocks.

(ii) Skeletal or immature soil:

This is only partly weathered material lacking maturation.

(iii) Transported or secondary soil:

The transported soils are those in which the parent materials have been shifted to different places by the agency of glaciers, streams and rivers (Alluvial soil) the gravitational forces as landslides (Colluvial soil), wind (Aeolian soil), sand storms (sand dunes), standing water and wave action (Lacustrine soil) and oceanic waves (marine soil).

The second important process involved in the soil formation includes addition of organic matter, humification and mineralization. Organic matters are added to the embryonic soil by various living organisms. Plants and animals living in and on the soil after their death are decomposed and the decomposition product is mixed with the soil. All organic plant debris fallen recently to the ground is called litter.

The litter is composed of dead leaves, twigs, wood, dead roots and various plant products. Just below the fresh litter often occurs the material derived from preceding season’s litter in which decay or microbial decomposition has set in. This is called duff. The litter is decomposed by soil microbes such as bacteria, actinomycetes and other fungi.

The products of decomposition include various types of inorganic and organic plant nutrients. They are all incorporated into mineral particles which then become dark in colour. The residual finally divided amorphous, incompletely decomposed black coloured organic matter added to the mineral matter of the soil is called humus and the process of its formation is referred to as humification.

Humus includes two types of organic matter the partially decomposed organic matter derived from litter and excreta of soil animals like centipedes, millipedes, earthworms, mites, grass-hoppers, etc. which feed on the litter of plant material. Gradually the humus is completely decomposed into simple compounds like carbon dioxide, water and minerals salts by a process called mineralization.

Depending upon the organic content, soils are generally classified as follows:

(i) Mineral soils which are rich in mineral particles.

(ii) Peat and muck which are rich in organic matter in wet areas.

(iii) Mors which are low in basic minerals.

(iv) Mulls which are rich in base contents.

Muller (1879, 1884) has recognized two kinds of humus mor and mull. Mor humus is acidic, abounds in fungi and is low in bacterial content. Fungul mycelia may help to bind together particles of humus and decomposing litter into matted layers.

A well developed mor can be differentiated into the following three layers:

1. Surface litter or L-layer composing un-decomposed leaves and twigs.

2. Fermentation or F-layer in which decomposition has proceeded some way towards development of humus. This layer is just below the L-layer.

3. Humus or H-layer which is found just below the F-layer. In this layer degraded humus fraction is accumulated. Mor is deficient in calcium and it develops on sandy soil under conifers. Mull is, on the other hand, slightly alkaline or neutral and abounds in bacteria. It does not show layer differentiation because of abundance of earthworms which promote mixing of organic and mineral materials. Intermediate between two types of humus is moder which has a richer and varied fauna.

Soil Profile:

Soil profile is the term used for the vertical section of mature soil up to the parental material to show different layers of soil. Soils commonly become stratified into layers or horizons at different depths. The layers of soil at different depths show different compositions and natures.

Normally three main horizons or groups of horizons can be recognized. These are the upper horizons or horizon, the middle horizon or zone and ‘C horizon (Fig. 2.10). A and B horizons collectively constitute true soil. Some workers recognized an additional horizon called organic or ‘O’ horizon above the A horizon, and D horizon below C horizon. These horizons are often divided by the use of appropriate subscripts.

‘O’ horizon:

The upper most layer of soil profile is called O horizon. It is also called litter zone or Organic zone. O horizon is present in forest soil but absent in the soil of deserts, grasslands and cultivated fields O horizon can be divided into two sub-layers normally A₀₀ or O₁ horizon and Ao or O₂ horizons.

A₀₀ or O₁ horizon:

It is the top layer formed by fallen leaves and things on the forest floor which are un-decomposed.

A₀ or O₂ Zone:

A₀ or i₂ horizon lies below the A₀₀ or O₁ horizon. It is a layer of partly decomposed organic matter; the upper portion which is partially decomposed is called the duff and the lower part is completely decomposed and the organic structures have lost their identity to form black amorphous material called humus.

‘A’ horizon:

‘A’ horizon designates the top stratum which is subjected to marked leaching. It is a layer of greatest biological concern as the plant roots, small animals, and micro-flora and fauna are found here most densely. In this zone the concentration of the organic matter is highest, and hence it is the dominant reservoir of plant nutrients. It is divided into three subzones which are denoted by A₁, A₂ and A₃. These can be distinguished by distinct colours, textures and organic contents.

A₁ horizon:

In this rich humus is mixed with minerals. It contains fungi & bacteria in abundance. It is dark coloured.

A₂ Horizon:

It is light coloured zone with less humus and maximum leaching of silicates, oxides of iron and aluminium.

A₃ Horizon:

It is transition zone between A and B horizons, slightly darker than O horizon. The B horizon or the subsoil lying under A horizon has little organic matter, very few plant roots and a sparse micro-flora and fauna. In it, iron and aluminium compounds are often accumulated. It is also divided into B₁, B₂ & B₃ zones. A and B horizons collectively represent the true soil.

C horizon:

At the bottom of B horizon is the C horizon which contains the parent materials of the soil. In this layer the organic matters are present in small amounts and little or no life is noted.

The essential components of most garden soils are as follows:

(i) Mineral particles of variable sizes obtained by weathering or breakdown of the rocks 40—50% by volume and 95% by weight.

(ii) Soil water: 5.30%;

(iii) Soil air or soil atmosphere: 25-30%;

(iv) Organic matter or soil humus arising from the death and decay of the parts of plants and animals or added manures: 1 to 5% by weight.

(v) Soil organisms, including both soil flora and soil fauna, as for example, protozoa, nematodes, earthworms, bacteria, fungi, algae, etc. Fig. 2.11 gives a rough idea about the percentages of various components of garden soil.

The important edaphic factors which affect the vegetation are as follows:

(i) Soil moisture;

(ii) Soil reactions;

(iii) Soil nutrients;

(iv) Soil temperature;

(v) Soil aeration or soil atmosphere; and

(vi) Biotic components of the soil.

(i) Soil moisture:

Plants absorb a small quantity of rain water and dew directly but they take a large quantity of water from the soil.

Water held in the soil is found in the following forms (Fig. 2.12):

(i) Gravitational water;

(ii) Capillary water;

(iii) Hygroscopic water;

(iv) Water vapour;

(v) Combined water.

Gravitational water is free water which percolates downwardly through the pore spaces between soil particles and accumulates in the pore spaces in the form of ground water. The amount of water present around the soil particles and held by surface tension and attraction force of water molecules is called capillary water. Capillary water remains readily available to the roots up to a certain soil moisture tension.

Water which is adsorbed on the soil particles and held on the surface of particles by forces of attraction and cohesion of its molecules is called hygroscopic water. This is the moisture remaining in air-dry soil. It cannot be used by the plants. The soil atmosphere, like external atmosphere, also contains moisture in the form of water vapour. Water of chemical compounds is called combined water.

Total water content of the soil is called holard. Water of the soil which is easily available to plant is termed as chresard and water which is so strongly held by the particles as to be unavailable to the plants is termed as echard. The availability of soil moisture is influenced by many conditions, such as distance of water table from the soil surface, the rate at which water percolates downward, the sizes of soil particles, the amount of annual rainfall, and the distribution of precipitation throughout the year.

Water tends to promote the stratification of soil. Soil’s available water is the chief factor responsible for local differences between plant communities. Heavily water logged soil is injurious for the growing plants because heavy accumulation of water in the soil reduces the soil aeration. Low water content in the soil is also injurious because it causes either temporary or permanent wilting of plants. Soil water is not only important in connection with direct fulfillment of water requirements of plants but it is also the medium by which mineral salts essential in the nutrition enter the plants in dissolved state.

(ii) Soil reaction:

This edaphic factor influences the growth and distribution of the plants. The soil may show acidic, alkaline, or neutral reactions. The growth and productivity of many species of plants are critically related to soil acidity. Species of Rhododendron, cranberries are acid loving. Most of the field crops, such as barley, maize, soybeans, tomato, rye, potato, flourish in slightly acidic soils.

Many ferns and beech trees thrive best in slightly alkaline soils. Soil acidity is important in many ways; in the behaviour of soil solutes, and in the relation of roots to the soil. Soil acidity affects the availability of iron, manganese, phosphate and other ions. In the acid soils, iron and manganese are available in appreciable quantities, but in the neutral or alkaline soils they are available in meagre quantities to green plants.

The accumulation of calcium, sodium and magnesium salts in the soil results in alkalinity. The reaction of soil influences the absorption of water and soil solutes by roots through its direct effect on inhibition and permeability of root cell membranes.

(iii) Soil nutrients:

The nature and availability of soil solutes are fundamentally important from the standpoint of plant nutrition. Normally, inorganic solutes are absorbed by the plants in the ionic forms. Different species of plants, no doubt they require the same ions for their normal development, require them in varying quantities. In saline soil where the percentage of salt is high, only halophytes (salt loving plants) grow. Some plants require lime and grow in calcium rich soils. Such plants are called calcicoles or calciphytes. Some plants do not thrive well when they grow in calcium rich soil. They are called calcifuges or oxylophytes.

Humus, a dark amorphous substance formed by partial degradation of dead organic remains is an important source of mineral and organic nutrients of plants. The fertility of soil is usually correlated with its humus content. Humus is the main source of nutrients for soil micro-organisms and green plants.

(iv) Soil temperature:

Soil temperature in combination with other edaphic factors influences the properties of soil itself and plants as well. Low temperature reduces the rate of water and solute absorption by roots. Root injury due to low temperature in the winter is more common in sandy soil than in clay.

Soil temperature is also important in the sense that it plays important role in determining the geographical distribution of plants on the earth. Soil temperature is affected by air temperature, the intensity of sunlight, the angles at which sun rays strike on the surface of soil, daily duration of sunlight, the amount of soil moisture and many other factors.

(v) Soil atmosphere:

In the soil, the spaces left between soil particles are called pore spaces. These spaces contain air. Soil air contains slightly lower proportion of oxygen and higher one of CO₂ than atmospheric air contains. Water logged soils are deficient in oxygen. Normally, plenty of oxygen for the soil is necessary for the life of micro-organisms and other soil inhabitants. It is also necessary for the respiration of underground parts of higher plants. Oxygen content of the soil is also an important factor in seed germination. The germinating seeds respire rapidly and usually they require large amount of oxygen.

(vi) Soil organisms:

The plants, animals and microbes inhabiting the soils show marked effects on the soil fertility. Decomposing agents, such as bacteria, fungi, and many others convert dead organic matters into humus, free organic compounds and organic ions and thus make the nutrients available to plants. Some soil organisms secrete essential or beneficial substances including growth hormones and some of them secrete toxic substances in the soil which show marked effects on the growth and distribution of plants.

Some of the blue green algae, like Nostoc, Anabaena, Cylindrospermum are beneficial to the higher plants because they fix atmospheric nitrogen into nitrogenous compounds that are utilized by the higher plants. Singh (1961) found that algal films that developed in paddy fields of Uttar Pradesh and Bihar were active nitrogen fixing blue green algae.

The animals of burrowing habit also play important role in the soil by turning over the soil. Earthworms increase the fertility of soil by adding excretory matters to it and also by making it loose. Local distribution of plant communities in different regions also brings about changes in the composition and fertility of the soil in those regions.