A. Physical Properties:
The physical properties include texture, structure, and colour.
Soil Texture refers to the particle sizes composing the soil. These particles are classified as gravel, sand, silt and clay in decreasing order of size. Four textural types are recognised. These are sand, sandy loam, loam and clay.
All the textural types are combinations of different sizes of particles. Soil texture determines the water condition of the soil affecting the pore space size. In sand as both the particles and the pore spaces are large, it drains rapidly. The particles and pore spaces in clay are small, hence drainage is very slow. Both are poor for plant growth for which loam texture is best.
The proportions of the different sizes present vary from soil to soil and from layer to layer Standard textural classes can be defined according to the ratio of sand, silt and clay.
On the basis of soil texture soil can be classified into three groups:
(i) Loamy Soil:
It is a mixture in which no one of the three grades (sand, silt and clay) dominates over the other two. It contains 20 per cent or less of clay and 30 to 50 percent of sand. Loam is termed silty loam where silt predominates and clay loam if clay predominates.
(ii) Sandy Soil:
A particular soil whose components has 65 per cent sand, 20 per cent silt and 15 per cent clay is called sandy soil. Sandy loam contains 20 to 50 per cent silt and clay and remainder sand.
(iii) Clay Soil:
It has 30 percent sand and 70% per cent silt and clay. Clay loam has 33.33 per cent sand, 33.33 per cent silt and 33.33 per cent clay.
Soil texture is important because it largely determines the water retention and transmission properties of the soil. In sand as both the particles and the pore spaces are large, it drains rapidly.
The particles and pore spaces in clay are small and hence drainage is very slow. Both are poor for plant growth from which loam texture is best. The intermediate loam textures are generally best as agricultural soils because they drain well but also have favourable water retention properties.
Agricultural soil scientists also use a measure of soil-water storage termed as Wilting point. Soil water in an amount less than the value at the wilting point cannot be absorbed by the plants rapidly enough to meet their needs. At this point the foliage of plants not adapted to drought will wilt. The wilting point depends on the soil texture.
2. Soil Structure refers to the arrangement in which soil grains are grouped together into larger pieces. An individual natural soil aggregate is called a ped. Soil structure is described in terms of the shape, size and durability of peds. Major types of structure are blocky, granular, columnar, prismatic, crumb, and platy. Soil structure influences the absorption of water by the soil, its erodibility, and ploughing.
(i) Platy Structure:
Platy soil structure consists of plates-thin flat pieces-in a horizontal position. It is mostly seen on the surface layers of virgin soils though it may characterize the sub soil horizon as well. This type is often inherited from the parent material especially those laid down by water or rice.
(ii) Primatic Structure:
In prismatic structure, peds are formed into vertical columns, often flat-sided, which may be 0.5 to 10 cm (0.2 to 4 in), across. They usually occur on sub soil horizons in arid and semi arid regions as well as in some poorly drained soil of humid regions when the top of the Prism is, round, it is called Columnar’ and where the top of prism is plane, level and clean, it is called prismatic.
(iii) Blocky Structure:
It consists of angular, equidimensional beds with flattened surfaces that fit the surfaces of adjacent peds. The blunt edges of the cubes is called sub-angular blocky structure’.
(iv) Spheroidal Structure:
It consists of peds more or less rounded in outline with surfaces that do not fit those of adjacent peds. In the granular variety of spheroidal structure the peds are small and the soil is very porous.
Agricultural Importance of Soil:
Soil structure is a physical property of great agricultural importance because it influences the ease with which water will penetrate a dry soil, the susceptibility of the soil to erosion and the ease of cultivation. Soils with a crumb structure are best for seed germination and are said to have a good tilth. One can improve the soil structure by forking and raking and on a large scale by ploughing and harrowing.
3. Soil Colour:
Soil Colour is a minor physical attribute but it is the most readily observed. It indicates the origin and composition of the soil. Increasing quantities of humus produce a range from white, through brown, to black. Black and dark brown colours are typical of soils in the cool and humid areas of temperate latitudes. Soil in the steppe lands and deserts are light brown and grey. Red and yellow colours are quite common and both are due to the presence of iron oxides and hydroxides.
4. Soil Temperature Regime:
Soil temperature is an important factor in determining the characteristics of a soil. Below the freezing point, 0°C (32°F) there is no biologic activity; between 0°C and 5°C (32° and 42°F) root growth of most plants and germination of most seeds is impossible; though water can move through the soil and carbonic acid activity may be important. A horizon as cold as 5°C (41 °F) acts as a thermal barrier to the roots of most plants.
The temperature of the soil is influenced by its colour, composition, slope and water content. Dark soils absorb more heat as compared to lighter soils. Sandy soils absorb and lose heat more quickly than fine-textured soils because the latter retain more water than the specific heat of water is four to five times more than that of soil particles.
5. Soil Density:
The mass of soil per unit volume is called soil density. Soils having larger particles are normally heavier in weight per unit volume than those with smaller particles. Density of soil is of two types: bulk density and particle density.
(i) Bulk Density:
Dry weight of unit volume of soil inclusive of pore spaces is called bulk density. It is calculated as:
Bulk density = Weight of soil / Volume of soil
Bulk density of soil changes with the change in total pore space present in the soil and it gives a good estimate of the property cf soil. Organic soils have low bulk density as compared to mineral soils.
(ii) Particle Density:
It is another type of soil density which excludes the non-solid or pore space fraction of the soil. In other words, density of solid portion of soil is called particle density. It is, in fact, the 3um total of densities of individual organic and inorganic particles. The particle density is calculated as:
Particle Density = Weight of solid portion of the soil / Volume of soil
6. Soil Water Regime:
It is one of the most important elements involved in pedological processes and plant growth. There are three basic forms:
(i) Hygroscopic Water:
Water adhering in thin films (4-5 million micron) by molecular attraction to the surface of soil particles and not available for plants is termed hygroscopic water. It is held at a tension of 31 atmospheres or more.
(ii) Capillary Water:
Water forming thicker layers and occupying the smaller pore space is termed as capillary water. It is held at a tension ranging from 1/3 to 31 atmospheres. Since it is held against the force of gravity, it is permanently available for plant growth and it is this type of soil water which contains plant nutrients in solution.
(iii) Gravitational Water:
Water in excess of hygroscopic and capillary water is termed gravitational water, which is of a transitory nature because it flows away under the influence of gravity. It is free water held at a tension ranging from 1/3 to 31 atmosphere. When the excess has drained away, the amount of water retained in the soil is termed its field capacity, when some of its pore spaces are still free of water. It represents the water held at a tension of 1/3 atmospheres.
7. Soil Porosity:
It is the volume of water which can be held within water which can be held within a rock or soil expressed as the ratio of the volume of the voids i.e. the pores to the total volume of the material, e.g. a material containing pores equal to half its total volume would have a porosity of 50%.
Different soils have different porosity. Alluvial gravel 25-35%; till 20-40%; conglomerate 5-25%; slate 0.001-1%. Where there is high organic content in soils the porosity is high (40-60%) but organic matter is reduced by agricultural cropping, thereby lowering the soil porosity.
B. Chemical Properties:
Chemical Properties of soil include soil water, chemical composition, soil colloids, and humus and soil air. A soil’s chemical composition can be tested only in a laboratory. Some tests for specific purposes, however such as checking acidity are routinely done in the field.
(i) Soil Colloids and Cation (Ion) Exchange:
Clay mineral particles of colloidal dimensions are chemically active in the soil because of their great surface area. A colloid is that amorphous state of a substance that does not form a true solution if mixed with liquid substance.
A colloid is a physical stage of an insoluble substance where it is light enough to remain suspended in water. The colloids may be organic, made up of very finely divided hymns, or mineral, in which case they are referred to as clay minerals.
Together, the two types make up a clay humus complex. Most soils have more clay minerals than organic colloids. The clay minerals are minute thin flakes but are of great importance because they are in a stage of continuous chemical change, which is fundamental to soil formation.
(ii) Base Status of Soils:
As soils are stratified into ‘Status’ levels and are classified into major groups on that basis. Status in soils is determined by the Percentage Base Saturation (PBS) defined as the percentage of exchangeable base cation with respect to the total exchange capacity of the soil.
A value of 35 per cent has been used by soil scientists as a dividing number separating one class of soils as high base status (PBS greater than 35%) from those of another class of low base status (PBS less than 35%). Soils of high base status have high natural fertility for food crops. Base status of soils thus has enormous impact on human food resources.
It is an important chemical constituent of the soil and is the non-living organic matter. It is developed through the slow oxidation of vegetative matter. Humus gives a dark brown or black colour to the soil and its particles hold ions in the soil. In cold humid areas, most soils contain a relatively high humus content and are generally darker whereas in arid areas, little humus is present and soils are light brown or grey.
(iv) Soil Solution:
Both air and water combine to form the soil solution, which comprises the environment for chemical reactions affecting the solid fraction of the soil. The soil atmosphere consists basically of air that enters pore spaces in the soil, diffusing into all interconnected openings. Fluctuations in barometric pressure are believed capable of inducing soil air to move alternately inward and outward resulting in some degree of circulation.
Three of the atmospheric gases present in soil air play an active role in soil processes: molecular oxygen (O2), molecular Nitrogen (N2) and Carbon dioxide (CO2). These active roles require that gases be dissolved in water; neither nitrogen nor oxygen is directly involved in chemical reactions affecting clay minerals and carbonate minerals in the soil.
Carbon dioxide, on the other hand, is of major importance in direct reactions because it combines with soil water to form a weak solution of carbonic acid. Complex organic acids, produced during the decomposition of organic matter, are also important reagents in the soil solution. An acid serves as the active agent in attacking the bonded atoms of the crystal structure of clay minerals.
(v) Acidity and Alkalinity:
As soil water absorbs carbon dioxide from the air, a weak solution of carbonic acid is formed. In addition, soil water absorbs acid materials formed by the decomposition of organic and inorganic matter.
These weak acid solutions react with soluble bases to form insoluble compounds and water. For example, calcium hydroxide (lime) reacts with the carbonic acid to form calcium carbonate (the chief component of limestone) and water. Thus, acid ground water removes soluble bases from the soil. The removal of soluble materials from the surface soil to lower levels by water is called leaching.
Most plants grown best in soils that are neutral neither acid nor alkaline (basic) or nearly so. Acidity usually occurs to some degree in soils of humid regions. Where acidity is high enough to cause trouble, lime is usually added to the soil. Alkaline soils with an abundance of soluble bases are most common in the drier parts of the world, where little leaching takes place.
Acidity and alkalinity are measured in terms of pH, a numerical value ranging from 0 to 14. Soil pH is the most important chemical properties of a soil and is generally related to the concentration of free hydrogen ions in the soil matrix.
Soils with a relatively large concentration of hydrogen ions tend to be acidic. Alkaline soils have relatively low concentration of hydrogen ions. A pH of 7 indicates a neutral soil; values slightly below 7 indicate mild acidity and values slightly above 7 mild alkalinity. Maximum soil fertility occurs at the range of 6.0 to 7.2.
C. Biologic Processes in Soil:
The total role of biologic processes in soil formation includes the presence and activities of living plants and animals as well as their non-living organic products. Living plants contribute to soil formation in two basic ways.
I.e. the production of organic matter the biomass both above the soil as stems and leaves and within the soil as roots. It provides the raw material of organic matter in the O horizon and in lower horizons. The decomposer organisms process this raw material, reducing it to humus and ultimately to its initial components carbon dioxide and water.
(ii) Nutrient Recycling:
It involves the cycling of nutrients from the soil in dead plant tissues. Nutrient recycling is a mechanism by which nutrients are prevented from escaping through the teaching action of surplus soil water moving downward through the soil.
Animals living in the soil play an important role in biologic processes of soil. e.g. earth worms rework the soil not only by burrowing but also by passing the soil through their intestinal tracts. Small tubular soil openings are also formed by many burrowing insects. Large openings are made by larger animals-moles, gophers, rabbits, etc. The soil micro-organisms can be classified into:
(i) Microflora: (i) bacteria (ii) actinomyc actinomycetes (iii) Fungi (iv) Algae
(ii) Microfauna: (i) Protozoans (ii) Nemaloates
Human activity is also a potent agent in influencing the physical and chemical nature of the soil.