Soils

Soils may be defined as a thin layer of earth's crust serving as a natural medium for the growth of plants. It is the unconsolidated mineral matter that has been subjected to, and influenced by, genetic and environmental factors, climate, organisms and topography all acting over a period of time. Soil differs from the parent material in the morphological, physical, chemical and biological properties. Some soils are red, some are black: some are deep and some are shallow; some are coarse-textured and some are fine textured.. The components of soils are mineral material, organic matter, water and air, the proportions of which vary and which together form a system for plant growth.

Soil-Forming Materials

There are three main kinds of rocks:

1) igneous rocks

2) sedimentary rocks

3) metamorphic rocks

 Igneous rocks: They are formed by the cooling, hardening and crystallizing of various kinds of lavas and differ widely in their chemical composition. They contains feldspars, maphic minerals and quartz. Rocks containing a high proportion of quartz (60-75%) are classified as acidic, whereas those containing less than 50% quartz are classified as basic.

Sedimentary rocks: They are derived from igneous rocks and are formed by the integration of fragmentary rock materials and the products of their decomposition deposited by water. The common sedimentary rocks are conglomerate, sandstone, shale and limestone. Alluvial, glacial and aeolian deposits form the unconsolidated sedimentary rocks.

Metamorphic rocks: They are formed from the igneous or sedimentary rocks by the action of intense heat and high pressure or both resulting in considerable change in the texture and mineral composition. The common metamorphic rocks are granite, quartzite from quartz or sandstone, marble from limestone and slate from shale.

Particle shape and size: The soil particles vary in shape from spherical to angular. They differ in size from gravel and sand to fine clay.

Textural classes

The principle textural classes are: Clay , sandy clay, silty clay, clay loam, sandy clay loam, silty clay loam, loam, sandy loam, silt loom, sand, loamy sand and silt.

The physical properties and the chemical composition of the small and large particles differ greatly. The coarse fraction, gravel and sand which are mainly composed of rock fragments or primary minerals act as individual particles. They have low specific surface and are relatively non-reactive. They do not hold large amounts of water or nutrients.

The silt particles are intermediate between sands and clays. Mineralogical, the particles of silt are similar to those of sand, as they are largely composed of primary minerals. They are more reactive than sands because of the higher specific surface.

The clay fraction controls most of the important properties of the soils. They are chiefly composed of secondary minerals-crystal line alumino silicates.

The Textural classes differ not only in the particle analysis but also in their bearing on some of the important factors affecting plant growth, such as 1) the movement and availability of water, ii) aeration, iii) workability, and (iv) the content of plant nutrients.

The moderately fine textured soils, e.g. loams, clay loams or silt loams are by far the excellent soils for crop growth, since they have the advantages of both sands and clays.

Color:

Red, yellow or brown colors are mostly related to the different degrees of oxidation, hydration and diffusion of iron oxides in the solids. Darks colors of soils are associated with one or a combination of several factors, including impeded drainage conditions, content and state of decomposition of organic matter, the presence of titaniferous magnetite.

Munsell chart consists of coloured chips arranged according to hue, value, and chroma, the three simple variables that combine to give the color. The hue refers to the dominant spectral color, the value to relative lightness of color and chroma to relative purity.

Density:

Soils having larger particles are usually heavier in weight per unit volume than those with smaller particles. True density of a soil is based on the individual densities of soil constituents and according to their proportionate contribution.

The bulk density or a apparent density is the weight per unit volume of dry soil as a whole i.e. particle and pore space, and hence it is lower than the true density.

Pore space:

The pore space of soil is the portion occupied by air and water, and it is determined largely by structural conditions. Sands have low pore space of about 30 percent, whereas clays may have as much as 50-60 percent. Good loams may have pore space of 40-50 percent

Plasticity and cohesion:

Plasticity is the property that enables a moist soil to change shape on the application of force and retain this shape even when the force is withdrawn. So, the sandy soils may be considered to be non-plastic and clayey soils to be plastic. Plastic soils are cohesive.

 

Soil temperature and heat:

It controls the microbiological activity and all the processes involved in the growth of plants. The temperature needed for seed germination, root growth and other biological activities varies from crop and varieties. Microbiological activities are retarded by low soil temperature. As a result, the nitrification processes is slowed down and plant nutrition and growth adversely affected.

Temperature of the soil is influenced by its color, composition, slope, aspect and water content. Dark soils absorb more heat than those of lighter hues. Sandy soils absorb heat during the day and lose it during night quickly than the fine-textured soils, because the latter retain more water and the specific heat of water s 4 -5 times more than that of the soil particles.

 

Soil Air:

It is important to know the content of the soil air and its composition as restricted soil aeration adversely affects root development, processes of respiration and other essential biological processes.

The occupation of nearly one-third of the pore space in soil by air and two-thirds of it by water constitutes the most favorable condition for plant growth.

The composition of air reflects a dynamic balance between two competing processes. The consumption of oxygen and the liberation of carbon dioxide by plant roots and the soil organisms tend to increase the difference in the composition of the soil air and the atmosphere above the soil surface. Gaseous diffusion tends to reduce the difference in composition.

Like atmospheric air, the soil air is composed largely of nitrogen and oxygen, but differs from the former in containing more moisture, more carbon dioxide and a little less oxygen.

 

Soil Water:

The greatest influence on the growth and yield of a crop is water. It is needed in much larger quantity than that of any other substance that contributes to growth and yield. Water serves the following functions in relation to plant life.

1) It is an essential part of plant food. It constitutes nearly 90 percent of plant tissues.

2) It servers as a solvent and carrier of plant nutrients.

3) It maintains cell turgidity and regulates temperature.

Water is held in the following forms:

1) Hygroscopic water: It occurs as a thin film (4-5 millimicron) and is held tenaciously with a tension of 31 atmospheres or more

2. Capillary water: It forms a continuous film around soil particles (outside the file of hygroscopic water) and in the micropore space. It is held by surface tension. Capillary water is held at a tension ranging from 1/3 to 31 atmospheres.

3. Gravitational water: It is free water held at a tension below 1/3 atmosphere. It saturates the soil and percolates downwards under the influence of gravity.

The maximum amount of capillary water remaining in the soil after the removal of gravitational water is called its field capacity. Capillary water held at tensions greater tan 15 atmospheres is not available to plants.

Water table lying more than 2-3 metres below the root-zone is not of much use to crops.

 

Soil Structure:

Soil structure refers to the arrangement of soil particles, both primary and secondary. In the field, the structure is described in terms of 1) type-referring to shape and arrangement, 2) class-referring to size 3) grade-referring to the degree of aggregation.

Type of soil structure

1. Platy: With particles arranged around the plane, generally horizontal

2) Prism-like: With particles arranged around a vertical axis and bounded by relatively flat vertical surfaces. This type includes prismatic and columnar types.

3) Block-like: With particles arranged around a point and bounded by flat or rounded surfaces which are the casts or the moulds formed by the faces of the surrounding peds. This type includes angular and sub angular types.

4) Spheriodial: Particles arranged around a point and bounded by curved or very irregular surfaces that are not accommodated in the adjoining aggregates. This type includes granular and crumb types.

 

Classes:

There are 5 classes recognized in each of the primary type. They are very fine, fine, medium, coarse and very coarse.

Grade:

The grade of structure, representing the degree of aggregation is determined in the field mainly by noting the durability of the aggregates and the proportions between aggregated and unaggregated material that result when the aggregate are displaced or gently crushed. Grades are termed structureless, weak, moderate, strong and very strong, depending on the stability of aggregates when disturbed.

Factors affecting structure:

Structure is influenced by texture. It is influenced by:

1) Soil Management: A good soil management, with a proper system of crop rotation has the effect of maintaining the soil in a good state of aggregation.

2) Absorbed Cations:

Sodium and potassium ions on the clay complex have the tendency to disperse the soil. Calcium has favorable effects on the aggregation.

3) Micro-Organisms:

The growth of fungi and the microbial decomposition products of organic matter have a binding effect on soil particles and it favors aggregation.

4) Variations of soil moisture:

Variations of soil moisture due to drying and wetting influence the structure. Drying of soils forms crack and big clods.

Poorly drained soils, usually have an unfavorable structure.

The crumb and granular structure (spheriodial) is considered very favorable to plant growth.

The structure and physical qualities of soils can be improved by adopting a suitable system of soil management, including legumes in the rotation system , green manuring and regularly supplying the soil with organic manure.

 

Chemical Properties:

Mineral matter and its composition:

The principal minerals occurring in the earth's crust are:

1. Feldspars:

2. Quartz:

3. Mica

4. Limestone

5. Hornblende and Augite

6. Olivine and Serpentine

7. Clays

8. Other Minerals: They include tourmaline- a boro alumino silicate with alkali metals and iron or magnesium; rutile-titanium oxide, zircon-zirconium silicate, glauconitie-hydrated silicate of iron and potassium, apatite-calcium phosphate, sulfur-bearing minerals, etc.

 

Inorganic components:

The soil supplies all the following essential mineral elements required by the plants:

1. Macros: Phosphorus, potassium, calcium, magnesium and sulphur.

2. Micros: Iron, manganese, zinc, copper, molybdenum, boron and chlorine.

Soil is also the source of nitrogen for plants, but the ultimate source of nitrogen is traceable to atmosphere from where nitrogen fixation takes place physico-chemically and biologically.

Ion Exchange: It is a reversible process by which cations and anions are exchanged between solid and liquid phases and between solid phases which are in close contact with each other. The exchange of cations and anions is termed cation exchange and anion exchange respectively. Ion exchange is the most important of all the processes occurring in a soil.

The capacity of soils to adsorb and exchange cations and anions varies greatly with the nature and amount of clay, and the organic matter. The cation-exchange capacity (CEC) is defined as the amount of a cation species bound at pH 7 (neutral pH) and is expressed as milliequivalents (m.e.) per 100 grammes.

Base saturation:

A soil saturated with calcium and magnesium is considered normal and fertile. If a soil has more than 15% exchangeable sodium, it is considered to be an alkali soil. On the other hand, if the soils are base-unsaturated, i.e. the proportion of exchangeable hydrogen is more, the soil tends to be acidic.

 

Organic Matter

Organic matter, though forming a small part of mineral soils, plays a vital role in the productivity and conditioning of soils. It serves as source of food for soil bacteria and fungi which are responsible for converting complex organic materials into simple substances readily used by the plants. The intermediate products of decomposition if fresh organic matter help to improve the physical condition of the soil. It also increases the fragility of the soil.

Humus: The organic matter in the soil consists largely of plant remains, the residues of soil micro-organisms feeding on them and several products of their decomposition.

The composition of humus is

Material	Percentage
Carbon	50
Oxygen	35
Nitrogen	5
Hydrogen	5
Ash(containing phophorus, potassium, sulphur and other elements	5

Humus is soluble in water only a very little extent but can absorb gases as well as water. Lime has a precipitating action on humas.

Content of organic matter:

The content of organic matter varies with the kind of the soil, vegetation, climate, cultivation, manurial and rotation practices, and biological activities. The vegetation determines the quality and quantity of organic material added each year, whereas the climate, temperature and moisture conditions determine the rate of decomposition.

The amount of organic matter and humus decreases from the surface downwards. Under acidic conditions, humus becomes dispersed and is carried down into the subsoil where it may form part of the hard accumulated layer of mineral and organic colloidal matter.

Carbon -nitrogen ratio:

Nitrogen plays a major role in the growth and reproductive processes of plants. The organic combination of nitrogen, with organic matter in the soil constitutes the principal store-house from which nitrogen is slowly made available to the crops.

Micro Organisms of the soil.

Raw organic matter in the soil is not directly used by the plants as food. It must be broken down first into humus and then into simpler products before it can be so utilized. This work is done by different kinds of micro-organism, which inhabit the soil by billions. The decomposition of organic matter primarily forms part of the feeding and growth processes of these microscopic plants and animals. Sugars, starches and proteins are broken down first; then cellulose and fatty substances (lipoids); and lastly, lignin or woody substances. When organic matter become chemically and biologically inert, the soil become infertile, despite its presence. So, all organisms are not beneficial.

 

Kinds of soil organisms:

Microflora:

1) Bacteria

2) Actinomycetes

3) fungi

4) algae

Microfauna

1) Protozoa

2) Nematodes

 

Density of Population:

The soil organisms vary in number from a few hectare to many millions per gramme of soil. The density is determined by food supply, moisture, temperature, physical condition and the reaction of the soil. Bacteria dominate over other types of microscopic life. If the soil is acidic and rich in organic matter, fungi predominate.

As among the soil microflora, generally bacteria form about 90 per cent of the total population, actinomycetes about 9 per cent and the fungi and algae together about 1 per cent.

Among the soil fauna, protozoa are the most abundant, followed by nematodes, worms and insects.

 

Bacteria activity:

The soil microflora typically produce ammonia from organic compounds when they set free more of nitrogen from the organic matter on which they are living than what they can assimilate and convert into their own protoplasms. Ammonia is converted into nitrites by one group of organisms called Nitrosomonas and nitrites are further converted into nitrates by another group of organisms called Nitrobacter. The process of conversion of ammonia into nitrate is called nitrification.

Bacteria are most active at temperature between 25 and 35C.The bacteria cease or reduce their activity when the pH value of the soil falls below 5.

 Two other bacteria intimately linked with nitrogen problem of the soil take up free nitrogen from the air and convert it into nitrogenous compounds for the use of crop plants. One group (genus Rhizobium) functions in symbiosis with leguminous plants, and the other (belonging to genus of Azotobacter) fixes free nitrogen independently of the legumes.

Azotobacter and other non-symbiotic nitrogen-fixing bacteria work independently of nay host corp.. Under optimum laboratory conditions, Azotobacter has been found to fix a considerable amount of nitrogen

Actinomycetes: They are similar in size to bacteria but resemble moulds in their growth and physiology. They can grow in the deeper layers of the soil and under drier conditions, and need less nitrogen.

Fungi: These organisms produce microscopic threads called mycelia. which may be found in the disintegrating organic matter on the surface of the soil or on plant roots in the upper strata below the surface. Many of the fungi are harmless saprophytes, living on dead organic matter; others are parasites which attack live plants and produce highly destructive epidemic diseases.

Algae: They are microscopic or larger plants containing chlorophyll. They are found in large numbers in the top layer of constantly moist soils.

Protozoa: Soil protozoa are unicellular animals, feeding either on soil organic matter or on bacteria, thus regulating the number of the latter in the soil.

Worms and Insects:

Most of soil contains worms, insects and other animals of different sizes. They feed on plant remains, which are ingested along with large quantities of earth. Their excrements in the form of casts is brought up and deposited on the surface.

 

Soil Surveys

Sols surveys are done to furnish comprehensive information about soils and an inventory of soil resources of the area.

Types of surveys:

Two main types of surveys are:

a) reconnaissance surveys

b) detailed surveys

 

Soil Classification:

The modern system of classification "Soil Taxonomy" developed by the USDA is used worldwide. This is a multi-category system which has six categories, namely, order, sub-order, great group, sub-group, family and series. The system is hierarchal in that an order is divided into sub-orders, and sub-orders into great groups, etc up to the series level. Thus, the number in the higher categories is fixed, whereas in lower categories, it is variable.

 

Major Soil Groups

Alluvial soils

It include the deltaic alluvium, calcareous alluvial soils, coastal alluvium, and coastal sands.

Black Soils

Black soils have a high degree of fertility, are highly argillaceous, very fine-grained and dark and contain a high proportion of calcium and magnesium carbonates. They are very tenacious of moisture and exceedingly sticky, when wet. Owing to considerable contraction on drying, large and deep cracks are formed. These soils contains abundant iron and fairly high quantities of lime, magnesia and alumina. Potash has a wide range. They are poor is phosphorus, nitrogen and organic matter.

Red Soils:

The ancient crystalline and metamorphic rocks on meteoric weathering have given rise to the red soils. The color of the soil is due to the wide diffusion of iron rather than to a high proportion of it. It is generally poor in nitrogen, phosphorus and humus. These soils are poorer in lime, potash, iron oxide and phosphorus.

The red soils are divided into two broad sub-groups:

i) red loams, characterized by argillaceous soils with a cloddy structure and the presence of only a little concretionary material; and ii) red earths where the top soil is loose and friable and rich in secondary concretions.

Leterties and Lateritic soils:

It is a compact to vecular rock composed essentially of a mixture of the hydrated oxides of aluminum and iron with small amounts of manganese oxides, titania etc.  It is derived form the atmospheric weathering of several types of rocks.

Desert Soils:

The desert sand contains quartz in well rounded grains, but feldspar and hornblende grains also occurs with a fair proportion of calcareous grains.

Problem Soils:

Problem soils are those which owing to land or soil characteristics, cannot be economically used for the cultivation of crops without adopting proper reclamation measures. Highly eroded soils, ravine lands, soils on steeply sloping lands, shallow soil depth, deep gullies, steep and complex slopes etc constitute one set of problem soils. Their reclamation may involve massive earth-moving operations, terracing, afforestation or plantation to maintain permanent cover with grasses, depending upon the intensity of the problem and the nature of the terrain and soil conditions.

Acid Soils:

Although soils having pH below 7 are considered to be acidic from the practical standpoint, those with pH less than 5.5 and which respond to liming are acid soils. In the classification of soils both the percentage base saturation and the pH are used as criteria to distinguish acid soils from non-acid ones.

Lime requirement.

Acidity in the soil systems can be conveniently classified as active or potential acidity. Active acidity includes hydrogen ions in the solution phase and is determined by pH measurements. The potential acidity may be considered to be the exchange acidity and makes up the bulk of the total acidity which is many times greater than active acidity.

The lime requirement of a soil necessary to neutralize the total acidity may be defined as the amount of liming material that must be added to raise the pH to some prescribed values. The value is usually in the range of pH 6 to 7.

The liming material should be evenly broadcast and worked into the soil several weeks before sowing a crop to allow time to completing the reaction. Keeping the land moist hastens the exchange process.

Saline and alkali or sodic soils.

Classes of saline and alkali soils.

1. Saline Soils: The soils containing toxic concentration of soluble salts in the root-zone are called saline soils. Electrical conductivity in the saturation extract of such soils taken as a measure of salts in greater than 4.0 mmhos/cm. Exchangeable sodium percentage is less than 15 ad the pH is less than 8.5. Because of the white encrustation due to salts, the saline soil is also called while alkali

2. Non-saline alkali or sodic soils: The soils do not contain any large amount of neutral salts and , as such, the electrical conductivity is less than 4 mmhos/cm. The detrimental effect of alkali soil on plants is largely due to toxicity of a high amount of exchangeable sodium and the pH. Alkali soils have an exchangeable sodium percentage more than 15 and ph greater than 8.5.  Because of high alkalinity, resulting from sodium carbonate, the surface soil is discolored and black and hence called the non-saline alkali soil.

3. Saline-Alkali Soils: This group of soils is both saline and alkali. They have appreciable amounts of soluble salts, as indicated by the electrical conductivity value of more than 4 mmhos/cm. The exchangeable sodium percentage is greater than 15. The ph is likely to be less than 8.5

 

Soil management

Successful farming does not merely depend on the knowledge of physical, chemical and biological properties of the soils. It is a matter of both soils and soil management. Correct application of the relationship between the soils and the crops to be grown  is the most important consideration.

Good soil tilth is first feature of good soils management. It means a suitable physical condition of the soil and implies, a satisfactory regulation of soil moisture and air. The maintenance of soil organic matter which encourages granulation is an important consideration. Good tilth minimizes erosion hazards.

The choice and sequence of adaptable crops and crop rotation is also very important considerations. A proper sequence of crops and varieties greatly influences the soil physically, chemically and biologically. Cropping patterns should be evolved according to land-capability and should be based on the principles of soil and water conservation and efficient moisture utilization. In irrigated areas, special management practices become necessary to avoid salinity, alkanity and water-logging. The productive capacity of the soil should be improved and maintained by providing adequate organic manures and plant nutrients through fertilizers and by including legumes in the rotation.

Recommended practices regarding the application of fungicides, insecticides should be used. Unless the recommended practices are economically profitable, they are not likely to be adopted.

 

Soil testing

For maximum production and rational soil management, a knowledge of the fertility status and physical properties of the soil are important.

In the soil-testing laboratory, soil samples are analyzed by using standardized rapid methods for the following items.

1) ph or the soil reaction which indicates whether the soil is acidic, alkaline or normal.

2) Total soluble salts, as determined by electrical conductivity which indicates the degree of salinity, alkalinity etc of the soil.

3) Organic carbon which is a measure of available nitrogen

4) Available phosphorus

5) Available potassium.