Here is a list of seven physical properties of soil: 1. Soil Texture 2. Soil Structure 3. Density of Soil 4. Pore Space 5. Soil Consistence 6. Soil Colour.
The relative size of soil particles is expressed by the term texture; more specially the texture is the relative proportions of different size, groups or separates.
Classification of Soil Separates:
There are a number of system for classification of soil separates namely United States Department of Agriculture (USDA) System. International system, British system and The European system. Out of these systems, the International System is commonly followed in India.
The soil separates are classified into Sand, Silt and Clay according to their size. According to the size of soil particles, the texture may be coarse and fine. The average size of the soil particles increases when the proportion of sand in the soil is increased and the resultant soil becomes coarser in texture. On the other hand, the average size of the soil particles decreases when the proportion of clay in the soil is increased and the resultant soil becomes finer in texture.
Soil Textural Classes:
Textural classification of soil means classification of soils on the basis of their texture into different groups or classes such as Sand, Sandy-loam and Silty loam. The broad and fundamental groups of soil textural classes are recognized as Sands, Silt and Clays.
2. Soil Structure:
The arrangement of particles in the soil mass is called soil structure. Soil structure also be defined as aggregates into which soil breaks up.
Structure is strictly a field term descriptive of the gross, overall aggregation and arrangement of primary soil separates. The primary soil particles such as sand, silt and clay usually occur grouped together in the form of aggregates. Natural aggregates are called peds whereas artificial aggregates are called clods. Clods are formed due to disturbance of the field by ploughing or digging. The words fragmentation and concretions are often used in connection with the structure of soil.
Fragment is a broken peds whereas when salts dissolved in percolating water precipitate, it results in the formation of concretions. The soil conditions and characteristics such as water movement, heat transfer, aeration, bulk density and porosity will be much more influenced by structure. In fact, the important physical changes imposed by the farmer is ploughing, cultivating, draining, liming and manuring his land are structural rather than textural.
3. Density of Soil:
Density represent the mass (or weight) of a unit volume of soil. One means of expressing soil weight is in the terms of density of the soil particles making up the soil.
The soil density is expressed in two manners as follows:
i. Particle Density:
The density of soil solid is known as the particle density of soil. The weight per unit volume of the solid portion of soil is called particle density. It is concerned with solid particles only. The weight of the soil solid has often been expressed in terms of the specific gravity. Specific gravity is the ratio of the weight of soil solid to the weight of an equal volume of water. In metric system, particle density is usually expressed in terms of grams per cubic centimeter.
Thus, if one cubic centimeter of soil solids weighs 2.6 grams, the particle density is 2.6 grams per cubic centimeter. Generally the particle density of normal soil usually vary between the narrow limits of 2.60 and 2.75 grams per cubic centimeter. The particle density will be higher if large amount of heavy minerals such as magnetite, limonite and hematite are present in the soil. The particle density decreases with the increase of organic matter in the soil. Particle density is also termed as ‘True density’. (T)
ii. Bulk Density:
Bulk density is defined as the mass (weight) of soil solid per unit volume of dry soil. It is also called as apparent (A) density. This volume includes both solid and pore space (voids).
The relationship between apparent density (A), True density (T) and the pore space (P) is as follows –
P = [(T – A)/T] x 100
It is expressed in percentage.
In this case, the total soil space (space occupied by solid and pore-spaces combined) is taken into consideration. Bulk density is determined by the quantity of pore-space as well as soil solids. The bulk density of a soil is always smaller than its particle density. In metric system (c.g.s), it is expressed in gram per cubic centimeter (gm/c.c.). Bulk density of sand is about 1.6 grams per cubic centrimeter.
Calculation of Density of Soil:
Factors Affecting Bulk Density:
(i) Soil Texture:
Bulk density normally decreases as minor soils become finer in texture. The sandy soils have high bulk density as the particles of that soil lie in close contact with each other. The low organic matter content of such soil increases the bulk density further. The particles of finer textured surface soil such as clays, clay loams and silt loams do not lie close together and hence have more pore space and well granulated.
Granulation encourages a fluffy, porous condition which results in low bulk density values. But sand and sandy loam show high bulk density.
(ii) Soil Structure:
Soil structure affects bulk density by influencing the Porosity of soil. The crumb soil structure shows low bulks density than that of platy soil structure.
(iii) Depth of the Soil:
Bulk density varies with the depth of soil. Surface soils usually have low bulk density. Because surface soils contain more humus than sub-soil. On the other hand, the underlying lower horizon of soil has higher bulk density due to lower content of organic matter, less aggregation and root penetration and a compaction caused by the weight of overlying layers of the soil.
When organic matter decomposed to form humus, some organic compounds are formed. This organic compound binds the primary soil particles such as sand, silt and clay to form the soil aggregates. Pore space occurs within and between soils aggregate. As a result, the weight of unit volume of soil decreases. When the percentage of humus increases, the bulk density decreases. The addition of farm manures in huge quantities lower the bulk density value.
(v) Nature of the Crop:
The bulk density of grass land decreases as the grass roots bind the soil particles to form soil aggregates and humus is formed due to decomposition of grass roots and other materials. The intensive cultivation increases the bulk density value.
4. Pore Space:
The pore space of a soil is that portion which is occupied by air and water.
The pore space is not occupied by soil particles. The amount of pore space depends on the arrangement of solid particles. The total porosity of sands and compact sub-soil is low as the soil particles of such soil lies in close together. On the other hand, medium texture soil rich in organic matter has high pore space per unit volume of soil. Pore space is necessary for retention of soil moisture, aeration of plant root and for proper drainage.
Kinds of Pore Space:
The size and shapes of pores and pore spaces vary considerably.
In general, pore spaces are of two types as follows:
(i) Macropores or Non-Capillary Pores:
Macropores are those pores through which water and air movement can take place easily. Macropore do not hold much water under normal condition. Light soil such as sands and sandy loam soil has macropore or non-capillary pore. Macropore is necessary for proper drainage and aeration of plant root. The average diameter of macropore is greater than 60 microns.
(ii) Micropores or Capillary Pores:
Micropores are those pore through which movement of air is difficult and movement of water is restricted largely to slow the capillary movement. Heavy soil such as clay, and clayey loam soil have micropore or capillary pore. Micropore is necessary for the retention of moisture which is necessary for the plant. The average diameter of micropore is less than 30 microns.
Porosity refers to the percentage of soil volume occupied by pore space. The existence of approximately equal amount macro and micropores or suitable for the growth of the plant which influence aeration drainage. Permeability and water retention favourably.
The derivation of the formula used to calculate the percentage of total pore space of soil is as follows –
Factors influencing total pore space of a soil are as follows:
(i) Soil Texture:
In sandy soil, total pore space is less as the pore of such soil is quite large. On the other hand, in clayey soil, the total pore space and micropores are high, because clay particles unite to form soil aggregates within which micropore space occurs.
(ii) Soil Structure:
A soil having compound structure has greater pore space than the single grain soil. Granular or crumb structure has more pore space than plate like structure.
(iii) Organic Matter:
Soil rich in organic matter having good granulation and aggregation has more total pore space and micropore space.
(iv) Depth of Soil:
The pore space of the surface soil is usually much more than that of the sub-soil, because plant roots and organic matter occurs more in surface soil than in subsoil.
(v) Soil Organism:
Soil organism such as earthworm and insect increases the macropore in the soil.
(vi) Nature of Crops and Cultivation:
Intensive Cropping tends to lower the total pore space in comparison to virgin soil. Continuous cropping and cultivation reduces the total pore space and the size of the macropores as the organic matter content of a soil decreases.
5. Soil Consistence:
Soil consistence is defined as a term to designate the manifestation of physical forces of cohesion and adhesion acting within the various level of moisture content. Soil consistence is defined, another way, as the physical condition of the soil at various moisture content as evidenced by the behaviour of that soil toward mechanical stresses or manipulation.
The two forces responsible for soil consistence are cohesion and adhesion which act within the soil. Adhesion is the attraction between soil particles and water molecules. Cohesion is the attraction between soil particles or between water molecules.
The manifestation as stated in the definition includes as follows:
(i) Behaviour of the soil towards gravity, pressure, thrust and pull.
(ii) Tendency of the soil mass to adhere to foreign bodies.
(iii) Tactile quality of soil on rubbing between the fingers.
Tillage is the physical manipulation of soil with tools and implements to result in good tilth for better germination of seeds and subsequent growth of crops. Tillage is tilling of land for bringing about conditions that are favourable for the cultivation of crops. Tilth implies to the physical condition of soil in its relation to plant growth. Tilth is brought out by tillage. Tillage is the primary function of cultivation and it is a laborious and expensive cultural practice.
Tillage helps to replace natural vegetation with useful crop and is necessary to provide a favourable edaphic environment for establishment, growth and yield of crop plants. Tillage helps to improve the physical condition of soil, control of weeds, insect-pests and diseases and also bring the nutrient available to plant. The cultivation is not possible without tillage operation. The crop production depends on good tillage operations.
The word manure has originated from the French word “Manoevrer”, which refers to ‘Work with soil’, that is why the word tillage and manure which synonyms as it clear by the statement of Jethro Jull (1700 BC) “Tillage is manure”. There must be sufficient moisture in the soil for good tillage.
Tillage is quite impossible in fully dry soil. On the other hand, tillage in wet land results in puddling of soil. Tillage in wet soil having sufficient water brings puddling condition of soil which is favourable for cultivation of transplanted paddy, onion etc. Tillage practices must be carried out at optimum soil moisture content to maintain a good soil structure.
Soil compaction is the process of increasing dry bulk density of soil, reducing the pore space by expulsion of air through applied pressure on a soil body. Under very high pressure, the soil particles may themselves be compressed but only slightly. Compaction due to the machines used for tillage of land, inter-cultivation, harvesting and threshing of crops has adverse effects on the normal growth of plants due to reduced aeration and increased bulk density of the soil. Compaction of the soil may also be due to grazing of animals, human activities in the field and intense rainfall as well as irrigation.
The colour of soil is probably the first soil property for the human perception and it is one of its obvious property. The colour of a soil is an indication of the nature of an individual soil. The colour of soils is due to the colour of their constituents. The colour of a soil is inherited from its parent rock material. As for example, red soil developed from red sand stone and sand developed from quartz.
Black coloured soils are rich in organic matter and therefore fertile. Red coloured soils are rich in ferric oxide, highly weathered and of poor fertility. When superphosphate is applied in red soil (pH 5.3-6.0); the phosphorus is converted to iron and aluminium phosphate. The phenomenon is called ‘Phosphate fixation’, soil colour influences greatly the soil temperature. The dark coloured soil absorbs heat more readily than light coloured soils.
There are many factors which are responsible for soil colour as follows:
(i) Organic Matter:
Organic matter is black in colour. A soil, which contains high organic matter and if it is alkaline, will be black or brown in colour. Colour indicates, approximately, the organic matter content of the soil.
(ii) Free Oxides of Iron:
A soil which contains high percentage of free oxides of iron (2Fe2O3 . 3H2O) will be yellow red in colour as the ferric oxide is red in colour. When the oxides of iron are dehydrated, the colour of the soil will be yellow. On the other hand, the colour of the soil will be red when the oxides of iron are highly hydrated.
(iii) Texture of the Soil:
The intensity of colour of the soil depends on the texture of soil. The fine textured soils are dark in colour and more fertile than coarse texture soil (i.e. sandy soil). Because, in fine textured soil (i.e. clay soil), the total surface area will be more than sandy soil. The effect of soil colour can be seen by the fact that black vertisols in dry state are heated stronger than all other soils.
(iv) Moisture Content of Soil:
The wet soils are darker in colour than dry soil. This is due to the interaction of organic matter and water. The water logging condition has also influence on the colour development of soil. Soils which are imperfectly and poorly drained are nearly always mottle with various shades of grey, brown and yellow, specially within the zone of fluctuation of water table.
Soils are generally described by its colour. Such as black, red, yellow, etc.
The three variables which combine to give colour as follows:
(i) Hue – It refers to the dominates spectral colour (rainbow, such as red, yellow, etc., It is related to dominant wavelength of light.
(ii) Value – It refers to the relative lightness of colour and it is a function of total amount of light.
(iii) Chroma – It refers to the relative purity of the spectral colour.
Soil colour indicates the nature and properties of soil.
By knowing the colour, one can get some idea about the type of soil as follows:
(i) Dark in Colour:
It indicates that the soil contains high percentage of organic matter. Black coloured soil is richer in clay. As a result, the water holding, nutrient retention capacity of this soil will be more and soil will be granular in structure and therefore will be more fertile than lighter colour soil. Dark colour soil absorbs much temperature than light colour soil. For this, dark colour soil is warmer.
(ii) Red or Yellow in Colour:
It indicates that the soil contains high percentage of oxides of iron (2Fe2O3. 3H2O) and this soil is low in water holding capacity, nutrient retention capacity. Super phosphate, if applied in this soil, will be fixed as iron phosphate, which is not available to plant. Rock phosphate, bone-meal etc. are the safe fertilizer for this soil.
The temperature of soil is very important, because plant growth as well as the chemical and biological weathering are greatly influenced by the soil temperature.
(i) Solar Radiation:
The main source of heat is the sun. The temperature of the soil is primarily dependent upon the amount of radiant energy received from the sun. The sun rays reach the earth after they pass through the atmosphere and warms the surface of the soil on which they fall. A moist or cloudy atmosphere prevent much of the sun’s radiation from reaching the earth. A part of soil temperature is lost to the air by radiation.
The heat is absorbed from solar radiation by surface soil and is conducted down the depth of the soil. The interior of the earth is very hot, the conduction of this heat to the soil is very slow.
(iii) Biological and Chemical Reaction:
In a soil, various biological and chemical reaction occurs. As a result, some amount of heat is liberated in the soil due to biological and chemical reaction.