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Current Category » Principles of Agronomy

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Soil Property- Density Of Soil

The density of the soil i.e. mass per unit volume can be expressed tn two ways: The density of the solid particles of the soil, called as particle density and the density of the whole soil including pore space, is called as bulk density. Particle density is also called as true specific gravity and bulk density is called as apparent specific gravity.

1. Particle density: It is the weight of the soil solids (g) without pores per unit volume (cc). It varies from 2.6 to 2.7 g/cc in most of the mineral soils with average of 2.65 g/cc. It is not affected by texture and structure of coil and it is static property.

2. Bulk density: It is the mass (weight) per unit volume of the soil inclusive of pore spaces in its natural structure.
It varies from1.3 to 1.7 g/cc in sandy soils and 1.1 to 1.4 g/cc in clay soils. However, it is affected by texture, structure, organic matter and depth of the soil. Surface soils have low bulk density than lower surfaces.
Soil porosity: Soil has spaces which are occupied by water and air. The amounts of water and air present in pore spaces vary and depend upon their relative amounts. The amount of pore space depends upon the arrangement of solid particles, organic matter content, granulation and aggregation (texture), depth of the oil, cultivation and cropping pattern of the soil.
The pore spaces are of two types: 1) Macro or non-capillary (more than 0.06 mm) and 2) Micro or capillary pores (less than 0.06 mm) having bigger and smaller sizes, respectively. Pore spaces between the aggregates of soil particles are macro spaces which are occupied by air and those between the individual particles of the aggregates are micro pores which hold the water. Macro pores allow rapid movement of air and water as water than micro pores. Proportion of macro and micro pores is important than total porosity.
Porosity of the oil can be calculated by formula: 
Porosity = 100 – (Bulk density * 100)/ particle density = 100 ( 1 – BD/ PD.

Sandy soils have 30 – 40%, clayey 50 – 60% porosity.
Soil colour: Colour indicates, approximately, the organic matter content of soil. The soils have various shades of black, yellow, red and grey colors. It may vary with the depth or horizons. Factors responsible for colour are:
1. Parent material from which soils are formed. E.g.: Red sandstone impart red colour to the soil.
2. Organic matter content imparts brown to blackish colour to the soil.
3. Minerals present in soil. E.g.: titanium (darker), Iron compounds like hematite (red) and limonite (yellow), silica or lime (whitish or grayish).
4. Accumulation of alkali – salts. E.g.: White or black depending upon type of salts.
Soil colour is useful for classification, to indicate organic matter and fertility, aeration, drainage, salt accumulation.

Soil air: In the gaseous phase of soil, water and air compete for the same pore space and their volume fractions are so related that an increase of one generally decreases the other. The amount and composition of soil air are believed to affect plant growth. Field air capacity is the fractional volume of air in a soil at field capacity which depends on texture. E.g.: sandy (25% or more), Loamy (15-20%) and clayey (below 10%). The circulation of air in soil mass is known as soil serration which is influenced by temperature water and diffusion. Soil air greatly varies in composition or CO2 than N and O2 gases present in atmospheric air i.e. 0.2 to 0.3% in soil air and 0.33% in atmospheric air. This needs continuous exchange of gas to avoid accumulation of CO2 in the soil.

Soil temperature: It affects the crop growth and activity of micro organisms. Optimum soil temperature requirement for germination and plant growth varies with crops i.e. 9°C to 50°C. The functioning of micro-organisms in the soil is very active within a certain range of temperature (27-32 °C). The major source of heat is sun and heat generated by the chemical and biological activity of the soil is negligible. Temperature can be controlled by maintaining optimum moisture content, providing drainage, mulching, organic matter, cultivation practices.

Soil consistence: It refers to the degree of resistance of a soil material to deformation or rupture or crushing which depends upon the degree and kind of forces (adhesion, cohesion) which attract one molecule to another. Adhesion is the force between similar materials.The consistence of soil is influenced by nature of clay minerals, exchangeable bases and humus. Thus helps to decide the time and type of tillage operations required to bring the soil at good tilth. The consistency also depends on plasticity of soil which is the ability to the kolded into different shapes when a certain amount of force is applied and then to retain even when the forces is removed.

Soil strength: Soil strength or mechanical resistance indicates the resistance offered by the soil to root penetration. It depends on soil moisture i.e. increase with decrease in soil moisture content and vice-versa. Soil compaction and bulk density also affect the soil strength.Soil compaction and soil crushing are also other physical properties of soil. These reduce the bulk density. These are useful for proper agril. Implements for land preparation, germination of seed.

Soil organic matter: It is mainly derived from the dead parts of vegetation and animal i.e. plant and animal residues. It forms a very small but important portion (5%) of the solid phase of soil. Its composition varies with type of vegetation, nature of soil population, drainage, rainfall and temp. Condition and the land management practices. The role of organic matter in maintenance, development and improvement of soil is well known as it enhances microbial activity, improves physical condition and fertility of soil and thereby soil productivity, enhances buffering capacity, prevent loss of nutrients, improves water retention and holding capacity etc. organic matter of soil cn be increased by addition of residues, green manuring, crop rotation etc. It influences the C?N ratio of soil. It is affected by climate which decides the nature of vegetation.

Current Category » Principles of Agronomy