Physical Properities of Soil

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Physical Properities of Soil

Physical Properties of Soil

Physical properties of a soil type depend on the size of particles in it. Particles above 2.0 mm are generally classified as gravel or stones and others as sand (between 0.05 and 2.0 mm), silt (0.002 to 0.05 mm) and clay (less than 0.002 mm). The texture of a soil depends on the percentage of sand, silt and clay in it. Based on this yardstick, soils are designated as clay, sandy clay, clay loam, silty clay, silty clay loam, sandy loam, silty loam, loamy sand, sand and silt. Soil particles occupy roughly more than half the space in soil. The remaining space between the particles, called the pore space, is occupied by water and air.

The different horizons of Soil profile (Courtesy: Soil Survey Directorate, IARI, New Delhi)

The bulk density of a soil is defined as the mass of a unit volume of dry soil which also reflects the total pore space of a soil. The pore size or porosity of soils together with bulk density determine the structure of soil. Sandy soils have single grained structured because of the general uniformity in particle size whereas in sandy clay or silty clay soils, the particles are grouped into aggregates.

The stability of soil aggregates depends on the organic matter content of individual soils and the nature of the microbial products which bind the particles together. Soil aggregation is a vital factor in plant growth, since movement of air, water and transfer of energy are interlinked with the porosity of soil.

Soil temperature is a very variable factor but nevertheless influences plant growth depending on the intensity of sunlight, day length, seasonal variations, rainfall and colour and texture of soil. Soil crusting is an important physical feature os soil which influences the emergence of seedlings. Soil crusts are formed by desiccation and may cause injury to the stems of seedlings resulting in crop failures.

The soil triangle showing the relationship between contents of clay, silt and sand in determining the different kinds of soil

Other physical factors in soil affecting plant growth are pH and moisture holding capacity. Retention of water in soils is related to pore space and capillary action of soil particles while pH is dependent on the chemical condition of soil.

Acid or alkaline soils are not generally as suitable for plant growth as neutral soils, since solubility and availability of plant nutrients are related to soil pH. Acid soils are usually characterized by excess availability of aluminium, iron, manganese, copper and zinc which may even prove toxic to plants.

The reverse is generally true of alkaline soils and in such soils. plants show symptoms of deficiency to many of the elements listed above. Neutral soils, in particular, favour the growth of such microorganisms which are responsible for the conversion of organic forms of nitrogen, phosphorus and sulphur into inorganic forms which can be absorbed by plants.

Changes in temperature affect not only the physiological reaction of microbial cells but also the characteristics of soil environment. A sudden fall in growth rate of microorganisms can be the result of thermal denaturation of proteins and changes in the permeability of membrane at high as well as low temperatures. However, specific proteins of flagella and ribosomes of themophilic bacteria are more thermostable than those of mesophilic bacteria which grow at ambient temperatures.

At low temperatures, microbes alter their cell structure especially the shape of proteins because cold hardiness of genes in ribosomes control the genetic coding of proteins and hence an organism’s ability to function at low temperatures. Growth of most psychrophilic bacteria stops at zero degree temperature despite the fact that some species are capable of growing below the freezi8ng point because of a mechanism to keep the cytoplasm unfrozen.

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