Soil structure refers to the way in which soil particles are grouped or bound together
to form lumps or aggregates. There are two main types of soil structure, single
grained and compound structure.
The structure of soil can be changed, modified and improved or damaged depending
on the various soil management practices adopted like tillage, manuring, liming,
rotation of crops, irrigation, drainage etc.
The density of soil that is weight per unit volume can be expressed in two ways
viz; the density of ‘solid’ (particle density), particles of the soil and the density
of the ‘whole’ (Bulk density) soil that is inclusive of pore space. Generally soils
with low bulk density have better physical condition than those with higher bulk
densities. Texture and structure of a soil, its total pore space and organic matter
content are all related to bulk densities.
In between the particles there are empty spaces which are occupied by air and water
and are termed as pore spaces. Pore spaces between the aggregates of soil particles
are macro-pores and those between the individual particles of the aggregates are
micro-pores. The percentage of macro pores is more in sandy soils these soils never
remain water logged and allow water to percolate down so rapidly that their moisture
content is very low,crops suffer due to shortage of soil moisture. The percentage
of micro pore is more in clayey or fine texture soils. In this soil water logging
condition causes adverse effect on respiration of roots and bacterial activities.
A proper balance between the macro and micro pores can be maintain by timely cultivation
and addition of organic matter so as to that crop neither suffers from shortage
of excess of water.
It is a combination of properties that determine the resistance of the dry soil
to crushing or pulverising action by implements and when wet its ability to be moulded
or changed in shape. All soils have cohesive and adhesive properties. Sandy soils
have these properties to a much lesser degree than fine texture clay soils. A good
tilth soil has both the macro and micro pores in more or less equal proportion,
ensures adequate retention of water and also free drainage of excess water. The
effect of such physical condition of soil on germination of seed and growth of plant
is very beneficial.
Soils have various shades of black, yellow, red and gray colours useful in soil
classification. Parent material e.g. red sandstone, organic mater, presence of certain
minerals e.g. titanium compounds imparts darker colour, hematite give red colour,
limonite-yellow colour, predominance of silica or lime. Soil colour is indirectly
helpful in indicating many other properties of soils e.g. a dark brown or black
coloured soil indicates its high organic matter content and fertility. A red or
yellowish soil shows good aeration and proper drainage. A white or black colour
due to accumulation of certain salts of alkali indicates deterioration of soil fertility
and its unsuitability for normal growth of many crops.
Soil temperature and plant growth
The biological processes for nutrient transformations and nutrient availability
are controlled by soil temperature and soil moisture. Soil temperature has a profound
influence on seed germination, root and shoot growth, and nutrient uptake and crop
growth. Seeds do not germinate below or above a certain range of temperature but
Micro-organisms functioning in the soil are very active while a certain range of
temperature, which is about 270 to 320C. It is necessary to know whether the soil
temperature is helpful to the activities of plants and micro-organisms and the temperature
could be suitably controlled and modified. The various factors that control the
soil temperature are soil moisture, soil colour, slope of the land, vegetative cover
and general tilth of the soil. Soil temperature can be controlled by regulating
soil moisture, proper soil management practices viz. good drainage, proper mulch,
good crumby structure, addition of sufficient organic matter help in keeping the
soil sufficiently warm and help in the chemical and biological activities in the