Definition and Composition of Soil

  Soil is the top layer of the earth crust in which plants have their roots and from which they obtain water and mineral nutrients. When soil is being formed during the process of weathering, the mineral rocks is broken down into particles of various sizes by the action of weathering agents such as water and wind. Humus or  organic  matter derived from the dead remains of animals and plants are latter added to the mineral particles to produce soil which is suitable for good plant growth.

   The soil is divided into three layers.

(I) Topsoil (soil surface), where most most plants roots are found. The top soil is much darker brown than the soil beneath it, I.e. the subsoil. It contains many parts of dead plants and animals, which latter form the humus. It is this humus that gives the top soil its characteristic dark colour.

(ii) Below the top soil is the sub soil. This is relatively infertile layer.

(iii) Underneath the subsoil is the parent rock.

  These three layers I.e. topsoil, subsoil and the parent rock together form the soil profile. A soil profile can be examined by making a vertical section through the soil down to the parents material I.e. the rock

Composition of soil

A good agricultural soil is made up of a mixture of the following substances; the numbers representing percentage by volume.

Solid particles.....................40%

Air.........................................25%

Water....................................25%

Humus..................................10%

These percentaged vary with different types of soil found in different places.

Experiment 1: to determine the composition of garden soil.

The composition of a garden soil can be determined as follows:

Method: fill a glass jar with soil to a depth of 10cm and then add water. Shake the jar thoroughly and allow both the water and the soil to settle. What do  you observe? Measure the depth of the sand and clay and calculate the percentage of each. The amount of humus and plant remains cannot be determined in this way. Make a chart showing the results.

Results:  sand particles being  being heaviest will sink to the bottom first. There will be a clear line between between the bottom layer I.e. sand and next layer, clay. Some of the plant remains will float on top.

Experiment II: to demonstrate the presence of air in the soil.

Method: turn an empty tin, which is open at one end, and press firmly into the ground until the tin is filled completely with soil. Gradually scrape the soil from the tin into water in a cylinder.

Result: as this being done, there will be bubbles of air in the water as the soil is falling into the cylinder of water. This experiment demonstrates the presence of air in the soil.

Use of air, water and solid particles in the soil.

Air- air is present in the pore spaces in the soil which are not occupied by water. Air provides oxygen for the respiration of  living organisms in the soil, and without air, seeds cannot germinate.

Water- the major source of water in the soil is rainfall. Other sources include supply from irrigation and underground water. The water in the soil is held is held by many forces , on the basis of which soil water can be classed as (I) free (drainage) water, (ii) capillary water, and (iii) hygroscopic water. Only capillary water is available to plants.

   Water in the soil may be lost by evaporation from the soil surface, transpiration from the opening ( pores and stomata) in plants and runoff from Soil surface in to streams and rivers. 

    Soil water is important to plants in the following ways:

(a) it is an important ingredient in the manufacture of food for plants (photosynthesis);

(b) water is used for transporting many food nutrients such ad starch, protein and fats to their parts of the plant where they are needed;

(c) the mineral nutrients in the soil are carried by water to other parts of the plant where they are absorbed to perform various metabolic and physiological functions.

(d) Enzyme activities in the soil and plant takes place in the presence of water in the soil;

(e) All living organisms in the soil require some amount of water for survival and to Carry out life activities. Water is an important constituent of the plant cell. without water, plants cannot grow.

Solid particles - the soil is a mixture of solids such as sandstone , gravel , clay and limestone. The largest particles in the soil are the stones and with many such large particles, the soil cannot retain water for agricultural purposes.

   Sand also forms coarse soil particles and therefore does not retain much water. Crops grown in such soils sometimes suffer from lack of water. The coarse solid particles however, prevent soil erosion because they are not easily washed away by water or blown away by wind. Sandstone contains silica which gives the soils its strength. Because of their large air spaces,  coarse particles are well aerated and therefore extremely light. Clays on the other hand are not as coarse as sandstone or gravel. They retain water tenaciously. Crops grown on such soil are usually poor and tend to ripen slowly. Clays , however supply some mineral to the soil like kaolin and montmorilonite. Limestone contains lime and when present, gives the soil its alkaline reaction.