Soil  is composed of air, water, organic matte, living organisms and mineral matter in different proportions.


The figures represented in the pie chart are just averages.

The percentages will keep changing from time to time and form place to place depending on the following factors.


For example places that receive a lot of rainfall have a lot of soil water.

Nature of vegetation

This greatly influences the amount of organic matter and living organisms in the soil e.g. land covered by forest is high in organic matter and land covered by grassland has a lot of soil air due to the crumb structure.


This influences the nature of drainage so that on steep slopes there is less soil water and most of the pore spaces are occupied by soil air. Mineral matter is easily leached from steep slopes.

In low lying areas, there is accumulation of organ ie matter and the soil tends to be high in water.

Mans activities

For example man adds organic matter or manure to field, he carries out irrigation and drainage and these two will both change soil air and soil water. By cultivation, man introduces more air in the soil.



Soil air occupies the pore spaces between solid particles if they are not already occupied by water. The larger the soil parties the greater the pore spaces and therefore aeration is free e.g. sand.

soil air

When soil parties are very small, they are closely parked leaving little pore spaces therefore aeration is limited e.g clay.

soil air 1

Soil Air differs from atmospheric air in several ways one way is that it is not every distributed. It varies from composition from place to place e.g. Air in a place where legumes are growing or have just been harvested will be richer in Nitrogen.

It has more carbon-dioxide and less oxygen than atmospheric air.

Soil Air contains more moisture that Atmospheric air.

Importance’s of soil air.

  1. It is required for respiration of plant roots
  2. It is required for respiration of soil organisms that break down organic matter to release nutrients for plant use.
  3. It contains Nitrogen which is an essential nutrient for plant growth.
  4. A well aerated soil will prevent the build up of carbon-dioxide to toxic levels.

Experiment A.

An experiment to find the amount of air in the soil sample.


  • Dry soil
  • Two measuring cylinders


  • Measure out 100cm3 of dry soil in a 250cm 3 cylinder
  • Add 100cm3 of water to the soil




Soil water is also found in pore spaces between the soil solid particles. It varies from place to place depending on the following;

  1. i) Size of soil particles

Sandy soil with large pore spaces will allow most of the threw and very little will be held clay soil has small pore spaces so water cannot drain through first – it has a high water holding capacity.

  1. ii) Man’s activities.

For example, irrigation, Drainage, Bush burning mulching, addition of organic matter and deep ploughing.

iii) The amount of rain fall and frequency irrigation

  1. Relative humidity

High relative humanity leads to loss of soil water through evaporation.

  1. Soil Temperatures.

High soil temperatures lead to rapid evaporation soil water.

Steep slopes are well drained and there is rapid surface rain fall so the soil retains water.

Flat areas are poorly drained so they tend to get water logged.

viii) Nature of vegetation.

Bare soil loses a lot of water through evaporation and places that are well covered for example the forest the remains moist.

On the other hand there are some plants that draw a lot of water from the soil living it dry e.g. Eucalyptus tree.

Importance of soil water to crop production.

  • It makes the soil easier to cultivate and root penetration is also made easier.
  • It dissolves mineral salts and transports them by capillary attractions through the soil so that plant roots take them in.
  • It is essential for germination.
  • Water is an essential raw material in the manufacturing of starch.
  • Food that is manufactured in the leaves is transported to other parts of the plant with water solution.
  • It keeps the plant cool through transpiration and the soil also remains cool through evaporation.
  • About 70 – 90% of the plant is water.
  • It keeps the plant cell turgid in the absence of water the plant will wilt and eventually die.


Water in the soil is held by many forces for example

Capillary water

This is water that rises above the water table and is held in fine and medium pore spaces. It can easily move in soil and the finer the pore space the greater the capillary attraction.

It can easily be taken in by plant roots.

Gravitational water

When there is excess water in the soil, it can easily move down wards being pulled by gravity this water is not available for plant use.

Hydroscopic water

When there is very little in the soil, it is held strongly by the soil particles and can not easily be taken out by plant roots. If soil has high content of organic matter or clay it easily absorbs water form the atmosphere in time of drought.

water cycle

Water is continuously being gained and lost from the soil as shown on the diagram of the water cycle. Water is lost from soil surface and water body evaporation and also from plants through transpiration. Animals also lose water as they breathe and sweat all this contributes to water vapour in the atmosphere when water vapour reaches condensation level it forms clouds.

Clouds keep increasing in size until they lose water through precipitation (rain, nail, slit water will infiltrate into the  soil and move further down as gravitational water if there is a lot of rain. As it moves down it carries some mineral nutrients a process known as leaching. Man can also add water into the soil by irrigation.

Some water will be lost from the irrigation channels by evaporation.

In soil, when water comes across a resistant rock, it moves over it sideways into water bodies and this process is known as percolation surface runoff flows into large water.

Plants can take in soil water by capillary action and on hot days lose some of this water through transpiration then the cycle continues.

Experiment B

Experiment to determine the amount of water in soil sample


  • Evaporating dish
  • Fresh garden soil
  • Source of heat
  • A balance / weighing machine
  • Steering
  • Descator


  • Weigh an empty evaporating dish.
  • Half fell the dish with garden soil and weigh again.
  • Heat the soil for about 40 minutes while stirring occasionally.
  • Cool in the descation and weigh the soil and the dish when cool and record your results.

Results and calculation

  • Mass of empty evaporating dish =       38gm
  • Mass of dish + fresh soil =       108 + 38 = 65gm
  • Mass of fresh soil alone =       (103 + 38) – 38      = 103gm
  • Mass of dry soil alone =       38 + 83 = 103 gm
  • Mass of dry soil alone =       (83 + 38) – 38 = 83gm
  • Mass of substance lost during heating
  • Mass of fresh soil – Mass of dry soil      =       103 – 83 = 20gm

Calculate the percentage of substance lost.

soil water

Soil Organic Matter

Soil organic matter is a mixture of plant and animal residue, loving and dead micro organisms at various stages of decay.


It is well decomposed organic matte.

Factors affecting the rate of decomposition of organic matter.

Plant or animal material being decomposed;

Its age, its chemical composition etc e.g. cut young grass will decompose easily because a lot of water and has less fibre than a piece of timber.

 Soil factors, i.e. its aeration

Moisture content, temperature e.t.c

Soil animal plant population

E.g. bacteria, fungi, protozoa, termites e.t.c which will aid in decomposition.

Climate factors

E.g. if there is too much rainfall it will reduce soil aeration and therefore reduce on composition. Rice in temperature will increase there rate of decomposition up to a certain level.

Importance of soil organic matter in crop production

  1. Fresh organic matter provides food and shelter for soil organisms which in turn improve soil aeration are meets organic matter within organics matte of soil.
  2. Fresh organic matter covers the soil surface and reduces the rate of water loss through evaporation.
  3. Humus impacts a dark colour to the soil improves on the heat absorption.
  4. A warm soil is desired for germination, root development and soil organisms.
  5. It is source of many plant nutrients especially nitrogen, sulphur, phosphorus and many micro nutrients and as the organic matter is being decomposed carbon-dioxide escapes into the atmosphere from where it is taken in by plant leaves to manufacture starch.
  6. Humus has binding properties (it sticks) so it will hold sandy particles together to improve on the water holding capacity and because of its spongy nature it will improve on aeration and drainage in clay soils.
  7. Erosion will be minimized since the particles are stuck together.
  8. Because organic matter humus has a large surface area, it absorbs water easily and due to its spongy nature, the soils become more porus. It will be easier for the roots to expand and absorb more nutrients.
  9. The organic acids released during decomposition will help in the weathering of rocks to release nutrients.
  10. Fresh organic matter make phosphorus more readily available ii) acidic soil.
  11. It buffers or stabilizers soil PH
  12. Humus prevents leaching of nutrients since it holds the nutrients iron on its surface.

Experiment C

Experiment to find the percentage of Humus content in the soil.


  • Evaporating dish
  • Fresh garden soil
  • Source of heat
  • Stirring rod
  • Balance


Collect flesh garden soil with plenty of Humus.

Weigh n empty evaporation dish.

Put the soil in dish until it is half full and weigh.

Place the dish over source of heat while stirring for about 40min


The changes in soil.

Cool the soil in a descator and weigh again.

Repeat the heating until you get a constant mass.

Results and calculations

Mass of empty dish ————————– x gms

Mass of dish  + fresh soil ————————- x +y gms

Mass of  soil alone ———————-(x + y – x) = ygms

Mss of dish + soil after strong heating ————–(x + s)

Mass of soil alone after strong heating  ————- = (x + s  – x) = sgms.

Humus lost during heating is (y – s)gms




The are either plants (soil flora) e.g. bacteria, fungi, algae and plant roots or they are soil animals (funa) nematocks, earthworms, termites, moles, snakes, mice etc.

Importance of soil living organisms

  • When they die they decay to form human which is a source of plant food.
  • They produce many organic acids that can dissolve some part of the rock to assist in wreathing e.g. algae and lichens.
  • They are great decomposers of organic matter to form humus which will provide plant nutrients as well as improve on soil structure.
  • Barrowing animals e.g. earthworms and moles improve on soil aeration and water infiltration.
  • Earth worms in addition to improving soil aeration produce the cast that has a cementing effect on soil particles and so will improve on soil structure as well as adding nutrients like calcium and nitrogen.
  • Some special bacteria fix nitrogen from the atmosphere to the soil and others will convert it into soils which are usable to others.
  • Large organisms e.g. termites mix organic with the inorganic part of the soil so will help in the development of soil.
  • Some are pathogens so they will cause a great deal of damage to the soil e.g. bacteria, nematodes and some classes of fungi.


This is the inorganic push of soil it is composed of small rock particles and minerals of various kinds. It varies in particles size and chemical composition.

Experiment E.

An experiment to show that soil is made of different sized particles.


  • Fresh garden sub-soil.
  • Measuring cylinder (250cm)
  • A solution to disperse soil a aggregates e.g. sodium, hydroxide / sodium carbonate.


  • Put about 50gm of garden soil in a 250cm3 measuring cylinder.
  • Add about four times its volume of water containing sodium hydroxide / sodium carbonate to help disperse soil particles.
  • Cover the mouth of he cylinder with your hand and shake vigorously for about two minutes.
  • Put the cylinder on the table for about 1 hour to allow the content to settle.




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