The breakdown of organic matter can be divided into two distinct processes, each carried out by different soil micro‐organisms. The two processes are:

  • The breakdown of Carbohydrates.
    • The breakdown of Proteins.


Carbohydrates are made up of Sugars, Starch, Cellulose, Fats, Oils, Waxes, and Lignin. Cellulose makes up the cell walls in young plants but turns into Lignin when the plants mature. The rather hard, woody stems of mature ripe maize consist mainly of Lignin.

Figure 1: A plant cell


As you know from your Botany Lectures, in all green plants which contain chlorophyll, there are two processes going on at the same time; one of these is photosynthesis and the other is respiration. Photosynthesis is the process whereby the green plant obtains energy from sunlight and uses this energy to build the simple chemicals of water and carbon dioxide into sugar and oxygen. This is a building up process. The reverse of this process is Respiration, whereby sugar and oxygen is broken down into carbon dioxide and water and energy is released; this energy is used by the plant for growth. The combined processes of Photosynthesis and Respiration are together known as the Carbon Cycle and can be represented by the diagram on page 2.

As you know from the previous lecture, organic matter is broken down by soil micro‐organisms, mainly bacteria. The simple Carbohydrates, (starch, sugar and cellulose) are used by the bacteria as food and are broken down very rapidly. Remember that, because the bacteria contain no Chlorophyll, they cannot use the process of Photosynthesis to obtain their food; they live on raw organic matter.

The more complex Carbohydrates, (oils, fats, waxes and lignin) are also broken down by bacteria but much more slowly. The result of this breakdown is the formation of humus in the soil. This process can be explained simply by the following diagram:

Table 1: The process of humus formation


The Carbon Cycle has already been dealt with in the first part of this Lecture. The Nitrogen Cycle is the method by which the protein part of the plant residue is broken down (as you know from your Botany Lectures) all protein contains nitrogen. In the Nitrogen Cycle, the plant protein is broken down by bacterial action into Amino Acids and then these are further broken down into the gas ammonia. This is a Breakdown Process.

Some of the Ammonia Gas escapes from the soil. The remainder is built up by bacterial action, firstly to form nitrites and then to form nitrates. Nitrates are the form in which Nitrogen is taken up by living plants which, in turn, build this Nitrogen into Amino Acids and then plant protein. The Nitrogen Cycle can be represented by the following diagram:

Figure 2: The Nitrogen Cycle

The breakdown of plant protein into ammonia gas and the build up into nitrates for use by other plants is a slow process, and the nitrogen is slowly released from organic matter. This is the main difference between using an artificial fertilizer as a source of nitrogen and using organic matter. The nitrogen from artificial fertilizers is quickly available, and the result can be seen in a crop after about two weeks, provided rain has fallen and the fertiliser has been washed into the soil. Nitrogen from the breakdown of Organic matter is available more slowly but is longer‐lasting and will help the crop throughout its growing period.

Figure 3: The Carbon Cycle

Source: theclimatehub


Having seen how organic matter is broken down in the soil into Humus and Nitrogen, we can now look at its properties. However, it is important to distinguish between raw organic matter, that is crop residues or green crops that have just been ploughed into the soil, and Humus. Humus is material that has been broken down by bacterial action; in other words, it has decayed.


This is the freshly ploughed‐in material, and its properties are mainly physical. It helps mainly to aerate soils (i.e. to allow air to penetrate the soil), especially heavy clay soils. It helps to drain soils and to allow the water to run down through the soil, again especially in heavy clay soils. And of course because it is raw organic matter, it encourages the activity of the soil Micro‐organisms by providing plenty of food for them.


Humus is a Complex mixture of very fine organic substances. It contains substances derived from lignin and also from the micro‐organisms that break down the lignin: e.g. dead bacteria. It is no

longer recognizable as plant material, and it consists of very fine particles thoroughly mixed with the mineral particles of the soil; it is an inherent part of the soil. It is dark in colour and contains carbon, nitrogen; calcium, potash and phosphate. It is found mainly in the topsoil and seldom penetrates to the subsoil. Humus has the following properties:

  • Humus being black makes soils darker. The darker colour increases the heat‐ absorbing capacity of the soil and so it warms up faster in the spring.
    • Humus helps the structure of the soil by holding the soil together; the soil particles join together to form small crumbs. In this state, a soil is said to have a good crumb structure. All this helps the formation of a good tilth.
    • Humus is colloidal and has colloidal properties.
    • The Humus Particles absorb water and swell and so help to hold water in the soil. This is  most important in sandy soils.
    • Humus Particles hold some nutrients on the surface of the particle in the same way as clay particles. Nutrients held in this way are mainly, Calcium, Magnesium, Potash, etc. The Humus particles also help to control the soil acidity and alkalinity, and the soil Base Exchange (see Lecture 4).
    • Humus makes mineral nutrients available to the plant.


It is extremely important to maintain organic matter in the soil because this helps both Soil Fertility and Soil Structure. Organic matter can be maintained in the soil by the following methods:

  • Ploughing in Leguminous green crops. This also adds nitrogen to the soil as well as organic matter.
    • Ploughing in other green crops, particularly grass pastures.
    • Ploughing in any type of manure eg: Kraal, Pig or Poultry manure.
    • Ploughing in Crop residues such as straw, stubble, maize stover, etc.
    • By the use of sound rotations, so that organic matter is not used up and never depleted. Monoculture is the growing of the same crop continuously on a land and must be avoided. Where possible, use a grass break; 2 ‐ 3 years down to grass does a tremendous amount to build up reserves of organic matter.


It is important at this stage to be certain that you know the difference between soil texture and soil structure. Texture refers to the actual composition of the soil, and the size of the particles. Soils are classified by Texture into Clay, Sandy, Peat, and Calcareous. The texture of the soil cannot be influenced by the farmer except, perhaps, by adding sand to a clay soil. However, this is not really practical, as it would take a tremendous amount of sand to have much influence on a clay soil. The texture of the soil governs completely the cropping of a farm: e.g. tobacco grows best in a sandy soil.

Soil structure is concerned with the Arrangement of the soil particles. Soil structure controls the following factors in the soil:

  • It determines the total pore space and pore size distribution (see Lecture 8), which in turn, control the water and air supply to the plant roots.
    • It influences the penetration of plant roots in the soil. Plant roots can penetrate more easily and more deeply into a soil which has a good structure.
    • It improves the stability of sandy and organic soils by helping the small particles to bind

together into larger aggregates. In other words, it improves the crumb structure of the soil.

  • Good soil structure prevents crusting on the soil surface.
    • In a soil with good structure, the soil particles should be built into water stable, aggregates measuring from 1,5 mm in diameter and containing both clay and organic matter particles.

Soil structure can be greatly influenced by the farmer. Soil structure can be damaged by:

  • Ploughing or cultivating at the wrong time. If a clay soil is worked when it is too wet, it becomes plastic and bakes into hard lumps on drying out and the good structure is lost.
    • The soil surface can be compacted by the trampling of livestock or by running over it with heavy implements. This drives out the air and makes the surface plastic therefore water is unable to penetrate into the soil. It also harms the soil drainage.
    • Continuous monoculture destroys the humus in the soil and with it the water and nutrient‐ holding capacity of the soil. Light land becomes like desert sand and, under dry conditions, the soil will dry out and blow away
    • Where a soil is flooded by sea water, the structure is completely destroyed. The calcium in the soil is replaced by sodium from the sea water and this causes the formation of a very sticky and unworkable type of soil.

Soil structure can be assisted:

  • by adding organic matter in the form of manure, green crops that are ploughed in, low stocking rate, etc:
    • by preserving humus from good rotations and using the grass break.
    • by ploughing early to allow for weathering and frost action which helps to break the soil down into a good tilth:
    • by cultivating heavy land when the soil is drying out to allow the clods to crumble. On light land, cultivating before the soil is dried out prevents moisture loss.
    • by liming to allow for the flocculation of the clay and humus particles.
    • By good drainage, to allow water the quickest passage through the soil and in doing so to draw air into the soil Pore spaces.