Soil is an Important Component of Agronomic Systems

Importance of Soil

1. Soil contains and holds plant roots:

Plant roots grow between the soil (mineral matter). The soil supports the roots and helps plants to anchor and stand strong. Soil mineral matter is about 45% of the soil volume and also attracts nutrients for plant uptake.

2. Soil contains and holds plant nutrients:

Plants absorb and feed on nutrients from the soil. They absorb the nutrients through their roots. These nutrients formed from the slow chemical weathering of soil, dead plants and animals. The dead plants act as nutrients to plants when decomposed and also add to the organic matter content of the soil, which makes up about 5% of the soil volume.

3. Soil contains water:

Plants consist of up to 90% water. Plants absorb water through their roots, and roots take water from the small spaces between soil. As they absorb water, plants also absorb nutrients in the water.

Water is present in the soil as a thin layer that clings to the soil. It is also held by organic matter (dead plants). Water can make up about 25% of the volume of soil.

4. Soil contains air:

Plant roots need oxygen to breathe. Air is present in the spaces between soil. Air makes up about 25% of the total volume of the soil. The oxygen allows soil organisms and plant roots to breathe. An example of a volume of soil is illustrated below.

 

 

5. Animals are also dependent on the soil:

Animals eat the plants that grow from the soil. We could not survive if there was no soil because we as humans eat both plants and animals.

The soil is man’s basic resource. Without it, there can be no agriculture, no plants, no animals, and no people. There are not many things that are more important than soil. Therefore, we must learn to conserve, improve, and care for the land.

 

Soil Constituents

1. Inorganic particles:

Inorganic particles (minerals) come from rocks that are broken down or weathered. Inorganic particles make up about 45% of the volume of soil. We can also refer to soil as grains or sand, depending on the texture.

Plant roots grow in the spaces between these particles. Soil particles are not all the same size. It can be separated using sieves with meshes of different sizes. Particles that are more than 2 mm in diameter are called stones or gravel.

Soil classification.

Soil	Size
Coarse sand	Particles more than 0.2 mm in diameter.
Fine sand	Particles more than 0.022 mm in diameter.
Slit	Particles more than 0.002 mm in diameter.
Clay	Particles less than 0.002 mm in diameter.

 

The texture of soil depends on the sizes of the particles in it. For example, soil with a lot of sand in it has a coarse texture whilst soil with a lot of clay in it has a fine texture. The texture of soil describes how coarse or fine it is.

Soil textures are therefore classified as follows:

• Sand: 10% clay.
• Loam sand: 10-15% clay.
• Sandy loam: 15-20% clay.
• Sand clay loam: 20-35% clay.
• Clay loam: 27-40% clay.
• Sand clay: 35-55% clay.
• Clay: 55% clay.

2. Organic material:

The organic material found in the soil refers to any biological material originally produced by living organisms in the soil. The organic matter may consist of living or dead plant material, living or dead animal material, or living or dead insects.

Dead plants and animals decompose (rot) in the soil as it is eaten by fungi and bacteria. After a while, you can no longer see what it used to be because it is broken down. Living plants absorb the nutrients, dissolved in water. Organic material helps to make the soil more fertile. Soil rich in organic material has a dark brown colour.

Remember, the best thing you can do for soil, is to mix organic material in it.

The benefits of organic material:

• It increases soil fertility by releasing plant nutrients.
• Holding water so that the soil does not dry out quickly.
• It binds soil together into loose crumbs that allow air to penetrate through the soil.
• It gives a better structure to the soil.
• Provide nutrition to soil organisms and insects.

3. Living organisms:

Earthworms, beetles, ants and termites live in the soil. There are also many fungi and many bacteria. Bacteria are called micro-organisms. They are very small. (A few grams of soil can contain about five million bacteria.) The fungi and bacteria feed on the remains of dead plants and animals and cause them to decompose. 

 

The Importance of Clay Percentage

Clay in the soil is very important because it not only determines the texture and structure of soil but also can attract and retain plant nutrients, especially positively charged nutrients since the surface area of clay particles is negatively charged.

For example:

 
The negative surface area of clay attracts positive loaded nutrients.

The structure of soil depends on how well the particles cling together. This has a big effect on the properties of soil, the plants growing in it, its ability to retain nutrients as well as the permeability of the soil.

 
Soil structures with their permeability.

 

Soil Characteristics and Classification of Soil

There are various types of soils in South Africa. Scientists classified these different types by specifying the type of soil in the upper level (A horizon) on top of the type of soil in the subsoil (B horizon).

 
Different soil layers.

Topsoil (A horizon):

• Is often dark in colour, due to humus.
• Has a depth of 5-30 centimetres.
• Contains plant roots.
• Is usually fertile.
• Supplies water, nutrients, and support for plants.
• Should have a good crumb structure.
• Should be well-drained and aerated.

Sub-soil (B horizon):

• Lies below the topsoil.
• Is lighter in colour and has little humus.
• Has never been cultivated.
• Has few plant roots or soil organisms.
• Is often poorly drained and aerated.
• Allows deep-rooted plants to absorb water and some nutrients.

Weathered parent material (C horizon):

• Often referred to as weathered rock below the B horizon.
• Can be deeper than 1 meter.
• Produces the subsurface and topsoil by weathering.
• Has coarse particles and rocks present.

Parent Material or (Bedrock):

• Occurs very deep below the C horizon.
• Is sometimes called the foundation or parent material.
• Is the original rock from which the topsoil was formed.
• Occurs sometimes when drilling holes are drilled.
• Underground water accumulates on it.

The A-horizon is also known as the topsoil and is of extreme importance for crop production. Topsoil is the surface layer of soil, extending in most gardens to a depth of 15-30 cm. It is in this region that annual plants form most of their feeder roots and find most of their food. A more compact layer lies below the topsoil and is often markedly different in colour and texture. It may contain plant food but is not ideal for crop growth because it lacks humus and has a poor structure. The drainage of soil often depends on the nature of the subsoil or underlying layer (bedrock). If it is clay it forms an impermeable layer preventing or slowing down the drainage.

Soil is classified into different soil forms consisting of separate soil families. Soil forms are classified based on the different soil horizons which are identified according to the soil’s distinct characteristics. These horizons or layers of soil should be identified in the soil profile with their distinct colour variation that can be determined with a Munsell soil colour chart. The naming of the soil forms will depend on the sequence of the soil horizons, where a typical example will be that of an orthic A horizon overlying a red apedal B horizon which is called the Hutton soil form. For simplicity, the soil family classification has been omitted here. When examining a freshly exposed soil profile, it is usually easy to recognise different soil horizons. Soils are always changing and so should our knowledge about soil.

Soil is classified under these main types:

• The taxonomic system of soil classification involves the classing of soils into soil forms and families depending on the sequence and characteristics of the horizons.
• The family is a category of the system of soil classification in the same sense as the order, great group, and subgroup with special attention given to the characteristics of soil horizons to determine the soil family of that specific soil form.