What is a soil?

Soil is the uppermost surface of the earth, which has been slowly transformed by decomposition due to the effects of weather, vegetation and human activities. The parent material from which soil is formed can be the underlying rock, deposits from rivers and seas (alluvial soils) or the wind (aeolian soils), or volcanic ash.

What are the soil constituents?

Soils is composed of:

• mineral particles
• the products of weathering
• organic matter
• water and air

Soil texture and structure are of special importance for soil fertility and plant growth:

• Solid particles are classified by size into gravel and stones, sand, silt and clay.
• Soil texture refers to the relative proportions of sand, silt and clay in the soil. Depending on the soil's texture, it is described as sand, sandy loam, loam, clay loam, clay, etc. Soil can also be characterized as light, medium or heavy based on its workability.
  
• Soil structure refers to the aggregation of the finer soil particles into crumbs or larger sizes.

What does soil do for plants?

Soil supports plants by providing a permeable layer for their roots. It stores plant nutrients and water.

Depending on their composition, soils differ in their ability to supply plant nutrients.

What determines soil fertility?

Contrary to what is widely believed, the colour of the soil reveals very little about its fertility.

How does soil hold nutrients and release them?

Decomposing rock material forms soils and releases plant nutrients. The original mineral content of this material - and the nature and intensity of the decomposition process - determine the kind and amount of nutrients released. Clay and organic matter retain nutrients in a plant-available form, that is, the nutrients are attached to the soil constituents.
Soil's ability to retain a certain amount of nutrients determines its natural fertility.

Nutrients, which carry positive and negative charges (cations an anions), are attracted by the clay and organic matter in the same way that metal filings are attracted by a magnet.

Soil water containing the nutrients in dissolved plant-available form is called the soil solution. Nutrients can only be taken up by roots in dissolved form. Therefore, they have to be released from the storing complex into the soil solution to be plant-available.

The process of nutrient adsorption (the attraction of water molecules and of ions on the surface of clay or organic matter particles) and release to the soil solution is very important. The difference in the adsorption strength of the cations and anions helps determine how and when fertilizers (particularly nitrogen fertilizers) should be applied to achieve the highest efficiency and avoid pollution by leaching.

Organic matter can adsorb more nutrients than a comparable amount of clay. It is therefore important to build up the organic matter, especially in degraded tropical soils with less ability to adsorb the mineral component.

Soil organisms increase soil fertility

The activities of soil organisms are indispensable for high soil fertility and good crop production. Most of these activities are beneficial for the farmer. Soil organisms decompose organic matter to produce humus; aggregate soil particles to provide better structure; protect roots from diseases and parasites; retain nitrogen and other nutrients; produce hormones that help plants grow; and can convert pollutants that find their way into the soil.

After being mixed into the soil and ingested by earthworms, the insoluble forms of nitrogen, phosphate and sulphur contained in the particles.

Farmers need good knowledge of their soils in order to improve their fertility.