soil fertility, carbon

Manage your soil fertility

Converting a field to organic production depends on how soil fertility is managed. Replenishing the soil is important as it determines the health of the plants.

Soil fertility isn’t simply a matter of adding synthetic fertilisers to the soil to provide nutrients to the plants. Fertile soil implies that the farmer treats the soil as a living organism.

The soil and all that grows in it is part of the cyclical agro-ecosystem. Plant roots grow well in healthy soil; with good root growth soil life is promoted. Healthy soil depends on the type and quantity of living organisms present, the humus and organic matter content, adequate drainage and air, the soil particle structure and the water-holding capacity of the soil.

Fertility and structure can be broken up into three categories; biological, physical and chemical.


The presence of bacteria, fungi, protozoa and soil ‘creatures’ indicate soil fertility. Soil bacteria break down organic matter which is then made available for plants through water. Bacteria produce a sugary gum that helps soil particles to stick together which enhances the water-holding capacity of the soil.

Fungi can break down some of the organic matter in soil that bacteria cannot.

Mycorrhiza fungi form a relationship with the roots of certain crops and assist in the uptake of some nutrients. Mycorrhiza also help to protect roots against pests.

Unwanted fungi on the other hand, can spread disease to plants. These fungi need to be understood in terms of what preys on them, the habitats they favour and so forth. Understanding the agro-ecosystem will help to eliminate the fungi without using chemicals.

There are larger soil creatures like spiders, earthworms, nematodes, little beetles and minor bugs, carrying things to and fro. These organisms help to break down material for the bacteria and the fungi to divide further. The intricate food webs created by these organisms help break down the materials faster so that the soil becomes healthier and stronger. Then the micro- and macro-populations of the soil benefit from the cycle created, because the soil has better capacity to provide shelter and food.


The arrangement of soil particles, the air spaces between them and the texture of the soil mainly determine physical soil fertility. The two extremes of soil structure are sand and clay. Sandy soil particles don’t stick together well and erode easily; clay soils stick together well but form hard clods that don’t allow absorption of penetration.

The best option is at a point midway between these two soil types. Dig organic matter thoroughly into the soil to improve the structure. Adding organic material allows for the movement of water and nutrients.

soil fertility
In heavy clay soil, the particles stick together in such a way that they do not allow absorption or penetration.

Because a good flow of nutrients and water is important for soil health, it will also, to some extent, determine resistance to pests.


This is determined by the quality and quantity of nutrients in the soil in relation to what plants need for growth. Plant nutrients in the soil are: primary minerals eg from erosion; secondary minerals obtained through chemical weathering processes; nutrients present as ions that are attached to the water molecules in the soil.

Soil organisms and biological life help to accelerate the rate at which primary and secondary nutrients break down. They can also make fixed nutrients available to plants.

Mycorrhiza fungi can cut phosphate applications by 5% to 20% in crops planted in fields where there is a low level of plant-soluble phosphates, but high level of fixed phosphates. Rhizobium bacteria fix nitrogen in the soil.

soil fertility
Adding organic matter to the sandy soil (left) can mean dramatic improvements in the physical properties of the soil.

The problem with synthetically derived fertilisers is that they only add minerals and no life to the soil. These products may be harmful to beneficial organisms. Without biology, you are also stuck with soil pH as the sole measure of nutrients available to plants.

If the chemical make-up or balance of the soil is disturbed, you’ll have to add fertiliser. Remember that every element works on every other element in an interdependent manner. Too much of one can make another one unavailable to the plant.

soil fertility
In healthy soil there is a balance between the biological, chemical and physical processes. Cultivation disturbs this balance, therefore it is the farmer’s responsibility to maintain and restore soil health.

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