1. Introduction

Global agriculture is facing a trend in yield decline for most crops. This is specifically applicable to crops that are practised under a mono-cropping system. It is a well known scientific fact that monoculture has a negative impact on soil biodiversity. This is applicable to sugarcane production (WWF Report; Sugar and the environment).

With mono cropping and overuse of land, it has become necessary for farmers to resort to more drastic measures to maintain yields. One such practise is to increase N, P and K chemical fertilisers at ever increasing costs, because the perception is that the higher the fertiliser levels the higher the yield.

This is often the result of a lack of understanding that soil is a living eco-system and that there is a difference between soil fertility and plant nutrition. To address this misplaced perception it is necessary to understand the difference between soil fertility and plant nutrition.


2. Soil fertility / Plant nutrition

Fertility refers to the inherent capacity of a soil to supply nutrients to plants in adequate amounts and in suitable proportions as well as oxygen, moisture and to maintain a healthy soil bio-diversity (active micro-biology, immune system). The focus here is soil health.

Plant nutrition refers to the manipulation of the nutrient status of the soil by adding organic and inorganic fertilisers according to the crop’s need. The focus here is on the crop’s needs.
The question can be asked what makes a soil fertile and what factors have a negative impact on soil fertility?

There are three basic soil components that contribute to soil-fertility. They are:

  • Soil micro-biology (soil bio-diversity)
Soil Physical characteristics
  • Soil Chemistry

All these factors are interlinked and no single soil type has all these factors in the ideal combination. It is further complicated by other factors like parent material, climate, slope, water quality (irrigated areas) and agronomic management practises.

Unfortunately soil nutrition was relegated to a simple recipe of four elements provided through chemical fertilisers e.g. Nitrogen (N), Phosphorous (P), Potassium (K) and Zinc (Zn) to meet only the crop needs at the expense of soil fertility. Very little attention was given to the important role of bio-diversity and active microbiology in plant nutrition. It is only in the last couple of years that there is a serious interest on this matter.


3. The role of bio-diversity

Active and healthy soil microbiology is able to:

  • Mineralise nitrogen and sulphur
Suppress nematodes, bacterial and fungal diseases
  • Actively decompose organic material
Improve root development with the result of better nutrient and water uptake
  • Recycle and keep nutrients available for plants, specially micro-nutrients
  • Improve soil physical and chemical conditions by increasing the humus content
  • Improve water holding capacity of soil
  • Less KWa power needed for soil tillage


4. Agronomic practises that contribute to the yield decline / plateau and loss of bio-diversity

  • Monoculture. Soils are nutritionally stripped and low microbial activity occurs.
  • Inefficient or no fallowing which result in increased levels of disease.
  • Burning of plant residues result in the gradual decline in microbial activity and N- mineralization in the soil.
Increasing pressure from weeds.
Increasing compaction through soil tillage and harvesting practises causing poor infiltration and soil aeration.
  • Poor irrigation practises. Over irrigation causes leaching of nutrients and increase in root diseases.
  • Decline in water quality in major river systems is causing a gradual build-up of salinity and sodicity.

In most cases yield losses occur because of a combination of these factors mentioned. The solution to low yields/yield plateau requires a “ back to basics” approach where sound agronomic management practises are adopted.

The following can be done improve soil bio-diversity and therefore yields:

  • Crop rotation
  • Fallowing and green-manuring
If there is not sufficient time to introduce proper fallowing or green-manuring practises compost can be applied to the soil


5. The role of compost in restoring soil biodiversity in commercial agriculture

Many books have been written about the role of compost in improving soil bio- diversity as well as the making of compost. It never became a standard practise in commercial agriculture for the following reasons:

It is bulky and transport costs did not make it viable
  • Practical problems with application
  • The value was always measured in terms of N, P and K content and in monetary terms.

Times have changed however and recent research across the world has shown that soil bio-diversity has great value in commercial agriculture both from a fertility as well as a plant nutrition perspective. Compost is a great and fairly quick way in restoring soil fertility although it must be made clear that it is a long term approach that is necessary.

The majority of third world countries do not have the economic means as well as infrastructure to import large quantities of chemical fertilisers. There is enough organic material on any farming enterprise that can be used to manufacture compost.

The marketing of our produce into European countries are going to force us to seriously reconsider using softer approaches to plant nutrition and pest control if we want to compete successfully in these markets.

It has become necessary to review the role of compost, manures, organic fertilisers and stimulants in a new perspective, as yields continue to decline even in the wake of increased chemical fertiliser levels.


Article written by F. Botha (B.Sc Hon Soil Science)
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