Why Composting is Important
Wine Grape Growing in South Africa

© Marinda Louw
A good compost heap should consist of green materials such as fresh grass cuttings, leaves and fruit as well as soil, old compost, weathered plant materials and manure.
After the growing season all the essential nutrients necessary for plant growth and soil structure have to be reapplied to the soil in the form of compost. Compost is decomposed organic material that is applied to serve as nutrients for the soil microbes but not the plant. This organic material is broken down by yeasts, fungi and other soil microbes in the soil until it is soluble in groundwater and can therefore be absorbed by the plant roots. In this way nutrients are supplied to the plant.

When compost is thoroughly broken down by microbes, humus is formed. It is the most stable form of carbon. The benefits of humus include the maintenance of minerals, water retention (the humus complexes absorb and bind water) and it helps to stabilize the soil’s pH by buffer action. Good compost is added to improve the soil structure, water-holding capacity and to increase soil life by providing organic matter for soil-borne microbes. Some producers also add compost to the soil with the planting of new crops.

The effect of compost on a plant’s growth can continue over several seasons. The amount of compost applied depends on the crop being cultivated, the quality of the crop expected and it is also determined by the soil’s needs and the nutrients in the compost. It is therefore important to determine the mineral content, pH and other properties of the compost by analysis prior to application. However, it should be remembered that deficiencies in the plant should be corrected with leaf nutrition and additives.

The most common form of compost is obtained from a mixture of animal manure (nitrogen) such as cattle manure and plant material (carbon) such as straw, leaves or (alien) trees. Waste products on the farm such as mulched yard trimmings, grape skins and chopped up cover crops can be used for composting. A specific ratio of carbon to nitrogen is required for the optimal degradation of raw materials into a well-balanced compost - the so-called C:N ratio. The microbiological process of composting requires moisture, oxygen and time as well as a temperature of 50 - 60°C for a certain amount of time to destroy weeds. Compost should be ‘well-digested’ as ‘raw’ compost can burn the plant roots and increase the loss of minerals.

A general recommendation for the application of compost where minimum tillage is applied, is 20 - 30 m³ / ha for vegetables and about 10 m³ / ha for vines, fruits and grains. Even as little as 5 m³ / ha of good microbial compost can make a difference in soil that is micro-biologically ‘dead’. Compost extract, a liquid mixture of compost and water that has been aerated with an air pump, can be applied to the soil through irrigation or with a sprayer.By Marinda Louw