Green Manure and Soil Health
Organic Farming in South Africa

Green manure is the planting of a crop with the sole purpose of improving the soil. Green manure (cover crops) improves the soil quality in the following ways:

Cover crops are ploughed into the soil to serve as green manure and increase the organic and microbial matter in the soil.

Increase organic material and build humus into the soil.
Supply nutrients for microbes.
Recycle nutrients.
Cover crops extract nutrients from deep in the subsoil and deposit them into the topsoil.
Legumes circulate phosphates and sulphur and increase soluble nitrogen (N).
The subsoil and clay layers are broken down, thus improving water and air penetration.
Cover crops decrease soil erosion.

Green manure is exactly what the name says: the producer buys seed to plant, which will never be harvested, but planted only to improve the soil’s nutritional quality. Examples include oats, chickpeas and lucerne. Note that if legumes are used as cover crops for green fertilisation, seed should be grafted with the right strain of bacteria in order to ensure nitrogen fixation.

Green manure can be planted between rows of different crops and then incorporated into the soil and does not necessarily have to be planted in an unutilised area. It will then serve as a weed control mechanism and ground cover.

If it can not be worked into the soil, the green parts can be harvested and used in the compost heap. The biggest disadvantage of green manure in an open area is that this area can not be used for one season, but it contributes to the quality of the soil and is used instead of fertiliser.

Green manure is at its best when it begins to flower and at this stage offers the most benefit for soil fertility. It is now chopped up and ploughed into the soil. If it can be sprayed with microbial inoculants beforehand, it will increase the micro-score in the soil and improve the humus-building properties of the soil.

Humus is formed with a process in which organic material is broken down into simpler components by bacteria and micro-organisms. An earthy, dark, crumbly substance is formed.

This substance is called humus and contains nutrients necessary for plant growth. Humus acts like a sponge, thus helping the soil to retain moisture and form plant nutrients. The crumbly structure helps to break down heavy clay soils and bind sandy soils into a more loamy soil. Wastewater and erosion are reduced by humus.

By Ananda Schoeman