By September, South Africa is moving into another growing season. Spring planting begins for a range of summer crops, while deciduous orchards and vineyards come out of winter dormancy as temperatures rise. Food gardens, community growing projects and landscaped spaces may also be preparing for planting and warmer conditions.
This brings greater attention to soil and what plants will need during the growing season. Plants rely on soil for water and nutrients, while its physical condition affects how water moves through it, how much air is available and how roots develop. Soil requirements vary according to factors such as its existing condition, what is being grown and how the land is managed.
Compost is relevant to this preparation because it adds organic matter and nutrients to soil. Spring also raises a useful question about the source of that organic matter and the potential value of materials currently managed as waste.
What organic matter is doing in the soil
To understand the value of compost, it helps to first understand what organic matter contributes to soil.
Soil has physical, chemical and biological properties that interact with plant growth. Its structure, for example, affects how water enters and moves through the soil, as well as aeration and the space available for roots. Its chemistry affects nutrient availability, while biological processes contribute to decomposition and nutrient cycling.
Organic matter is involved in several of these processes. Adding it can contribute to soil aggregation and porosity, with implications for infiltration, aeration and water retention. It also provides material for soil organisms and contains nutrients that can become available through decomposition.
Compost provides one source of organic matter, although not all compost is the same. Its nutrient content and other characteristics depend on factors such as the materials used to produce it and how the composting process is managed.
Vegetables such as leafy greens, tomatoes, peppers and brassicas have different nutrient requirements, and harvesting removes plant material and nutrients from the growing system. How compost is used needs to take the crop, existing soil conditions and the wider fertility programme into account.
Deciduous orchards and vineyards have their own requirements. Spring follows their winter dormancy period and brings renewed plant growth, so the role of compost needs to be considered in relation to what the soil and crop require and the characteristics of the compost being applied.
So, where does the organic material come from?
Food production and processing, retailers, hospitality businesses, households and landscaping activities all generate organic material. Think vegetable offcuts, food residues and suitable garden material. Once these materials are no longer required for their initial use, they are generally classified as waste. Their organic matter, carbon and nutrients, however, remain present.
How this material is managed determines whether those resources can be recovered. If suitable organic waste is mixed with general waste and sent to landfill, it is no longer available as feedstock for composting. Separating suitable materials allows them to be biologically decomposed and converted into a more stable material for use as a soil amendment.
This is what we mean by the “compost dividend”. Take something as ordinary as a vegetable offcut. It may no longer be suitable for consumption, but it still contains material that can be recovered through composting. If correctly separated, that offcut can become part of the feedstock used to produce compost.
In-vessel composting systems such as BiobiN provide one way to manage this process in a controlled environment. Suitable food and other organic materials can be kept separate from general waste and processed through composting.
The relevance to spring lies in the timing. Growers may be preparing soils for planting or renewed growth and considering whether organic amendments such as compost are appropriate for their soil and crop requirements. Meanwhile, food and landscaping activities continue to generate suitable organic materials that can be used to produce compost.
This creates productive use for materials that might otherwise be sent to landfill. Composting recovers suitable organic material and returns it to soil in a different form, linking organic waste management with an existing demand for soil amendments during the production season.
Image: Krista Glīzdeniece
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