Getting The Right Mix –Composting From The Microbiological Viewpoint

By definition a compost is a ‘made mixture’, with manure a common raw material. The aim of composting is to fuel the activity of micro-organisms by providing a source of organic carbon, to transform nutrients from the inorganic to the organic form. “Organic” implies that nutrients are locked up as components of cells. In the process of microbial growth, complex organic compounds are broken down with humic acids produced as a by-product. Water constitutes about 90% of the wet weight of a cell, with macromolecules constituting 96% of the dry weight (Madigan et al 1997). Macromolecules are synthesised within the cell, from the four main elements Carbon (C), Oxygen (O), Hydrogen (H), and Nitrogen (N). Hence the emphasis on the carbon-to-nitrogen ratio of a compost mix (see Table 1), and the importance of aeration and watering. However other nutrients are also required in relatively large quantities for microbial growth, namely Phosphorus (P), Sulphur (S) and Potassium (K), with Magnesium (Mg) to a lesser extent. If any of these elements are limiting in a compost mix, then microbial activity may be reduced. If a compost sufficient in organic carbon but deficient in other nutrients is added to a soil, microbial activity will be stimulated, drawing nutrients from the soil (Patriquin 1986). This phenomenon of ‘drawdown’ is not unique to nitrogen. Plants can be starved of the other major nutrients, due to the competitive advantage of microbes for nutrient uptake. For this reason, microbially active (immature) composts should not be used for potting mixes or as fertilisers for actively growing plants (Hoitink and Poole 1980).