‘Humus’ is the term used to describe the more stable forms of dead organic matter in the soil stemming, among others, from plant roots, crop residues and organic fertilisers. Some components of the organic matter supplied to the soil are broken down in a matter of weeks or months by soil fauna and microorganisms, whilst others remain as humus in the soil over decades or centuries.
In order to ensure soil fertility, the decomposition and subsequent delivery of humus must stay more or less in equilibrium, i.e. the humus balance must remain steady. Organic matter is contributed e.g. by dead bits of plants (including crop residues) and organic fertilisers (e.g. dung, compost). As soil organic matter is broken down by soil organisms, the energy locked in the organic matter and the available nutrients are used. At the same time, the soil organic matter is mineralised, i.e. converted into simple inorganic components (ultimately into carbon dioxide, water, and inorganic substances that can be reused as plant nutrients). Intensive tillage (deep primary tillage and all-over fine seedbed preparation, the norm e.g. when growing crops such as potatoes) speeds up mineralisation, whilst the cultivation of multi-annual temporary leys slows it down. Within the SAEDN, the humus balance is calculated according to the method of Neyroud (1997). You’ll find more information on the calculation below.