Green revolution led to increased dependency of cultivators on synthetic fertilizers that did not increase the long term fertility of soil. The result is the loss of nutrients from the top soil. If these nutrients are not replenished, the soil will suffer from nutrient depletion and will become unusable for further farming. The concerns relating to land degradation, threat to eco-systems over and inappropriate use of inorganic fertilizers, atmospheric pollution, soil health, soil bio-diversity and sanitation have rekindled the global interest in reducing the use of chemical fertilizers. The alternate solution to all the existing problems may lie in the conversion of nutrient rich waste into a value added product through low labor intensive and eco-friendly technology. Composting offers several benefits including soil fertility, increased agricultural productivity, improved soil bio-diversity, reduced ecological risks and a better environment.

What is composting?
Composting is the controlled decomposition and subsequent stabilization of mixed organic substrates under aerobic conditions that allow the development of thermophillic temperature as a result of biologically produced heat. Compost is a dark brown, crumble material obtained from a mixture of decayed or decaying organic matter. It is used to fertilize soil. The composition of feedstock materials, method of production, and size and time of production, affect the chemical, biological, and physical characteristics of finished compost.

Importance of compost maturity
Many uses of compost require a mature product that prevents the soil nutrients from being immobilized. Thus, mature compost is important because it will not adversely affect plant development due to reduced oxygen or nitrogen availability and/or the presence of phytotoxic compounds.

What is compost maturity and stability?

Maturity refers to the degree or level of completeness of the composting process. For compost to be mature, raw materials (feed stocks) should be sufficiently decomposed to produce a stable product.

Stability determines the potential impact of the compost material on nitrogen availability in soil or growth media.

If the decomposition of the organic materials is not complete, the aerobic microbial activity continues in the soil, resulting in oxygen deficit land and if the waste is completely decomposed, the resulting highly stable product does not provide sufficient nutrient to the soil.

Impact of immature compost on plant/soil health

Compost maturity refers to determine the grade of compost for product performance. Maturity is not related to quality, but to what stage in the composting process the material has decomposed.

Immature compost when added to soil continues to decay, a process that requires both nitrogen and oxygen. The high level of microbial activity in unfinished compost requires a large intake of oxygen, and the microbes may pull this from the surrounding soil, essentially suffocating the roots. Immature compost can eat up plants' oxygen supply

.It may stunt, damage, or even kill the plants, rather than having a positive impact on their growth.

Immature compost may have high C: N ratio (>30:1). A relatively large proportion of available carbon may affect the availability of other nutrients if they are present at limiting levels. The high carbon to nitrogen ratio (C:N ratio) of immature compost also means that, as the carbon compounds continue to break down, microorganisms will take available nitrogen of the soil making it N deficient. Such composts should be used as mulches and supplemental N may have to be provided to overcome short-term N immobilization.

Partially mature compost may have high content of phenol and organic acids that can harm the plant growth and may cause burn" leaves, or stunt growth. Many organic acids are phytotoxic, often causing immediate injury to plants that show white or chlorotic (bleached) leaves. In composts with a high C: N ratio (> 40:1), acetic acid is the primary organic acid formed along with some alcohols.

Immature compost with high level of electrical conductivity value > 3-4 mmhos/cm (highest in manure-derived composts) leads to phytotoxicity. Adding compost high in potassium can burn plant roots, as potassium is a soluble salt.

In composts with very low C: N ratios, anaerobic conditions yield ammoniacal products (putrid odors) that cause the toxicity. The risk of ammonia toxicity is greatest with composts having high nitrogen content (i.e low C:N ratio), especially if the nitrogen is in an available form. Such compost should be applied at or below the agronomic rates (i.e. no more nitrogen than the plant can use). High concentrations of ammonia can cause the death of plants. Lower levels cause burn on the margins of young leaves. On less sensitive plant, root tips turn brown.

Immature compost may have weed seeds and pathogens that can cause damage and disease to the crop plants

Compost should be tested for specific minerals before its application to soil because boron toxicity is the most widely known example that can cause crop losses. Excessive boron can get introduced with feed stocks like coal ash and cellulose insulation, etc.

Secondary pathogens and their toxins, e.g., spores and endo-toxins generated by bacterial and fungal growth within the composting process itself may also be present in unfinished compost. Volatile and semi-volatile organic chemicals of both synthetic and natural origin (including noxious odors) along with persistent, lipophilic organic chemicals may harm the crop plants

Tips for compost maturity

Its smell should be earthy and not sour, or like ammonia.
It should not produce heat after it is turned or wetted.
Its color is dark brown.
It should have a C:N ratio of 10-15:1
High level of CO2 and NH3 are often associated with volatile compost emissions and are predictor of instability. Their level should be in mature compost.

The composting process may last from few weeks to many months, depending upon the types of materials used, water requirements, and turning of the piles. A properly made heap will reach temperatures up to 160°F in several days. During this time, there occurs reduction in biomass.

Carbon is lost as CO2 and nitrogen as ammonia.
Though, compost is most indispensable input in ensuring good soil structure, in plant production, healthy root development and soil biological activities and nutrient retention but using immature compost can cause more damage than good to plants. When added to soil immature compost can temporarily reduce plant available nitrogen, add weed seeds, pests, diseases causing pathogens and root-damaging organic acids. Though, unsafe for cultivable land, immature compost can be used to build organic matter content in depleted soils.

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