• Eleanor Dring

What is Compost?

Updated: 2 days ago

Part 1

By E. Dring

Composting is the biodegradation of organic matter to form a mineralised humic product known as compost (Argun et al., 2017). Compost gives nutrients and minerals back to the soil. It is a natural process that takes place in the environment, such as trees shedding their leaf litter in autumn. The decomposition of the leaves replenishes the soil with organic matter. Composting is a valuable opportunity when moving towards a circular bio-based economy, reducing food waste whilst producing a useful product to help maintain food security (Razza et al., 2018).

The Composting Process:

Solid waste goes through several phases to become a stabilised product. The process of composting starts with organic components, such as garden and food waste, paper and sludge (Chadar et al., 2018). The organic waste begins in mesophilic conditions then undergoes self-heating and enters the thermophilic stage. Aerobic conditions allow for the biological decomposition of materials by microorganisms. When the compost matures, it returns to ambient temperature, this is known as a mature mesophilic stage. Compost rich in nutrition is formed as a result of the process (Figure 1).

Figure 1 - Aerobic composting process.

Aerobic: Composting mainly takes place in aerobic conditions, where oxygen is readily

available. In these conditions, microorganisms increase the temperature of the organic

material, in turn, removing pathogens and increasing the rate of disintegration (Argun et al., 2017).

Anaerobic: Anaerobic composting is slower and takes place when oxygen is not available. This process biodegrades organic waste into mud (Argun et al., 2017). Other products formed are organic acids and the release of biogases, these include methane, carbon dioxide and ammonia (Chadar et al., 2018).

Vermicomposting: Vermicomposting is an aerobic process where microorganisms and earthworms are introduced to break down the organic substrate. Lignocellulose (and other large ring structures) in green waste is broken down by fungi. Where earthworms are present, the diversity of microorganisms has been found to increase due to microbes moving through earthworm intestines. Vermicompost has been studied to have higher nutrient and lower lignin concentrations compared to solely aerobic compost (Cai et al., 2018).

Composting Conditions:

Nutrients: Breakdown of proteins into ammonia will increase the pH of the compost. An increase in pH increases the mineralisation of carbon and nitrogen (Neina, 2019). A pH value between 5.5 and 8 is suitable for microorganisms to thrive in compost and provide available nutrients for plants (Argun et al., 2017). The carbon to nitrogen ratio of a compost heap is important for microbiological activity. A recommended C:N ratio is 30:1. (Argun et al., 2017), with acceptable values laying between 20:1 and 40:1. This ratio can be achieved by adding the correct proportions of brown and green matter. Brown material provides the compost with carbon and increases porosity (important for air and water to move into different layers). Green material is a source of nitrogen, this can include food waste and manure. It is recommended to start with brown matter which absorbs water, such as dried leaves. The addition of calcium and phosphorus can help produce compost of high quality, these can be added in the form of limestone and granite dust (Chadar et al., 2018).

Water and oxygen: Moisture content is significantly important for decomposition. If the heap is too saturated, microorganisms cannot get the oxygen required to survive and nutrients are removed through leaching. The recommended water content during the composting process should be at around 55% (± 5%; Argun et al., 2017). During aerobic composting, oxygen is essential for microbial activity, such as the oxidation of carbon. As a result, energy is released which increases the temperature of the heap, in turn, increasing the degradation process (Chadar et al., 2018). In order to maintain sufficient gas exchange whilst sustaining optimum temperatures, irregular particle sizes ranging from 6 to 75 mm are recommended. If the compost has an unpleasant odour, there may not be enough oxygen available. Regular turning of the organic matter helps aerate the compost heap by increasing its porosity, this also reduces the risk of the compost overheating (Argun et al., 2017).

Health: To produce healthy compost, free from pathogens and viruses, it is important to

regulate what organic inputs go into the process. Garden waste should not contain diseased plants or faeces from pets. To remove most pathogens from the compost, PAS 100 recommends a minimum of 65°C, with a 51 % mass per mass moisture for 7 days (PAS 100, 2011). As a result of fewer pathogens, plants grown from the compost are less likely to be infected with soil-borne diseases (Mehta et al., 2013).

Why is Composting Useful?

Carbon Sequestration: In the EU’s Green Deal there are incentives by the European

Commission for farmers who follow practices that increase carbon sequestration (European Commission, 2020). Carbon sequestration increases the storage of carbon (which could be in soil) and reduces the amount of carbon dioxide in the atmosphere. Increasing the amount of soil organic carbon helps improve the sustainability of agriculture (Tautges et al., 2019).

When applying compost to land instead of synthetic fertiliser, the amount of carbon dioxide and nitrous oxide gases released from soil has been seen to decrease (Tautges et al., 2019). However, the gases released initially when producing the compost could offset carbon sequestration (Cayuela et al., 2012). Nevertheless, earthworms during the composting process are thought to reduce greenhouse gas emissions (Lim et al., 2016). Soil with a high organic matter content has a high respiration rate. Though, this is counteracted by producing healthy plants from the soil which remove carbon dioxide from the atmosphere through photosynthesis.

Food waste: If food waste is put directly into landfill, their anaerobic breakdown will drive a lack of oxygen leading to the release of methane. Composting is an inexpensive and straightforward alternate method of removing organic waste (Chadar et al., 2018; Krstic et al., 2019).

Nutrients: Compost stabilises soil and has a high water-holding capacity. If applied to land, the hydrological conductivity of the soil is increased reducing surface runoff. Therefore, the erosion of soil is reduced causing less soil-derived pollution from entering waterways (Argun et al., 2017). With climate change, extreme weather conditions such as droughts are becoming more frequent. The application of compost can improve the water retention of soil. Thus, reducing the need for irrigation, which reduces farm expenses and does not deplete freshwater environments.

By 2030, the European Commission aims for a 20% reduction in fertiliser use (European

Commission, 2019). Compost is a beneficial alternative that sustains soil nutrition.

Compost retains nutrients, such as phosphorus, nitrogen and potassium, which is useful for improving plant growth (Chadar et al., 2018). This stabilised matter can be used as nourishment in organic farming, enhancing the production of food to feed the growing human population. This is advantageous to farmers because the addition of compost is cheaper than synthetic fertilisers.


It is important to keep waste to a minimum, but some excess products are unavoidable.

Composting is a sustainable process of waste disposal. The activity of microorganisms to decompose organic residue results in a product that has several environmental benefits. Compost can be applied to land to restore soil nutrition and enhance food security. In order to avoid toxicity in plants and their crop, it is essential to regulate what inputs go into a compost heap. In the next part, I will investigate what by-products of leather can be composted.


Argun, Y.A., Karacali, A., Calisir, U. and Kilinc, N., 2017, Composting as a Waste Management

Method. Journal of International Environmental Application and Science, 12(3), 244-


Cai, L., Gong, X., Sun, X., Li, S. and Yu, X., 2018, Comparison of chemical and microbiological

changes during the aerobic composting and vermicomposting of green waste. PloS

one, 13(11), e0207494.

Cayuela, M.L., Sánchez-Monedero, M.A., Roig, A., Sinicco, T. and Mondini, C., 2012,

Biochemical changes and GHG emissions during composting of lignocellulosic

residues with different N-rich by-products. Chemosphere, 88(2), 196-203

Chadar, S.N., Chadar, K. and Singh, M., 2018, Composting as an Eco-Friendly Method to

Recycle Organic Waste. Department of Chemistry, University Institute of Technology-

RGPV Bhopal, India.

European Commission, 2019, The European Green Deal, Brussels

European Commission, 2020, A Farm to Fork Strategy for a fair, healthy and environmentally friendly food system, Brussels.

Krstic, I.I., Radosavljević, J., Djordjević, A., Avramović, D. and Vukadinović, A., 2019,

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vermicomposting technologies for organic solid waste biotransformation: recent

overview, greenhouse gases emissions and economic analysis. Journal of Cleaner

Production, 111, 262-278.

Neina, D., 2019, The role of soil pH in plant nutrition and soil remediation. Applied and

Environmental Soil Science PAS 100, 2011, Specification for composted materials: BSI

Razza, F., D’Avino, L., L’Abate, G. and Lazzeri, L., 2018. The role of compost in bio-waste

management and circular economy. In Designing Sustainable Technologies, Products

and Policies, 133-143

Tautges, N.E., Chiartas, J.L., Gaudin, A.C., O' Geen, A.T., Herrera, I. and Scow, K.M., 2019, Deep soil inventories reveal that impacts of cover crops and compost on soil carbon sequestration differ in surface and subsurface soils. Global change biology, 25(11), 3753-3766

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