Organic waste: what can be composted?

How does composting work?

To keep it simple, composting is defined as the transformation of organic waste of plant and/or animal origin into a product that returns to the soil. Whether this composting takes place outdoors in the vegetable patch, in a heap, in a silo or in a composter, or indoors, the principle remains the same, with a few nuances. Composting is, by the way, modelled on what happens in nature, on the scale of a woodland floor, for example. Plant debris, leaves or dead wood, or animals (fur, feathers, droppings…) settle on the ground and decompose. At the origin of this decomposition are micro-organisms, naturally present in the soil, which feed on these wastes and transform them into fertile matter, humus, rich in minerals (nitrogen, phosphorus, potassium, calcium…) useful for plants, and in carbon, essential for the soil.

A compost should comprise green and moist waste and dry and brown waste

Concretely, the organic matter deposited degrades into carbon dioxide and water, in the presence of oxygen. Bacteria are the first to act in consuming cellulose. Then fungi do their work by attacking lignin. Finally, epigeous worms or red worms complete the job by enriching the compost with their droppings. That is why an outdoor compost heap needs oxygen, a sufficient moisture level and above all a variety of wastes. It is therefore essential to turn it regularly and to add water if it is covered. The decomposition of the waste releases a certain amount of heat at the heart of the heap, around 50 to 60 °C.

To succeed with composting, it is essential to diversify organic waste. Not all wastes are equal and do not yield the same benefits. For an effective compost, you must fairly include so-called green or moist wastes, rich in nitrogen, which break down very quickly, and brown or dry wastes, with a high carbon content, which decompose more slowly. By alternating the layers, which you mix regularly, you obtain a well-balanced compost. Experts agree on an ideal carbon/nitrogen ratio of around 25 to 30, i.e., two parts green materials to one part brown, or at worst a fair mix of green and brown wastes.

For more information: How to successfully compost in 5 steps.

What green waste should be composted?

Green materials are very rich in nitrogen, useful to plants. There are many green wastes that can be added to compost, with a few precautions:

• Kitchen waste : All plant matter can be composted. Thus, fruit and vegetable peels will be added to the compost, provided they are not too large. You can chop potato peels, cabbage stalks, the skins of squash, melons, watermelons… often very hard and slow to decompose. Citrus can also be added to the compost, as can banana peels, provided they are organic. Egg shells can also be added, finely ground.
• Lawn clippings : They will be incorporated into the compost only if they are sufficiently dry. If they are too damp or compacted, they must be dried before putting them into the compost.
  

  Many green wastes can be added to the compost

• Garden cultivation waste : They go into the compost without issue. The annual flowering plants can also be incorporated at the end with their roots and, if present, the potting compost.
• Weeds or adventitious plants : They can also be included provided their root system is dry and their seeds have not formed. Otherwise, adventitious weeds without roots pose no problem.
• Fallen fruit from fruit trees: they can be added to the compost, in small quantities, if they are free from disease.
• Nettle shoots, comfrey leaves, fern, and burdock, which are considered excellent natural compost activators.

What brown materials should be added to the compost?

Brown waste is also very common in the garden, especially from pruning. They are rich in carbon, essential for the soil’s microbial life. Because they are tougher, more woody and thicker, these wastes should ideally be shredded before being added to the compost :

Prunings are perfect for adding carbon to the compost

• Dead leaves: all leaves can be added. The thickest leaves from Magnolias grandiflora, from Paulownias, from a ivy, from plane trees will be shredded. Other leaves from plants (walnut, rhubarb, oak, chestnut, poplar…roses) are cut up and added in very small quantities.
• Twigs, shoots and small branches from shrubs: they will also be shredded. Some may be left as they are to aid aeration of the compost.
• Flower stems such as sunflowers should be cut into small pieces.
• Perennial and grass cuttings: to be cut up and incorporated into the compost in March.
• Straw: to help its decomposition, it is recommended to moisten it slightly.
• Thuja branches: shredded only and in very small quantities.
• Kraft paper, unprinted, which wraps fruit and vegetables or cardboard boxes, unprinted and unglazed, cut into small pieces, egg boxes cut into small pieces, cheese boxes made of cardboard or unprinted, unglazed wood, the absorbent paper
• Tea bags and the coffee grounds with the filter.
• Herbivore animal litter (straw bedding from chickens or rabbits) but in very small quantities.
• Dead houseplants: to be incorporated with the potting compost.
• Manure.

Bad odours? Why?

Logically, a compost shouldn’t smell bad; only a woodland odour emanates from it. If nauseating odours emanate from it, it’s out of balance:

• An ammonia odour comes from an excess of green waste that produces too much nitrogen. You should incorporate brown waste.
• Odours of damp mud indicate a lack of oxygen and excess moisture. Therefore, aerate the compost.