Compost production for a vegetable garden

Medium-term fertilizer composts

An optimal mix of fresh organic material for composting should be close to the following values: water 50-60%, C/N ratio 30-40, pH between 6.5 and 8.5 (1). To get as close as possible to these values, the sources of organic matter must be varied and the proportions adjusted. According to the NF U 42-001 A1 standard, these composts are considered as organic fertilizers. The diversity of fermentable domestic biowaste, particularly from crop residues, lawn clippings and kitchen waste, means that it must be combined with structuring biomaterials of complementary composition in order to produce a medium-lasting humus that is compatible with market gardening. Far too much domestic vegetable waste is burnt or sent to landfill when it could be used to produce compost after being shredded. There are also other interesting sources.

Chips from untreated wood sawmills can also be used as structural biomaterials (nowadays, sawmill oils are biodegradable (2) and are disposed of in thermophilic composting). Sawmill residues consisting of fine chips mixed with wood powder can be purchased for a small fee. Composting sawmill residues, leaf litter and shredded hedge trimmings mixed with highly putrescible organic materials produces a humus comparable to mull. This requires approximately :

For a precise choice of the volume of each material in these 3 categories, it is not easy to present a model that is valid for everyone, because for each type of organic material, the C/N ratio varies considerably in relation to the method of cultivation, the time of harvesting of the residues, the climate, etc. As an indication, lawn clippings have a C/N of between 10 and 17, kitchen waste from 10 to 25, vegetable crop residues from 15 to 25, manure from 10 to 19, leaves from 20 to 60, cereal straws from 50 to 150 and sawdust from 150 to 500.

As far as the nutrient content of composts is concerned, the addition of dead leaves from large trees has a clear advantage. The roots of the trees go deep into the soil and they bring in trace elements from the decomposition of the parent rock which are found in the dead leaves in the autumn. For this reason, I strongly advise systematically introducing tree leaves into a medium or long term compost.

Composts that produce long-lasting humus.

Composts with a C/N higher than 40 are produced from a large volume of organic matter rich in cellulose and lignin. These composts correspond to the NFU 44-051 standard defining organic amendments intended for the maintenance of crop soils and the reconstitution of organic matter stocks. These composts are of interest in agriculture, as they are known to develop into a very stable, long-lasting humus. On the other hand, they provide very little fertilising elements to the soil in the short term. Unlike medium duration humus, humus with a high C/N ratio contributes to nitrogen storage (3). However, the nitrogen, phosphorus and potassium content must be less than 3% on the raw product and the sum of these elements less than 7%. The advantage of these composts is that they produce very stable CAC over time, thus indirectly improving the fertility of cultivated soils.

The decomposition of biowaste lasts longer when it contains a lot of plant debris rich in cellulose and lignin; at least one year or more. These composts, when incorporated into the crop soil, can cause 'nitrogen starvation', which refers to the absorption of all the nitrogen reserves in the soil by bacteria to metabolise the excess carbon, resulting in a nitrogen deficiency for plants. However, some of the fixed nitrogen is released by the death of the bacteria when the soil C/N ratio becomes more balanced.

Are nitrogen-enriched composts really organic?

Medium-term fertiliser composts are richer in mineral salts that can be assimilated by plants. However, due to nitrogen losses during composting, the contribution of these fertilisers to plant nitrogen nutrition remains low if it is not corrected by a mineral or organic nitrogen fertiliser at the time of composting. Fertilizers enriched with mineral nitrogen sold in garden centres are called organomineral fertilizers. Their advantage is that they offer a synergy of effects due to their organic support and their mineral complement. The NF U 42-001 standard requires that these fertilizers contain at least 1% organic nitrogen of animal or plant origin.

Compost enriched solely with organic matter receives various manures during the composting process, some of which are rich in nitrogenous matter such as poultry manure or guano. These composts, which are approved for organic farming (found in garden centres, agricultural COPs and websites), are not always truly organic, as most are produced from manures from conventional farms using plants grown with synthetic fertilisers and pesticides.

The nitrogen supply can also be supplemented during the spreading of the compost used as a bottom dressing, by adding a mineral fertiliser.

Composts that replace farmyard manure.

Farmyard manures are not required to reclaim garden land. Since the nitrogen content of lawn clippings is generally higher than farmyard manures, the latter can be replaced by composting lawn or grass clippings with more carbon-rich organic matter. Thus, straws can be recycled without the need to raise farm animals, which has another advantage: nitrogen losses are lower because of the disappearance of a step in the recycling of organic matter. For those who do not have enough lawn clippings to compost, the addition of pearl urea or ammonium sulphate (or ammonitrate for farmers) is more than enough to obtain a balanced compost.

Composting method and local regulations.

Before choosing a composting method (in a heap or with a composter), it is necessary to check local regulations (e.g. regulations of a group of municipalities or a housing estate) for possible restrictions. In France, home composting for recycling kitchen and garden waste does not require a special declaration. Many models of home composters have emerged in recent years, encouraged by local authorities. There are different models approved for family use, with some composters holding up to 900 litres of fresh organic material.

Ammonia and methane production from poorly controlled composting

Regardless of the method of composting, cold or hot, it is important to note that a poorly controlled composting process can release a lot of methane and/or ammonia. Ammonia is released into the atmosphere and/or dissolved in water with the risk of being carried deep into the water table. Methanisation is a process of degradation of organic matter by micro-organisms in the absence of oxygen. The poor choice of materials and the lack of periodic aeration are often the cause of these losses of environmentally hazardous materials.

It should be noted that losses of methane, ammonia and nitrous oxide cannot be avoided during composting. These losses are the cause of well-known environmental pollution from agriculture, which has increased in recent years.

Cold Composting

Most individuals compost cold by gradually accumulating organic matter. Composting takes place at a low temperature and is facilitated by the many beneficial organisms that colonise the bin. This composting is fed from time to time by kitchen waste, lawn clippings, crop residues, dead leaves collected here and there... This cold composting process has some major drawbacks:

When organic material is removed from cold composting and incorporated into garden soil, seed germination is often observed if the gardener has incorporated kitchen waste such as melon seeds, or lawn clippings, proving that this composting system is not very effective due to the lack of a thermophilic phase.

Properly maintained composting is carried out by a large number of microorganisms that consume a lot of water and oxygen. The processes involved are complex and also depend on the nature of the organic material, and secondarily on the outside temperature, especially for small volumes.

In a natural decomposition process, fungi are the first organisms to appear in a compost. White filaments (mycelium) can be seen inside the compost. Macro-organisms (earthworms, insects, mites, myriapods, crustaceans, etc.) come into play when the temperature of the compost is not very high. These beneficial organisms harbour colonies of bacteria in their intestines and it is assumed that these play an important role in the decomposition of organic matter. Cold composting can have a slight increase in temperature up to 30°, which is compatible with the existence of beneficial organisms living in a mesophilic environment.

Vermicomposting is a variant of cold composting with the same drawbacks: pathogens and weed seeds are not destroyed. Vermicomposting is not described on this website because it does not allow high temperatures to be reached, without which it is impossible to sanitise compost.

Hot composting (composting with thermophyll phase)

In France, large-scale high-temperature composting in windrows or with appropriate equipment is subject to regulations to protect the environment and nearby homes. These composting operations are carried out by local authority departments or companies. This type of large-volume composting, which requires specialised equipment, is not described in this study. Below I describe how to improve composting with approved composters for family use (or when small-scale heap composting is allowed) by adding a thermophilic phase to the composting process that has no adverse effect on the environment and the neighbourhood. In addition, composting with a thermophilic phase has the advantage of rapidly reducing the sources of bad odours often accompanied by an army of unwanted flies that are often found in composters approved for family use.

In the thermophilic phase, the organic material on the periphery must be moved to the centre from time to time so that all the organic material reaches a temperature of 55° to 65° for at least 15 days. Composting in a pile is best suited to this sanitation procedure. Depending on local regulations, you may have to switch from heap composting to a compost bin. Regardless of the composting method (in a heap or in a bin), the successive transfers of organic matter from the edges to the centre mean that the thermophilic phase must last at least 30 days. It should be noted that some pathogens are destroyed at 40° (such as the potato rhizoctonia fungus), but the vast majority of pathogens are destroyed at a minimum temperature of 47° (potato mildew, fungi causing tomato stem rot or cucumber root rot, white rot of garlic, etc.). In the laboratory, many nematodes are destroyed in a few hours at a temperature of about 45°, but some pathogens require higher temperatures and/or longer periods. For maximum pathogen kill, it is commonly accepted that an average temperature of 60° is required.

Temperature is not the only factor responsible for pathogen removal. Some substances produced during composting are toxic to pathogens. Antagonisms between pathogens and the dominant thermophilic flora also play an important role.

High temperature composting in the summer requires a volume of organic material in the order of 600 to 900 litres, which is the capacity of some composters authorised for family use. Below 600 litres, you will have difficulty maintaining a temperature above 50° for 30 days. For 600 litre composters, avoid bins that encourage heat loss (rectangular shape, too thin walls). Octagonal composters with thick walls and lids are preferable.

The larger the volume of the composter, the easier it is to achieve high temperatures. A 600 l composter can only be used in summer because of its small volume and provided that the highly fermentable organic material is changed frequently during the thermophilic phase. The 800-litre Picumus and Quadria composters distributed by some local authorities (4) are very suitable for this type of composting. It is possible to find on websites or in garden centres approved composters with the Afnor NF environnement label, such as the 900-litre Thermo-King composter that I recommend.

Some wooden or plastic composters should be avoided: the thickness of the wooden slats is ridiculous, the fastening system is too fragile and cannot withstand the pressure of the organic matter, and the compost outlet doors are too small when they exist. You may be less disappointed if you build your own compost bin with pallets and reclaimed wood (check local regulations).

High temperature sanitation composting is very effective in destroying most pathogenic micro-organisms (also called phytopathogenic organisms) and weed seeds. With a temperature of 55/60° the seeds are destroyed depending on their depth in the compost. At 30 centimetres, all seeds are destroyed in 24 days. At 90 centimetres, all seeds are destroyed in 3 days (5). With a few exceptions, as described in this article "Pathogens resistant to the sanitation phase", most pathogenic micro-organisms are neutralised (fungi, bacteria and parasitic nematodes). Viruses such as the one that causes bean mosaic are also destroyed, although they are more resistant to composting than other pathogens.

This type of composting requires a lot of fermentable material and nitrogenous products to reach a temperature of 55°/65°. Manures are not always sufficiently supplied with nitrogen, requiring the addition of a nitrogen supplement. The use of a mineral nitrogen fertiliser (urea or ammonium nitrate) is met with ideological resistance from environmental activists, even though it is a very reliable and inexpensive method of safely recycling and sanitising most organic matter of plant origin. Dried blood and roasted horn, which contains 13% nitrogen, can be added, but the cost of making compost will be much more expensive. The addition of a mineral nitrogen supplement helps to correct the inevitable nitrogen losses from ammonia volatilization during the composting process. Without the addition of nitrogen supplements, the final compost may be unbalanced (low in nitrogen and high in potash and phosphate). Compost from manure decomposition alone is known to have low nitrogen content. Nitrogen addition is all the more necessary when the C/N ratio of the organic matter is high.

The dose of pearl urea (urea nitrogen in 46% BUNIFERT granules (6)) added to the compost corresponds to about the volume of a good coffee cup for an 800 litre bin (3 times more for an addition of roasted horn). As soon as a loss of temperature is noticed, this nitrogen addition should be repeated as long as the temperature can be readjusted. Alternatively, 150 g per m² of calcium cyanamide can be applied to successive 20 cm layers of plant waste. Calcium cyanamide kills weed seeds, fungus spores, pathogenic bacteria and insect pests more easily. However, it has the disadvantage of increasing the limestone content, which is incompatible with soils that are too rich in limestone.

Many home composters do not allow aerobic composting to be carried out properly to completion because of the insufficient number of aeration openings. If the raw material coming out of a compost bin smells like sewage, this indicates anaerobic fermentation. In an anaerobic environment, toxic substances are formed. It is therefore important to avoid anaerobic composting if you do not want it to become a source of pollution for the cultivated land after spreading. Composters with many rows of holes on all four sides are more likely to engage in aerobic fermentation.

There is no need to use compost activators to start fermentation. A bucket of organic material taken from older compost is just as effective and less expensive.

In winter, due to heat loss, a mass of at least 1 m³ of fresh organic material should be collected. The organic material is enclosed in a screened frame, which allows for good aeration. In this type of composting, temperatures above 65° can be reached in winter if the pile contains enough highly putrescible organic material. The pile should be stirred every 2 weeks. A comparable result is obtained with 900 litre Thermo-King type composting bins.

The compost arm is 65 cm long. This means that the tool cannot reach the bottom of the composter if the height of the composter exceeds the length of the brass'compost. However, during the thermophilic phase, it can be seen that the ceiling of organic matter descends very quickly. This can be as much as 10 cm on the first day, which means that after a few days the deeper layers can be aerated with the brass'compost.

The brass’compost is 65 cm long. This means that the tool cannot reach the bottom of the composter if the height of the composter exceeds the length of the brass'compost. However, during the thermophilic phase, it can be seen that the ceiling of organic matter descends very quickly. This can be as much as 10 cm on the first day, so that after a few days the deeper layers can be aerated with the brass'compost.

In a compost bin, the introduction of pearl urea (unless there is sufficient highly fermentable organic matter) with 10 to 20 litres of water immediately starts the thermophilic phase. The temperature is checked with a laboratory thermometer through the tank vents. If the temperature does not exceed 50°, highly fermentable organic matter and/or pearl urea (or ammonium nitrate for farmers who can use this nitrogen fertiliser) should be added.

An interesting solution is to provide two compost bins, one for dry storage of organic material (or maturing compost), and the other to start composting with a thermophilic phase when the appropriate volume is reached. The bottom of the bin should not be watertight so that beneficial organisms can find shelter during the thermophilic phase. To speed up the movement of earthworms, stir the soil with a fork spade at the base of the container on at least two sides, the other two sides being reserved for the gardeners to carry out the aeration operations above the container requiring the movement of a stool, for example, in order to position themselves at the right height.

Some authors recommend covering the compost heap with an impermeable cloth to avoid moisture loss, to protect it from heavy rainfall, or to avoid the introduction of weed seeds and undesirable insects. But active compost still needs to be well aerated and consumes a lot of oxygen, especially during the thermophilic phase. Therefore, any obstacles that prevent air circulation should be avoided, or ventilation openings should be provided. In case of heavy precipitation, a folded tarpaulin is placed near the compost heap.

The different phases of composting with a thermophilic phase are described by clicking ici.

1) Godden 1995 – signalé dans Infos CTIFL N°225 octobre 2011
2) https://www.condat.fr/produit/huiles/huiles-de-lames/lames-de-scie/
3) COMIFER – Calcul de la fertilisation azotée ; guide méthodologique pour l’établissement des prescriptions locales -Edition 2013.
4) http://www.quadria.eu/produit/composteur-picumnus/
5) RAGDALE et al. (1992)
6) Azote uréique BONIFERT : vendu dans les COOPS agricoles par sac de 50 kg. Tenir compte de son dosage très élevé : en azote 46 % pour une fertilisation directe du sol censée corriger une carence en azote.