Compost production for a vegetable garden

Hot composting (composting with thermophilic phase)

Large-scale high-temperature composting in windrows or using appropriate equipment is subject to regulations designed to protect the environment and nearby homes. This type of composting is carried out by local authorities or companies. This type of large-scale composting, which requires specialised equipment, is not covered in this study. Below, I explain how to improve composting carried out using composters approved for domestic use (or when small-scale composting in piles is permitted) by adding a thermophilic phase to the composting process that has no harmful effect on the environment or the neighbourhood. In addition, composting with a thermophilic phase has the advantage of quickly reducing the sources of unpleasant odours, often accompanied by an army of unwanted flies, which are often found in approved household composters.

During the thermophilic phase, the organic matter located around the edges must be moved to the centre from time to time so that all the organic matter reaches a temperature of 55° to 65° for at least 15 days. Composting in piles is better suited to this sanitation process. Depending on local regulations, you may be required to abandon composting in piles in favour of a compost bin. Regardless of the composting method (pile or bin), the successive transfers of organic matter from the edges to the centre require the thermophilic phase to last at least 30 days. It should be noted that some pathogens are destroyed at 40°C (such as the potato rhizoctonia fungus), but the vast majority of pathogens are destroyed at a minimum temperature of 47°C (potato blight, fungi causing tomato stem or cucumber root rot, white rot of aubergines, etc.). In the laboratory, many nematodes are destroyed within a few hours at a temperature of around 45°C, but some pathogens require higher temperatures and/or longer periods of time. It is generally accepted that an average temperature of 60°C is required to eliminate the maximum number of pathogens.

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

High-temperature composting in summer requires a volume of organic matter of around 600 to 900 litres, which corresponds to the capacity of some composters authorised for domestic use. Below 600 litres, you will find it difficult to maintain a temperature above 50°C for 30 days. For 600-litre composters, avoid bins that promote heat loss (rectangular shape, walls that are too thin). Octagonal composters with thick walls and lids are preferable.

The larger the volume of the composter (over 600 litres), the easier it will be to achieve high temperatures. A 600-litre composter can only be used in summer due to its small volume and requires frequent renewal of highly fermentable organic matter during the thermophilic phase. The 800-litre Picumus and Compostys composters from Quadria, distributed by certain local authorities (4), are very well suited to this type of composting. Approved composters with the Afnor NF environment label, such as the 900-litre Thermo-King composter, which I recommend, can be found on websites or in garden centres.

Some wooden or plastic compost bins are best avoided: ridiculously thin wooden slats, fragile fastening systems that cannot withstand the pressure of organic matter, and compost removal doors that are too small, if they exist at all. You may be less disappointed if you build your own compost bin using pallets and reclaimed wood (check local regulations).

High-temperature sanitation composting is very effective at destroying most pathogenic microorganisms (also known as phytopathogenic organisms) and weed seeds. At a temperature of 55/60°C, seeds are destroyed depending on their depth in the compost. At 30 centimetres, all seeds are destroyed within 24 days. At 90 centimetres, all seeds are destroyed in 3 days (5). With a few exceptions specified in the article “Pathogens resistant to the sanitation phase”, most pathogenic microorganisms are neutralised (fungi, bacteria and parasitic nematodes). Viruses such as the one that causes bean mosaic are also destroyed, even though 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°. Manure does not always contain sufficient nitrogen, requiring the addition of a nitrogen supplement. The use of mineral nitrogen fertilisers (urea or ammonium nitrate) is met with ideological resistance from environmental activists, even though this method is very reliable and inexpensive for safely recycling and sanitising most organic matter of plant origin. Dried blood and roasted horn, which contains 13% nitrogen, can be added, but this will make the compost much more expensive to produce. Adding a mineral nitrogen supplement compensates for the inevitable nitrogen losses due to ammonia volatilisation during the composting process. Without the addition of nitrogen supplements, the final compost may be unbalanced (low in nitrogen and rich in potash and phosphate). Composts derived solely from the decomposition of manure are known for their low nitrogen content. A nitrogen supplement is all the more necessary when the C/N ratio of organic matter is high.

The dose of granular urea (46% urea nitrogen in BUNIFERT granules (6)) added to the compost corresponds to approximately the volume of a good coffee cup for an 800-litre bin (three times more for a supply of roasted horn). As soon as a drop in temperature is noticed, this nitrogen supply should be renewed until the temperature can be readjusted. You can also spread 150 g per m² of calcium cyanamide on successive 20 cm layers of plant waste. Calcium cyanamide more easily destroys weed seeds, fungal spores, pathogenic bacteria and harmful insects. However, it has the disadvantage of increasing the limestone content, which is incompatible with soils that are too rich in limestone.

Many domestic composters do not allow for proper aerobic composting due to an insufficient number of openings for aeration. 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 prevent composting from entering an anaerobic phase if you do not want it to become a source of pollution for cultivated land after spreading. Composters with multiple rows of openings on all four sides are more likely to undergo aerobic fermentation.

To start fermentation, there is no need to use compost activators. A bucket of organic matter taken from older compost is just as effective, and this process is less expensive.

In winter, due to heat loss, it is advisable to gather at least one cubic metre of fresh organic matter. The organic matter is enclosed in a wire mesh frame, which allows for good ventilation. With this type of composting, temperatures above 65°C can be reached in winter if the pile contains enough highly putrescible organic matter. The compost should be turned every two weeks. A similar result can be achieved with 900-litre Thermo-King compost bins.

The compost turner is 65 cm long. This tool cannot therefore reach the bottom of the composter if the height of the latter exceeds the length of the compost turner. However, during the thermophilic phase, the level of organic matter drops very quickly. This drop can reach 10 cm on the first day, which means that after a few days, the deeper layers can be aerated with the compost turner.

in a compost bin, adding urea pellets (unless there is sufficient highly fermentable organic matter) with 10 to 20 litres of water immediately triggers the thermophilic phase. The temperature is checked with a laboratory thermometer through the bin’s ventilation holes. If the temperature does not exceed 50°C, highly fermentable organic matter and/or urea pellets (or ammonium nitrate for farmers who have the option of using this nitrogen fertiliser) must be added.

Some authors recommend covering compost piles with a waterproof sheet to prevent moisture loss, protect them from heavy rainfall, and prevent the introduction of weed seeds and unwanted insects. However, compost in the active phase always needs to be well aerated and consumes a lot of oxygen, especially during the thermophilic phase. Any obstacles that prevent air circulation must therefore be avoided; otherwise, ventilation openings must be provided. To cope with possible heavy rainfall, a folded tarpaulin is placed near the compost heap.

An interesting solution is to have two compost bins, one reserved for dry storage of organic matter (or compost in the maturation phase), and the other for starting composting with a thermophilic phase when the appropriate volume is reached. The bottom of the bin should not be waterproof so that beneficial organisms can find refuge during the thermophilic phase. To speed up the movement of earthworms, turn the soil with a spade directly above the bin on at least two sides, with the other two sides reserved for gardeners to carry out aeration operations above the bin, requiring the use of a stool, for example, to position themselves at the right height.

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