Hot Composting

by Grant Douglas,
April 12th 2010
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The foundation of all the soil fertility, plant health, bountiful flowers and health giving food crops is humus. Humus is the base of the complex biochemical activity that manufactures plant food. It is the sponge that holds moisture in balance with air in the soil, but it needs constant replenishment by organic manuring.

Compost making is controlled rotting down of organic waste to create humus. Nature rots things where they lie; we gather them together in a heap where temperature, moisture and aeration can be controlled to achieve rapid conversion by fungi, bacteria, insects and worms.

The conditions necessary for hot composting (reaching temperatures between 100 and 150°F) are:

• Aeration - Necessary for microbal and insect activity but if there is too much, heat and moisture will be lost, as will contact between materials.
• Water - Also needed for bacterial, fungi and insect activity, but too much will waterlog the heat, creating anaerobic conditions (best consistency of squeezed out sponge). Remember green material is mostly made up of water.
• Nitrogen - Carbon nitrogen ratio (C/N). Microbes need nitrogen to reproduce and break down carbon, therefore the ratio of these two elements is critical. The ratio varies in different materials, for example animal manures (this also varies between manures) are high in nitrogen (fertiliser such as blood and bone, and in the extreme sulphate of ammonia) as are soft green material such as lawn clippings. Straw/sawdust are extremely low in nitrogen and high in carbon, therefore it is essential to mix the materials that are used to attain a rough percentage of 10 to 20% nitrogen to carbon.
• Neutrality - Microbal and insect activity is at its optimum in a neutral acidity environment so a light dusting of lime every 30cm of compost heap height is often recommended, but may not be necessary if neutral materials are used in the making of the heap.

Making the heap - A sandwich technique is used, using a variety of material and attaining the percentage of nitrogen to carbon.

Dimensions: A width and length of at least 1.5m is really necessary for containing heat and moisture. Height can vary but a minimum of 1.3m is recommended.

Site: A warm, sunny site is preferable, avoiding hollows where moisture will collect after rain. The heap must be sited on soil, not concrete.

Size of material: Although larger material such as corn or cabbage stalks will break down, it is preferable to chop these into smaller pieces to increase their surface area where fungi and bacterial activity can take place.

Type of material: Avoid materials that may contain hormonal herbicides, extreme cases of disease such as tomato or potato blight, noxious weeds such as oxalis, and materials which may contain industrial dyes or chemicals such as glossy paper.

Turning the heap: A well-made heap will rapidly rise in temperature up to 150°F due to microbal activity. This can take anything between 10 days to three weeks. When temperatures begin to drop, it is time to turn the heat over and form a new heap, placing the outsides and top into the centre of the new heap. Temperatures in the new heap will then begin to rise, although not as high as for the first heap. When the second heaps temperature begins to drop you can turn the heap, but this is not essential if you are willing to wait a wee bit longer for the finished product. At this stage insects and manure worms will take over from the fungi and microbes in breaking down organic matter to create humus.

Capping: The ideal capping for the top of the heap is used hay or straw biscuits. This allows moisture from the steaming to escape but traps any rainwater which might fall on the heap.

Finished product: It should have a crumbling nature but will still have some visible residue of the tougher organic matter which, if not wanted in the garden, can be added to the next heap.

Kitchens scrap composting

As can be seen from the above in hot composting, it is necessary to gather all the materials together at one time. This is not possible when composting garden waste and kitchen scraps, or where the heap is made over a longer period of time. It is not possible to attain the sort of temperatures that are possible when a heap is made in one go, but satisfactory results can be achieved using two or three contained bins and ensuring that a variety of material is added to the bin and correct moisture levels are maintained. If you have two bins together (that of 1.25m width and length), when one bin is full turn it into the second until you reach the layer of composted material. This material is ready to use and fresh material can be added to the other bin.

If food scraps all you have and very little garden waste, I would recommend using a worm farm.

Alternatives to composting

Green manuring, sheet composting, permanent organic mulching.