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Six Steps to Making Hot Compost

Originally published on February 23, 2017

Ross Mars, author of The Permaculture Transition Manual: A Comprehensive Guide to Resilient Living, is a scientist with a PhD in Environmental Science and a highly regarded permaculture teacher and designer. In today's blog, he shares his secrets for making successful hot compost piles.

Excerpt from the Book

Hot Compost

This is by far the best method, but it does have its secrets. You need to follow the Six M's: Materials, Moisture, Mixing, Microorganisms, Minerals and Mass.

1. Materials: the "right stuff"

For best results, a compost pile must be, as the word implies, a composite of different materials — a mixture of plant and animal material. To make everything work properly you need a balance of carbon and nitrogen substances. Carbon substances are “brown”, and these include plant materials and sawdust, straw, paper, cardboard and dried leaves. Nitrogen substances are “green” and contain protein substances, so again living plants and weeds, but also animal manures and food scraps. The ideal carbon to nitrogen (C:N) ratio is 30:1, where decomposition organisms require the carbohydrates and carbon substances to be balanced by a suitable proportion of protein or nitrogen. Most deciduous leaves have a C:N ratio of about 60:1, while grass clippings, manures and food scraps have a ratio of about 20:1 or less, and woody materials often range as high as 500:1. Too much nitrogen in a pile results in the formation of ammonia gas; too much carbon and the pile will take years to break down.

In a practical sense, about two shovelfuls of fresh animal manure to every wheelbarrow load of green plant material, such as weeds and lawn clippings, seems to work well. If you include some paper or dried leaves, add more animal manure. If you cannot access animal manures you can use blood and bone fertilizer or fish meal as your source of nitrogen.

C:N ration (weight:weight)
Lawn Clippings
Food Wastes
Chicken Manure
Cattle Manure

2. Moisture: add water

You need to add enough water so that when you pick some material up it is like a damp sponge, and when you squeeze the material a few drops of water drip out. About 50% of the pile should be water (you will be surprised how much water is required!). If there is not enough water (<40%), then little decomposition occurs, but too much (>60%), then less air is available in the heap and it becomes anaerobic (without oxygen). Under-watering is the largest single cause of slow composting.

Photo by Samara Doole on Unsplash

3. Mixing: the air that I breathe

Generally, the more oxygen then the more heat, quicker decomposition, and less smell (and less flies). We want to make the pile aerobic (with oxygen) to enable the pile to really heat up. Anaerobic composting, with very little air in the pile, causes problems such as smells often associated with decomposition (think septic tank).So, turn the pile as often as you can — once a week for the first three weeks and then leave alone. Turning with less frequency will also result in a good compost product, but will necessarily take longer. However, turning compost too much can waste valuable nutrients into the air and cause the pile to lose too much heat. Getting enough oxygen is crucial to the success of the hot compost process. A composting grate, at the base of the pile, is a useful strategy to help with this. Placing small stones, twigs or other coarse material on the bottom of the pile or bin allows air to passively move through the pile, providing microorganisms with the oxygen they need to enhance the decay process.

4. Microorganisms: the engine room

You don’t have to source the special “brew” of compost microorganisms (mainly bacteria) as these are always present in the air and surroundings. When you start a new compost pile, you can add some “aged” or cured compost from a previous effort. This inoculates the mixed materials with bacteria to “kick-start” the new pile. While you can buy commercial inoculants, it is far cheaper to use aged compost, dilute urine or herbs such as yarrow, comfrey and borage. It is believed that these herbs contain high levels of nutrients to help feed the microorganisms, which enables them to build up in high numbers very quickly, but little research has been undertaken on this. It’s up to you to investigate by trial and error.

5. Minerals: is that sand you’re adding?

Plants need soil. They need minerals in soil. You should never just grow plants in pure compost. I know you can, and I have many times, but a soil with only 5% organic matter is more than ample to support plant growth. Compost is expensive so it makes sense to only use it mixed with sand.

While you can just use sand, you should experiment with what you add to the compost pile. Small handfuls of rock or granite dust, loamy soil, crushed limestone and diatomaceous earth all add nutrients to the pile and these are beneficial to growing plants. Adding sand also helps earthworms — they use the sand grains in their gut to grind dead matter. But be warned: adding lime or too much limestone does more harm than good.Even though these substances add calcium to the compost, it is not required for composting to proceed. There is also no need to “neutralize” the acidic nature of the compost as it decays, as well-made compost goes through an acidic stage before it finally balances itself.

6. Mass: size matters

There are two aspects to this — the amount (volume) and particle size. The shredding of materials is important. The finer the organic material the faster the decomposition (large surface area to volume ratio (SA:Vol) so microbes can attack all sides). While it is best to shred the material you use in the compost pile, occasional bulky, woody material, such as wood chips and street-tree prunings, assist in aeration of the pile. Also a rounded pile has a low SA:Vol ratio so less heat is lost, whereas a long, thin pile has a large SA:Vol ratio and will cool down quickly. Secondly, piles require a certain critical mass — you need at least 35 ft3 but larger is better. This is a big pile. If the pile is any smaller, it cannot maintain heat, heat escapes and the pile cools down. Large piles contain the heat longer. This results in better pasteurization and the microbes have more time to degrade any toxins that may come with the raw materials. You might have to collect enough material over a few weeks before you activate the pile to start the hot decomposition process. The dedicated organic gardener will ask neighbors for their plant prunings and lawn clippings. Making a compost too big, say 50 to 70 ft3, makes turning and shifting the pile difficult and laborious. There is also a natural tendency to continually add material as you acquire it. The pile slowly builds up. However, this will usually result in passive or cold composting and rarely does the pile heat up or maintain the heat to sustain the compost process.

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