Making a good hot compost pile is like baking a cake with no specific recipe. Yeah you know the ratio’s of butter to sugar, flour to eggs, but you throw it together hoping that the ratio will turn out some good German chocolate cake, nice and crumbly, full of texture and rich smell. Well, that is similar to hot composting as the end product we want it to look like good German chocolate cake; crumbly, 70% cacao dark brown, producing the stable carbon compound known as humus. Unless done in a round cage with specific cage link interspaces as measuring tools, the recipe is done mostly by hand and feel. Even then seasonal variations and materials will produce different end results so it truly is an artisanal experience. It’s quite a science but a really fun farm task to take on and perfect over time. Having built countless compost piles in my ten plus years of Permaculture as well as studying and working with Dr. Elaine Ingham, I feel quite versed in this particular topic and urge you to bake the cake thus creating soil!
Basic Pattern Process of Hot Compost:
Materials are gathered and a pile is heaped together in layers in one or two days to achieve the ideal size and hot compost conditions. Ratios of materials are used to
formulate a pile that will heat up and produce a quality compost aimed at improving soil fertility and seeding the whole soil food web. The pile is monitored for moisture content, temperature and overall quality throughout the duration of the pile. When needed the pile is turned, or often called flipped, to decrease temperature and increase oxygen content. The pile goes through a hot process then turns to cold compost, which is aided further by compost worms. The compost is used in a myriad of ways in the garden, nursery, or broad acre applications through a liquidification process.
Materials of a Hot Compost:
All hot compost piles are made of the following four basic ingredients:
- Green material or those that are nitrogen rich
- Brown material or those that are carbon rich
The first ingredient, nitrogen or green material, is often broke into two categories within hot composting. First is your cut green material done so by hand primarily as machine shredding, like lawn clippings, has so much edge it becomes the second kind of nitrogen material, high nitrogen. Those cut by hand, with say a rice knife or scythe or hand-pulled garden weeds, break down much more slowly than the finely chopped clippings. Other high nitrogen materials include plant material from legume family if it is properly fixing nitrogen (check the nodules in the ground, should be tip of the pinky size and bright red when opened). This ensures that the plant is actually fixing nitrogen and in disturbed soils sometimes the rhizobium bacteria in charge of this fixation are simply not present in great enough numbers to push those legumes into high nitrogen status. Other high nitrogen material is animal manure, in particular horse and chicken. However from past experience (argentina 2012-2013), if animals are corralled for long periods of time like goats, they finely break down their pellets by stepping on them also giving them more edge. Moreover they also urinate on these heavily pushing it to a high nitrogen material especially when not bedded down with straw or sawdust.
Brown material, not manures even though they are brown, are carbon rich and based off of dead plant material that lived its full life cycle. For example straw is a brown material because it is cut when the plant has fully lived its life cycle. Perennial weeds that leave their stalks behind in the winter with their pithy stems are a great brown material example. This is part of the reason I always keep wild edges on the gardenscape and a healthy patch of Jerusalem Artichoke. However if hay is cut green, which it usually is and turns brown, then technically technically it is a green and can be a detriment to proper pile ratios of all of these materials. Other sources are wood chips (if chipped during the summer they will be have a green content as well), sawdust, and leaves. A tip for brown material in the pile is to soak it beforehand in wheelbarrows, buckets, pits or ponds as it becomes much more active in a pile if activated by a thorough water quantity.
Water is vital to a pile and its balance must be strictly monitored. As the pile develops and the green material starts to break down, it is easier to get a reading on the pile moisture. When you pick up a handful and squeeze tightly, one to three drops should fall out. If it is much more than that the pile can easily go anaerobic and the organic material begins to putrefy. If it is too little, decomposition and heat production will slow or even halt completely. In the drier summer months when turning, water is often added and piles sited in the shade or by a windbreak. Furthermore, in humid times the pile is covered but the tarp must rest slightly above the pile through adding vertical stakes so that anaerobic conditions do not occur. Water acts as a lubricant and life source for the microorganisms thus making it a vital observation during the life of a pile.
Air: These thermophilic compost piles are meant to facilitate aerobic decomposition which of course has a precursor of oxygen being present. Without air, the pile loses its potency as the by-product of the microorganisms that thrive in these conditions are not what facilitates the best growing conditions. In fact those by-products are a bit toxic and are easily identified through unwelcoming smells. Air is in relation to temperature and the hotter the pile gets the more oxygen is being consumed by the decomposing microorganisms. Bacteria are the main players here which feed mostly on the green material and the more high nitrogen there is the easier it is for a pile to overheat and oxygen levels decrease completely. Turning the pile facilitates the addition of air and also cooling temperature readings. Moreover, air stays in the pile around more chunky plant material, which I always add for this specific reason. However if materials are cut finer they will breakdown quicker so do take some time to cut some of the materials finer. For example straw or stringy perennial greens can be cut with a machete and a cutting block. This gives them more edge for the microorganisms to colonize but be careful of finely cut materials by machine as mentioned earlier.
Temperature Regines of a Hot Compost:
A hot compost pile is meant to get hot but too much heat is detrimental as is stated above. This heat is generated not from the sun but rather the by-product of microbial life. Their eating, movement, and reproduction all cause heat too build and when the proper “party conditions” are presented the compost pile heats up accordingly. Composting temperatures should reach 160 F (72 C) but not go any higher. This is when you know to flip the pile or better yet when it is approaching that temperature. If it goes higher than that, there is a small window where facultative anaerobes will proliferate, as the pile swings from aerobic to anaerobic at this edge. Once above this threshold, then the pile will go anaerobic and piles that are too high in nitrogen have actually been known, although rare, to actually catch fire. Piles will have a warmth to them for around 35 days and getting them above 130 F helps to kill pathogens and weed seeds. This is one of the main reasons why we hot compost besides accelerating succession and evolution in and through the pile. If the pile spikes to 160 F or 70 C within one or two days you most likely added too much high nitrogen. This is ok but requires near daily turning and can lead to anaerobic piles. It should get to that peak temperature in day three or four but a spike to maximum temperate in one or two days says that you put into much high nitrogen material. However, if the pile didn’t heat it could be from not enough green material or high nitrogen and also insufficient water content.
Monitoring the Hot Compost:
The temperature of the pile and its overall health is measured through various scientific and sensory measurements. For the most accurate portrayal of compost pile temperature, which again corresponds to oxygen content, is a compost thermometer. Usually they are sold as 1 meter or yard long and give very accurate readings. The pile should be read from various angles and depth to find out what is the peak temperature of the pile and a different times of the day. Getting it right in the center is a bit of an art but overtime you learn how to do this easily. Without the thermometer you can also of course measure with your hand. I find that if I wiggle may hand into the pile about a foot (30 cm) and I have to pull it out right away, well its time to turn the pile. Also smells are one of our best indicators. If you are smelling high amounts of ammonium, sulphur, rotten eggs, or that of vomit, turn the pile immediately as those are indicators of imbalance and probably anaerobic conditions. Ammonium smells often happen the first day especially if you put lawn clippings in. Also again you want to monitor for water content so do your squeeze test every time you turn the pile and add water accordingly. If it is too wet open the pile up on a sunny day and slowly turn the pile as to extract moisture content. Again it’s a feeling thing. These factors of temperature and moisture content should be recorded to begin to get an overall pattern on any particular site. I like to put mine in bar graphs for temperature and record each pile in PowerPoint, which allows you to easily interface with bar graphs. To compliment this, I note the pile ratios and my observations as the pile is built and matures thus creating a system of documentation and analysis. See slideshow below.
Size of the Hot Compost:
To achieve proper temperature for fast breakdown and killing of pathogens and weed seeds, a pile of at least one cubic meter or yard should be built. This requires quite a lot of material so collecting the material beforehand, especially the carbon material and getting it wet, greatly helps getting the size in one day. Green material should be collected just prior to building the pile but a weed pile from the garden maintenance can also be tapped into. If lots of hands are available such as when we teach this technique in PDC’s, all material can be collected at once. If you can build the pile slightly bigger than the one cubic meter or yard as part of the size in the beginning is lots of air pockets between materials. This will diminish quickly as materials breakdown. By having a pile this size it will create conditions of thermal mass and insulation to keep the pile hot enough to achieve proper temperature for an extended time period. I normally build these piles in a near perfect rectangular shape rather than inside a wire cage and feel the process out for the right recipe. By building it in a rectilinear fashion, the pile edge has been decreased so heat can not escape through an increased surface area from the normal oval shaped heap.
Ratios with the Hot Compost:
As stated before the pile is layered with green material, brown material and contains oxygen and water. Proper ratios will ensure success and can be geared to specific farm needs. If you are looking for a compost for the kitchen or market garden, soil biology succession dictates a balanced pile but slightly higher on the green side. For forest gardens and other agroforestry systems, piles with a higher carbon content will help to increase fungal counts when measured under the microscope and better support that level of succession. Pasture systems are slightly more fungal than gardens and in between the two above as we are relying on perennial grasses and forbs to feed our livestock. Thus a basic ratio is below and again the ultimate measuring tool for complete accuracy would be to build the pile inside a wire fence that has a way to use the fencing spaces as a calibration:
Gardens: 40 % GREEN, 10% HIGH NITROGEN, 50% BROWN
Flipping OR Turning the Hot Compost:
The maintenance of the pile for proper oxygen, temperature and water content requires turning or flipping the compost. This means that all material is moved from the initial location and reassembled in a spot next to the original. I like to build the pile on higher ground and turn downhill if possible as it exemplary of the slope part of energy efficient planning. Garden forks and curved hay forks aid in this process. Done properly, the pile contents from the outside should be first removed and moistened if need be. Then begin to turn the other part of the pile and put those ingredients from the outside onto the inside of the pile. This ensures all material is heated to the pathogen and weed seed killing temperature. Take a moisture test once you remove that first layer which often dehydrates in the warmer, drier months. If you need to add water do so gently but ensure you don’t overwater as this can lead to anaerobic conditions. If it is too wet because of over watering or intense rains and not covering the pile from that, then the flipping process should be done slowly over hours to let wind and sun pull moisture out. Also when turning the pile you should monitor for smells which anything foul means things are beginning to go or have gone wrong and are leading to anaerobic conditions. If you see lots of white ashy material, then you are getting to that threshold edge and the pile should be turned slow to let it really cool down. This will indicate that you need to turn the pile more often if you are not using a compost thermometer.
After around 35 days the pile will cease to have any marked temperature and will continue to cool down to ambient temperature. Keep monitoring the pile and if dry or too wet moderate conditions for that. This is also where I like to take a few handfuls of compost worms from the vermicopmposting bins and shove them in the center. Some people use compost after six weeks but I prefer to wait another six weeks and let the worms further work it. Also fungal counts can increase as they begin to work the remaining higher carbon materials.
Monitoring with the Microscope:
Beyond smell, texture and attentive monitoring during the initial compost phase the ultimate way to gauge your success in proliferating the complete and
diverse soil food web is to look at it under the microscope yourself or contract this service out to a soil food web lab. This allows you to accurately see if your compost contains the much-needed fungus that is far too often lacking in our modern soils. Also if you take this compost and potentialize it through making a tea or extract, it is a very good idea to examine it under the microscope. While this may seem a bit pretentious, it is part of the biological revolution that is unfolding as how we evaluate soils now is very Cartesian and materialistic to say the least. Examining the web present under the scope is pattern based all the while representing and giving voice to the critters that are unlocking the minerals and nutrients. Thus we should aim to stimulate them more than simply adding mined minerals because of a chemical reading.
Ultimately we go through all of this trouble to get a finished product. To keep costs down on the Permaculture farm we should aim to produce some or all of our own compost. Accordingly this important function should be supported by many elements and so one of those is hot compost. In a typical farm year I will build between four to eight of these piles depending on materials and labor. It’s a very involved process straight from the beginning and I have flipped piles that spiked suddenly on Sunday nights at dusk to ensure success. But in the end we get rich compost that can be used as a top-dressing for gardens and agroforestry plantings for direct fertility purposes. I also love to use it in seedling or potting mixes thus reducing our dependence on outside inputs such as commercial compost and peat moss.
Furthermore, the finished product can be put into a liquid solution known as compost
extract or compost tea. The difference is the brewing type and time but essentially they concentrate microorganisms by the billions for you to spread through the wonderful, life giving solution known as water. This facilitates the broad acre application in forestry systems and pastures but I regularly do this in gardens as well. Also after earthworks are implemented this is a wonderful way to rebalance the disturbance that would have taken place. This also includes any tilling that happens for new garden implementation. Hot compost and its liquid spreading is one of the most powerful regenerative agriculture tools we have in accelerating succession and evolution. This contributes to landscape healing, nutrient cycling that ensures plant and animal health, and completion of the hydrological cycle.
Written by Doug Crouch
Header art by Sien Verpoest