Hot Composting

Nine Steps to Hot Composting 

By Jim McNelly, Earth 911


Active Composting

Active, hot composting is a BATCH process. It differs from passive piles that just "sit there" seemingly forever or "continuous flow systems" where stuff is periodically dumped on top of the material already in the bin and removed from the bottom when it is dark and crumbly.

My recommended steps for creating active, hot batches of compost are:

1. Prepare the Composting Area
2. Choose Your Bin
3. Stockpile
4. Inoculate
5. Mix
6. Balancing the Ingredients
7. Water
8. Aerate
9. Active, Hot Composting

1. Prepare the Composting Area

Before beginning the active, hot composting process, the composting area must be properly prepared. Start with an empty bin or space on the ground where you wish to locate your compost pile. Make certain your space is well drained and that storm water runoff from the roof or the yard will not reach the composting area. Choose a point that has a higher elevation than the rest of the yard, but make sure that it is within easy reach of the garden hose because, like houseplants or garden vegetables, the compost pile needs occasional watering.

Appearance and aesthetics are usually as important as where it is in vicinity to the intended use for the compost. While many people place their bin near the garden where the compost will be used, others find that a place centrally located between the front and back yard is convenient. If you don't have a garden, the first compost pile may be a good incentive to start one. Many gardeners say that compost is the secret to larger and tastier home-grown vegetables. An area where a landscape project is contemplated is also a good location for the compost bin. The wise landscaper knows that compost-enriched soil yields long-term growth dividends for trees and shrubbery.

Avoid building bins against walls or fences that can rot and discolor. Choose a level spot away from drainage swales and roof overflow. Avoid low spots where water can stand or pond. Leave plenty of room to get into the area with a pitchfork and a wheelbarrow. The area should be from 6' x 6' up to 12' x 9' for multiple bins. A bin near the kitchen makes processing table scraps easier. It makes little difference whether the bin is in the shade or light, although a dark bin in the sun will warm up earlier in the morning during the springtime and fall. If you place a bin under a tree, you can expect the roots to climb up into the bin, making it difficult to remove the compost. Placing a bin ten feet or more away from the house will discourage bugs, especially termites that may come with wood mulch from moving into your home. Having the garage or storage shed nearby makes it easier to keep pitchforks, shovels, the screen and wheelbarrow clean and out of the weather.

 2. Choose Your Bin

With proper preparation, piles can be made to heat and decompose adequately without the need for bins. The main purpose of bins and enclosures is to help hold heat by keeping the composting mass as close as possible to the active composting center. A bin is like an oven, except that its purpose is to help hold moisture as well as heat. In small piles bins also allow air infiltration from the sides, assisting the "chimney effect" of warm air rising and aiding ventilation. Bins are an advantage since they help the pile look neat, containing unsightly matter within an enclosure.

Avoid using an existing building or wooden fence as a sidewall since an active composting pile will decompose walls, discolor paint, and destroy untreated wooden fence slats. Use galvanized metal, plastic, or cedar for rust and decay prevention in the construction of the bin. (Redwood and cedar are decay resistant but usually come from "old growth" forests.)

There are two basic types of bins: open-sided and enclosed. Enclosed bins tend to be smaller, but since they hold heat and moisture better than open-air bins, it usually take less time for the material inside to decompose. It is hard to believe, but small, enclosed bins may actually process more material over time than large, open-sided bins that may hold three times as much material. Small yards with a high percentage of table scraps often have the best results with enclosed plastic bins, preferably ones that can be lifted off, exposing the material inside for turning or curing. Plastic bins are often rodent resistant which is important if you will be composting a lot of table scraps. Large yards can use larger open-air designs or multiple plastic bins. For square or rectangular bins, large doors are better than small ones so that the material can be more easily loaded and unloaded. Three-stage systems are best, but only one or perhaps two bins are usually active at any given time.

The three stages of composting are stockpiling, hot composting, and curing. Curing piles and small stockpiles do not typically need bin enclosures. When making compost on a batch system, the bin or bins should be sized for the total amount of material to be made as a single batch at any given time. Batch composting is best suited for managing large volumes of grass clippings that otherwise would be mulched into the grass or left on the curb in bags. If using a bin for grass clippings, collect the grass in a way that lets you know how much volume you have in a single cutting. A thirty-gallon trash can holds approximately four cubic feet. A wheelbarrow typically holds either four or six cubic feet.

Pallets are an inexpensive (even free!) material to use to build a compost bin. Not all pallets are the same, however, and best results are achieved if pine, not hardwood pallets are selected. Believe it or not, most softwoods such as pine are more rot resistant than most hardwoods such as oak. Standard pallets at either 40" or 44" are higher than the standard recommendation of bin sides at 36". My experience is that a height of 32", about a man's waist, is the optimum. Hardwood pallets are rather heavy, making doors hard to open. When selecting pine pallets, find ones that have narrower spaces between the boards, which will help prevent material from spilling out. Check paint wholesalers as they tend to have the optimum size pallets for composting since paint cans need more flat surface than other products.

If you produce four to six cubic feet of grass clippings per week, stockpile a week's worth of material for making a batch the following week, which will produce a batch of compost every two weeks. Accounting for volume reduction and adding old compost and wood chips as a bulking material, then you will need two plastic bins sized between ten and fifteen cubic feet. You will also need an area capable of handling an additional two stockpiles of curing compost without bins. If you use open-sided larger bins, you will need two bins approximately twenty to thirty cubic feet each with one stockpile area that can hold numerous five to ten cubic piles.

Many composting books suggest making a bin 36" or even 48" high, but I have found that most people have a difficult time lifting material with a pitchfork more than 32" high. A problem I have with permanent bins, whether they are made out of blocks, boards, pallets, or whatever, is that they require an extra step to move material from one bin to the other. By this I mean that the material has to be lifted out of the bin through the door, then carried around the side wall, and then placed through the opening of the second bin and dumped. With easily removable sides or the new generation of smaller plastic bins, the bin can be removed like a "Jell-O" or "cake mold" and the material can be accessed from all sides, making the transfer process to the next bin, which could even be the same plastic bin, much easier. Many "do it yourselfers" pride themselves in making an elaborate permanent bin, but find that the work to actually use it for active, hot composting to be rather difficult.

Some experts recommend treated lumber, but others express caution that the wood treating and paint compounds may contain heavy metals such as copper, chromium, arsenic, or other toxins that could leach into the compost. The uncertainty of long-term disposal for treated lumber also suggests against their use. Under no circumstances should treated lumber be burned because dangerous arsenic compounds can be released into the atmosphere. One recommendation is to use recycled plastic or bins made with scrap cedar. Enclosures of chicken wire, hardware cloth, and other types of wire mesh are popular.

A wide variety of back yard compost bins are available, many of which have been proven to promote active, hot composting when used properly. Avoid bins with solid sides that keep out air and choke the compost. Several manufacturers sell cedar slat bins that assemble like "Lincoln logs" with open sides and can have modules added to expand into three separate bins. This type of bin is popularized as the original "Lehigh" bin in the 1950s.

While the three-bin system is recommended in many gardening books, their disadvantage is that the adjacent sides have no open area from which to draw air. An advantage of shared sides is that less heat is lost from the common sidewall. Fifty-five gallon drums set upon blocks with holes perforating the base and sides can also be effective compost bins.

A rotating barrel is another composting option. Barrel composters range in size from seven cubic feet up to twenty. The larger tumblers have been described by some users as difficult to turn due to their weight. Barrels are excellent mixers and condition organic material quite suitably in preparation for active bin composting. For those people who choose rotating barrel composters, a bin is still necessary for higher quantities of material, since most tumbling barrels have only a limited capacity. One writer on the Internet tells a story about placing a barrel perpendicular between the wheels of an upside down toy wagon, turning it between the wheels.

Once the bin design is chosen, a suggestion is to construct two or more freestanding bins separate from, but close to each other. Open air bin sizes range between 3/4 cubic yards to 2 cubic yards in volume, such as a 3' x 3' x 3 square, 4' x 4' x 3' square, 4' x 4' round, or a 3' x 3' or 2' x 2' five or six sided bin. The cubic yardage of a box equals the length in feet times the width times the height divided by 27 = L x W x H /27. The area of a round bin is calculated by multiplying pi (3.141) times the radius squared times the number of feet in height divided by 27 (R2 x pi x H /27) A bin, three feet round and three feet high, contains 21.18 cubic feet. (1.5 x 1.5 x 3.141 x 3) or .78 of a cubic yard (/27). A four-foot diameter round bin that is four feet high contains 50.24 cubic feet or 1.86 cubic yards.

Three foot widths are considered the minimum for open air sided bins if the pile is to hold sufficient mass to heat properly.

If no bin is desired, a "stand alone" pile of 5' in diameter by 5' high (1.7 cubic yards) is optimum, although a small area at the base of the pile should be cleared to allow one end of a passive air flow system to reach the open air. Since the air entering the sides of the pile is as critical as air entering from the bottom, a 5+ cubic yard pile that is 4' wide by 4' high by 9' long has more surface area and better air access than a pile that is 6' by 6' by 4' high.

A compost bin with easily removed sides is desirable. Some bins have slats that are removed by pins. Others have individual panels that slide out from the top or side. Still others have removable panels or hinged gates. Wire or plastic mesh systems often have fasteners that hold the two ends together. Removable sides also make turning and aerating the compost easier, so the householder is more inclined to actually use the bin and thus turn and mix material regularly.

In choosing a bin design, always remember that it is air, not the container, that is the main secret to successful composting. The more open area, the more air can enter the pile. While an underside passive airflow system can help, it is no compensation for inadequate initial mixing. Avoid airtight bin designs unless you are prepared to let organic material sit for over a year, possibly generating odors if disturbed.

Be certain that there is plenty of working area around the compost bins so you can get at the pile with proper leverage and without straining. A bin design with sides that can be disassembled and reassembled quickly will make pile turning easy. Holding bins are used mostly to make the composting area look neat and to keep leaves from blowing away.

Most yards do not generate sufficient organic matter to support more than one active composting bin. If there is insufficient material to fill a bin, the dedicated homeowner interested in organic gardening will pick up yard trimmings from a neighbor's yard in order to keep the bins fully active, keep material out of the landfill, build up the topsoil, and enjoy some exercise. Once you have chosen your composting area and constructed your bin(s), the active, hot composting process can begin.

 3. Stockpiling

There is a natural tendency to slowly fill the compost bin with fresh organic matter and hope for the best. This is the basis of passive composting. But the composter who desires a hot, active pile waits until there is sufficient material accumulated to properly start a "batch" of compost. Other composting guides suggest that the person stockpile right in the bin, layer by layer. The mixing stage is completed some time later when the pile is "turned". While this process can be effective for passive composting, it is easier to stockpile outside the bin and mix everything all at once at the start of a hot compost batch.

Separate from the active compost bin should be an area where grass clippings, leaves, and other soft stemmed yard debris can be stockpiled. Since active, hot composting requires that you fill the bin all at once, this usually requires stockpiling of at least half the volume of the size of your bin. Leaves, wood mulch, hay, shredded bark, oversized compost left over from screening, straw, pine needles and cones, old compost, and shredded paper products are easy to stockpile. Keep these small piles near the mixing area to avoid too much handling or moving of the material.

When considering how much material to stockpile, you first need to know the capacity of your bin. For an open-air design, it is important to have about twenty cubic feet, or two thirds of cubic yard of raw material. For plastic bins, you only need to gather about half that amount. The important consideration is to have plenty of fresh material on hand, knowing that the remainder required to fill the bin can be made up with a blend of compost recycled back from the most recent batch. This "minimum batch requirement" may require gathering supplemental material from the neighbors.

When stockpiling green material such as grass clippings, you may wish to immediately add wood chips and old compost inoculant to keep the material aerated while waiting to start the next batch. Once the active, hot composting process is established and there is an existing quantity of rich, active compost to use for blending, then fresh material can be added intermittently without as much need for stockpiling. Additional grass clippings or leaves should be watered and mixed with active compost culture in the mixing area before placing them on top of an active composting pile. This will help prevent the new material from restricting the natural aeration of the active compost underneath.

Many composters keep a stockpile of leaves from the fall to add to the summertime grass clippings. These holding bins are usually larger than the smaller active composting piles. They also keep a stockpile of wood chips for free air space and bulking. The other stockpile is cured or curing compost to use as an inoculant.

Grass clippings can be stockpiled for a week or so, taking some table scraps mixed in during the meantime. You will be impressed at how quickly table scraps decompose when added to a trench in the top of a hot, active pile. Some stockpiles may be left as small mounds near the bins; others such as leaves may require containment. Leaves may be left in bags, however in yards with many trees, you may have to stockpile leaves in large bins, often over forty cubic feet, or four feet by four feet square. Shredding leaves or running them through the mower can reduce their volume and make them a little easier to compost. Brush should never be placed into the compost bin, at least an active bin, without chipping or shredding. Not only does it take months, if not years, for full-size branches to decompose, unchipped brush clogs the composting mass, making it nearly impossible to fork out the material.

 4. Inoculating

Inoculating the compost pile is the process of distributing an active biological culture evenly throughout the material to be composted. In the past, many compost guides have recommended using soil as an inoculant, even recommending several 6' layers of topsoil. Clinical studies have shown that adding soil does more harm than good for the composting process since most soils are low in organic matter. The number of active organisms in even a rich topsoil with 7% organic matter is insignificant compared to an organic compost with up to 90% organic matter.

Some composting experts correctly point out that the essential micro-organisms are already present in leaves and grass clippings and will eventually decompose the organic material. But distributing high populations of organisms evenly throughout the raw material has been shown to accelerate the composting process tremendously over piles with no inoculant. In large commercial composting facilities, the percentage of compost recycled is closely controlled to keep the composting piles performing at their optimum.

Compost inoculation is like starting a yogurt culture or adding yeast to bread. The active culture must be thoroughly blended in order to produce the desired effect. Mixing the active culture at the start of the process is essential because all raw organic material must be inoculated with micro-organisms before the composting process can begin in earnest. An active, hot compost process requires immediate decomposition. There is not enough time to wait for organisms to migrate up from the soil or move down from a layer on the top of the pile.

Several compost inoculants are available on the market which claim to accelerate the composting process. Some are active bacteria cultures that may help the inoculation process when first initiating or re-starting the active composting process. The best commercial inoculants are bacteria concentrates cultured under laboratory conditions. Inoculants that are merely old compost in a small bag can be quite expensive.

Many composters report positive results using packaged inoculants. However, millions of active composting piles have been created and have generated wonderful compost without extracts and supplements. The existing beneficial micro-organisms in the raw material or the added culture inoculated from an active composting pile will provide all the essential micro-organisms sufficient to initiate active composting. Saving old compost to "re-seed" the next batch is a sure and inexpensive means of maintaining the active compost culture without having to purchase inoculants time and time again.

If you have no old compost pile to inoculate the first pile and your topsoil has little organic matter in it, you may wish to purchase some inexpensive bags of composted cow or other manure from the garden center. You could also harvest some old leaf mould from a wooded area to help start the first composting culture. Once you have made your first batch, however, you will have plenty of older compost to blend in with new materials.

You don't have to keep excess amounts of cured compost in the active bin area if you need the space for fresh material. Once a pile has cooled, compost can be taken out of the bin to finish curing on the ground. The bin is primarily for heating and aerating. Once you start the composting reaction and have an active culture of micro-organisms, you will discover that inoculating new piles becomes easier with practice and your piles will consistently heat and decompose rapidly with a minimum of odor or effort.