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  A Publication of UVM Extension's Vermont Vegetable and Berry Program

Low-Tech, High-Quality On-Farm Compost

by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension
Adapted from information presented at workshops by Dr. Will Brinton, Woods End Lab, Mt. Vernon, Maine.

Anywhere you go, there are waste products that are totally useless in farming that can be made into a useful product if proper compost recipes are developed. Some of these materials are more challenging than others, such as fish or paper mill waste, while others are easier to compost, like dairy or poultry manure plus bedding. On-farm wastes and yard wastes tend to be relatively free of contaminants compared to municipal wastes. With off-farm wastes, total source separation that results in "clean" residues is essential if the end-product is to be used in farming. Otherwise, a variety of contaminants may accumulate in the farming system over time. Composting for waste disposal and composting for production of a soil amendment can be conflicting approaches.

A book that is helpful with compost recipe development, compost siting, and compost marketing is On Farm Composting, available from the Northeast Agricultural Engineering Service, Riley Robb Hall, Ithaca, NY 14853, as well as from many Extension offices.

To succeed in making the highest quality compost with the least investment of time and equipment, it is important to understand that there are 4 different groups of  microbes involved in composting. Obligate aerobes are strict oxygen consumers that die off when oxygen is lacking. Facultative anaerobes are versatile organisms that can be simultaneously aerobic and anaerobic - in other words they adapt to the oxygen level of their environment. Aero-tolerant anaerobes grown anaerobically in the presence of oxygen - they don't use it but they can tolerate it. Obligate anaerobes are limited to oxygen-free environments and die off if it is present.

All of these organisms have a contribution to make to the compost process - there are no "good guys" and "bad guys". Contrary to popular belief, it is not optimal to manage composting as an entirely aerobic process - obligate aerobes do not normally dominate until the end of the compost process unless extreme measures are take to aerate the pile, such as frequent turning or forced ventilation, which are expensive and unnecessary. Obligate aerobes do generate heat, which is important if the compost ingredients are unsanitary (ie sewage sludge), and they work fast, which is helpful if you are in a rush for compost. But the facultative anaerobes are just as important to on-farm composting, because they function at lower oxygen levels, minimizing the need for manipulating the pile, and they are also responsible for generating the disease-suppressing abilities that have been observed with certain composts. The natural mix of organisms in compost makes the process very resilient to varying environmental conditions.

Some degree of anaerobic respiration during composting is also advantageous because it results in less loss of carbon than a fully aerobic process, which in theory converts everything to the end products carbon dioxide and water, although in practice a small amount of microbial biomass, or humus, also results. Because anaerobic respiration results in the production of a variety of end products such as organic acids and alcohol, the rapid loss of carbon as CO2 is avoided, and there is greater opportunity for conversion of the original carbon to humus.  Although the products of anaerobic decomposition can be toxic to plants, as the compost process continues and oxygen levels rise, aerobes will readily decompose these materials.

In other words, the extra metabolic steps that non-aerobic decomposition contributes to the compost process leads to more microbial biomass and byproducts to be generated, and these are the high molecular weight compounds that comprise finished compost or humus. The result is that carbon, and plant nutrients such as nitrogen, are more effectively recycled than with highly aerobic, "clean-burning" composting.

All of the microbes required for a healthy compost process are already present in most wastes that can be composted, and therefore the utility of adding compost inoculant is limited. A notable exception can be food wastes, which are often cooked (ie sterilized). In a study that examined the effect of inoculants, added microbes could not subsequently be found in dairy manure compost, apparently because the existing microbes excluded introduced organisms.

 Although most farmer-composters do not monitor the oxygen levels of their windrows, it is instructive to understand how these levels vary during the compost process. Immediately after raw materials are mixed together, the oxygen levels in compost are just below the ambient (outdoor) level of 22% but drop off very rapidly as aerobic decomposition takes off. The process soon shifts to a mixture of aerobic and anaerobic decomposition as oxygen levels drop down and then fluctuates between about 2% and 5%, which is a healthy range for facultative anaerobes. This level can be naturally maintained in a windrow with sufficient porosity. Toward the end of the compost process, oxygen levels start to climb back up, nearing ambient once the compost is mature and decomposition slows to a halt. That's because as internal oxygen demand subsides, diffusion of oxygen into the pile can catch up.

However, the porosity of a windrow tends to decrease as materials are decomposed, and for a short period of time, when oxygen demand is still high, a windrow may go excessively anaerobic until diffusion of oxygen can catch up. In many cases the best thing is to leave the windrow alone until it re-equilabrates. But if the pile is not porous enough, or its core is wet, then diffusion may be too limited, requiring turning to aerate the pile. The key is not to over-do it, because aggressive turning further reduces the porosity by breaking down the aggregates or small clumps of composting materials, increasing the need for subsequent turning. In other words, the need for frequent turning becomes a self-fulfilling prophecy in order to prevent the pile from becoming too anaerobic.

If the pile is extremely low in oxygen for extended periods of time, the result can be strong odors, a slow process, and the accumulation of phytotoxic compounds. To avoid this situation without having to turn frequently, compost piles should be built so that they are self-aerating. This is accomplished by using materials with sufficient porosity, managing moisture content to avoid soaking, and keeping windrows small enough to avoid internal compaction. Then, the natural oxygen demand of the compost process will assure a steady, low level of oxygen as it diffuses into the windrow to replace the oxygen that is consumed by the microbes.

In conclusion, some turning is usually needed to facilitate the compost process. The use of sufficient bulking material and low-impact turning methods (like a bucket loader versus a compost turner) can allow a windrow to remain self-aerating with a minimum of  turning.  At the outset, raw materials with sufficient porosity must be mixed together to distribute carbon, nitrogen, moisture, and oxygen evenly through the windrow. Then, after the initial burst of decomposition and oxygen consumption, a gentle turning can help keep the pile sufficiently aerated. Finally, as the process nears completion, turning helps homogenize the finished compost and mixes in materials on surface that have not been fully decomposed.

Windrow height, after settling, should be no more than 3 to 5 feet unless large amounts of porous materials like straw are being composted, otherwise the core will become compacted. Keeping the windrows small also avoids high-heat conditions which encourage the loss of moisture. The windrows must be kept moist with irrigation if necessary. If rains threaten to saturate the windrows, they must be covered.

Under this type of low-tech, "minimum tillage" management system, dairy manure or similar materials should compost to maturity in 4 to 5 months. The cost to produce compost using a bucket loader to turn 3 times is about $7 per ton. Obviously this figure will vary depending on material and labor costs.

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