Compost is a valuable soil amendment that has the potential to provide nutrients to crops, improve soil structure, promote microbial activity and suppress plant diseases. However, it is not a cure-all for soil problems, and its use should be part of your overall soil fertility plan to manage nutrients, rotate crops and minimize tillage. Making your own compost is one way to keep its cost down, but it still takes time and management to make high-quality compost.
Opportunities for on-farm composting
Most communities generate significant amounts of waste products that can be composted if collection and transportation issues can be addressed. Some of these materials are relatively easy to compost, such as dairy and horse manures with bedding, while others can be more challenging to compost, such as fish waste or paper mill waste. Yard waste, food processing waste and other off-farm, carbon-based residues may also be available. Most states and some municipalities have regulations that apply to larger-scale compost operations. Before you start composting, find out what state and local rules might affect you.
Even if the price is right for a given waste, it has to be suitable for agricultural or horticultural end use. In other words, the raw ingredients used to make compost need to be relatively free of non-compostable materials or toxic contaminants. On-farm wastes like manures are less likely to raise concerns about such contaminants compared to many off-farm wastes, especially when you know the farm or farms that the manure comes from. The use of off-farm wastes can be made less risky through clear communication with the generators of that waste about the standards you require to make high-quality compost.
When accepting a waste stream for on-farm composting, be sure to ask if there is a process in place to remove undesirable materials such as plastic, metal, glass or grass clippings from herbicide-treated lawns. When possible, inspect materials upon delivery before they are added to your compost piles. If contaminants are found, provide samples to the waste supplier as soon as possible and develop a plan for preventing future occurrences.
On-farm composting is commonly done in windrows, or long piles that are turned over several times. Piles may be turned by pushing them with bucket loaders, by loading the compost into a manure spreader and reforming the windrow or with a commercial compost turner. If the piles are not turned, they are called static piles. Instead of turning, these may be aerated with perforated pipe and sometimes a blower is used to push air through the pipe.
The composting process
Composers make music, but decomposers make compost. Lucky for us, decomposers are widespread in nature, and compost piles don’t need to be inoculated in order for the raw materials to start decomposing. In fact, a well-made compost pile heats up right away to 130 degrees or more, indicating a rapid rate of decomposition that can continue for several weeks. During this time the pile needs an adequate supply of air and moisture to remain active. Turning can keep the pile aerated while rain or irrigation may be needed if the pile starts to dry out. Eventually, the pile temperature will drop to 100 degrees or so as the compost continues to mature or “cure” into a finished product.
Many microbes do the work
To succeed in making the highest quality compost with the least investment of time and equipment, it’s important to understand the different groups of microbes involved in composting. Obligate aerobes are strict oxygen consumers that die off when oxygen is lacking. Facultative anaerobes can work in both aerobic and anaerobic conditions; in other words they adapt to the oxygen level of their environment. Aero-tolerant anaerobes grow 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 when it is present.
It’s not essential to manage composting as an entirely aerobic process, but it does speed up the compost process and it creates heat, which helps reduce the number of weed seeds and pathogenic organisms. If you have the time and your compost is made from ingredients with relatively low risk of weed seed survival, or food safety risk, such as a plant residues rather than animal manures, then the facultative anaerobes may be an especially important part of your on-farm composting strategy. They function at lower oxygen levels, minimizing the need for (and cost of) manipulating the pile. They’re also responsible for generating some of the disease-suppressing abilities that have been observed with certain composts.
The anaerobic problem
If a compost pile becomes extremely low in oxygen for extended periods of time, the result can be strong odors, a very slow process and the accumulation of phytotoxic (plant damaging) compounds. To avoid this situation without having to turn frequently, compost piles should be built so that they are as self-aerating as possible. This can be accomplished by using materials with sufficient porosity, managing moisture content to avoid soaking and keeping windrows small enough to avoid internal compaction from their own weight. 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.
Time to turn
The porosity of a compost 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-equilibrates. But, if the pile is not porous enough, or its core is wet, then turning may be needed to aerate the pile. Keep in mind that aggressive turning further reduces 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.
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 the surface that have not been fully decomposed.
Not too high and not too wet
Windrow height, after settling, should typically be no more than 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 excess heat conditions, which encourage the loss of moisture. If rains threaten to saturate the windrows, they should be covered. Use either a breathable compost cover, or if using plastic, do not completely cover the sides of the windrows. Leave an uncovered gap along the edge of the ground that will allow air to be more easily drawn into the windrows.
The end use of compost determines how important its quality is. Compost for potting mix must be the very best, while compost for roadside landscaping can usually be of lower quality than that required for food crop production. Typical quality targets for good compost are: a neutral pH, an organic matter content of 40 to 60 percent, a carbon to nitrogen ratio of 10 to 15, low-soluble salts, absence of phytotoxic compounds and weed seeds and good seed germination. You can easily test for some of these attributes yourself by doing a germination test of the compost to see what sprouts when you put a moist sample in a warm location, and by planting a known number of crop seeds in a compost-filled tray and observing germination and growth. Many university and private labs can test other attributes of finished compost.
Spreading the wealth
Compost is often referred to as a soil amendment rather than a fertilizer, but it does contain a significant amount of plant nutrients, even if they are not immediately available. It’s estimated that 10 to 30 percent of the nitrogen available in compost may be released in the year after application. Finished compost typically has an analysis of 1-1-1 (N-P-K). These numbers vary quite a bit depending on what ingredients were used, how they were composted and for how long. If you will use or sell a lot of compost it sure makes sense to have it tested so you or your customers know what they are applying per yard or per ton. (One ton of compost is equal to about 2.5 yards, again, with variation, depending on bulk density and moisture content.)
In general, high rates of compost, perhaps 20 or more tons per acre, can be applied to soils needing structural improvement and additional nutrients, and lower rates, around 5 to 10 tons per acre, can be used on soils with decent, but not great structure and moderate levels of fertility. On soils with very good physical condition and/or excessive levels of nutrients, the use of compost should be minimized, even avoided.
Editor’s note: This article was originally published in the August 2010 issue.