This article originally ran in the spring issue of New Farm Magazine, the magazine of Organic Farmers Association. All OFA members receive a complimentary issue of New Farm twice per year. Click here to sign up!
How-To for Farmers: A scientist shares insights and tips for managing your compost heaps.
By Jean Nick
Compost is a vital resource for organic farmers—as a soil amendment and a way to deal with many common types of waste. And making compost couldn’t be simpler. “Compost happens—just give it time,” says Rick Carr, compost production specialist at Rodale Institute. But if you pay attention to a few key details along the way, the process will be very efficient and you’ll get the best possible results.
BASIC INGREDIENTS Start with materials you have on hand, including carbon-rich crop residues and high-nitrogen barnyard manure. Add fall leaves, grass clippings, and other homeowner-provided yard waste, which many municipalities can provide for farmers. Local food processors (such as grain mills, breweries, and coffee roasters) and food service operations (at sites like hospitals, schools, and supermarkets) generate compostable waste that they may deliver to local farms to avoid the cost of disposal. Chip large logs or branches before adding them to your compost pile, but don’t bother chopping up other materials, Carr says. “It might speed up the decomposition process, but the return on investment is rarely worth it. Time is free, so let it do the work.”
BEST RATIO Achieving an effective ratio of carbon-based (brown) to nitrogen-rich (green) materials to maximize decomposition can be challenging for beginning composters, but with a little trial and error you can easily become a master. The trick is to always learn from your mistakes. Generally speaking, start by mixing browns and greens at a ratio of 5 to 1 and work from there. Keep in mind that carbon and nitrogen content will vary by feedstocks and shouldn’t be viewed strictly as green or brown but rather how much green is in a brown material and vice versa.
MINIMUM SIZE You can just layer the different feedstocks on top of each other as you gather them—they ultimately blend together as you turn the heap. Most composters suggest an initial minimum pile size of a cubic yard, but you can make it as wide and high as your equipment can reach to turn it.
Active Ecosystem Like soil, compost has a complex food web that includes a diversity of microorganisms. The most critical actors are the countless microscopic bacteria and fungi that feed on dead plant materials.
BACTERIA PROFILES The transformation starts with bacteria already present and surviving on the feedstocks. Heat is generated as feeding and breakdown accelerate in the pile. A shift in the microbial profile from mesophilic (thriving at moderate temperatures) to thermophilic (heat-loving) microbes begins to occur as the temperature continues to rise. Eventually, it may reach near 170°F and hold for a short while until food resources become exhausted. The temperature will then decrease, and this could be used as an indicator for when to turn. Turning compost temporarily adds oxygen to the pile but more importantly exposes new surfaces for decomposition. As microbes digest plant matter, they release carbon dioxide, humic acids, and more heat.
AIRFLOW NEEDS Aerobic bacteria are the fastest, most efficient decomposers during composting, and they release plant nutrients such as nitrogen, phosphorus, and magne- sium. They require oxygen to function and survive—regularly turning the pile introduces the air they need. When they don’t get enough oxygen, anaerobic bacteria outcompete them for resources. These microbes produce organic acids and amines (sour-smelling ammonia-like substances), and they leave behind nitrogen that’s unavailable to plants.
SUPPORTING ACTORS Actinobacteria—a more complex group of bacteria similar to fungi—work in a pile’s moderate heat zones, decomposing tougher materials such as cellulose, starches, and lignin. They also release nitrogen that plants can absorb from the compost. Actinobacteria give finished compost its pleasantly earthy aroma. Fungal mycelia also break down cellulose and lignin, taking over after faster bacteria have consumed readily available simple sugars. The fungi become the dominant decomposers at the end of the process, still feeding on woody materials.
Don’t waste money on so-called compost activators, Carr advises. “Everything you put into a compost pile—plus the air itself—is all the microorganisms need.”
ORGANIC STANDARDS The USDA’s National Organic Program specifies how certified-organic farmers must manage their compost heaps. The regulations state that the finished compost have a carbon-to-nitrogen ratio between 30 to 1 and 15 to 1. Just as critical, the pile’s temperature must remain between 131° and 170°F for 15 consecutive days, and the whole heap must be turned at least five times within that 15 days to ensure that any human pathogens present in the raw materials are killed.
Your composting process must be documented with temperature records. If the pile does not hit the required temperature marks, the compost should be treated as if it were raw manure. (See “Taking Temperature” at right for thermometers and hints that get you reliable readings.)
WEED KILLING Another reason to get your compost pile temperatures above 140°F is that the heat also kills plant pathogens (diseases) and weed seeds.
HEAT PATTERN Once your pile has enough volume, the bacteria begin generating heat. (Cold air temperatures slow this down.) After a few days or weeks, the pile’s temperature plateaus and starts to drop. Then it’s time to turn the heap to add air, redistribute moisture, and give the microorganisms new surfaces to work on. Mix the pile thoroughly, moving material in the center to the outer surface and material from the outside to the inside. The pile will heat up again but may not get quite as hot as before; then it will cool down, signaling it’s time to be turned again. Repeat until no reheating occurs.
DONENESS TESTS The composting process is complete when the pile no longer heats up after being turned. You should see that the volume has decreased from what you started with, and the original materials (other than large woody bits) are no longer identifiable. A handful of finished compost is dark brown and crumbly, almost like chocolate cake crumbs, and smells earthy and kind of sweet.
PROBLEM SOLVING If your pile isn’t heating up and the materials are dry, dampen them with water when you turn them. In very wet or very dry climates, cover the pile with a tarp to exclude or retain moisture. If your pile reeks and/or is wet and slimy, add more brown material or turn it repeatedly to dry it out.
COOK TIME Once you stop adding fresh matter to a pile, the decomposition time will vary from three months to eight months, depending on how often you turn and manage the material. It’s not uncommon, however, for composters to allow their piles to process for up to 12 months. To ensure a steady supply of compost, start a new pile at least once a year.
Rick Carr recommends a 36-inch Reotemp Super Duty Fast Response Compost Thermometer ($149, reotemp.mybigcommerce.com), which reads temperatures of 0° to 200°F.
Insert the probe into the center of the pile and wait until the needle settles. If you are a certified-organic farm, you need to measure and record the temperature daily until you have met the National Organic Program’s 15-day requirement. After that, or if you are using temperature just to know when to turn the pile, measuring once or twice a week is sufficient, Carr says.
See Rick Carr’s webinar on the science of compost management at RodaleInstitute.org/learn/webinars.