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

On-Farm Composting

by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

In most communities there are a variety of ‘organic’ (meaning carbon-based) waste products that have the potential to be transformed into a valuable soil amendment by composting. On-farm composting can be profitable, but it isn’t for everyone.

The Benefits of Making Compost.

Making your own compost is usually less expensive than buying it for field or greenhouse use. And unlike bagged fertilizer, applying compost improves soil structure while it provides nutrients for crop growth. Typically, finished compost has a nutrient analysis of about 1-1-1, although this varies depending how the compost is made. Much of the nitrogen and phosphorus in finished compost is slowly available over time, while the potassium is more soluble and more readily available. Since compost contains relatively low concentrations of slow-release nutrients it is not a good short-term ‘fertilizer’ for crops, but over time, compost applications can build up soil nutrient reserves. Applying compost is also a great way to maintain or increase soil organic matter levels.

If you make more compost than you need on the farm, it can also be a source of revenue. It’s relatively easy to sell pick-up loads to local gardeners and landscapers. Putting the product in bags requires considerably more labor, but can increase the price you get. Before you sell any compost, be sure to establish a regular testing program to assure its quality by sending samples of each batch to your state university or other lab that performs compost analysis.

The Drawbacks of Making and Using Compost.

To make good compost requires a significant commitment of time, equipment and space. If your management plate is already full, it probably makes more sense to buy compost than to make it.

When applying compost, if a little is good, a lot may not be better. Large or repeated applications of compost will add a significant amount of phosphorus to the soil, so if your soil already tests high in P, adding compost is not be advisable. Instead, focus on growing cover crops to enhance soil organic structure, using legumes to provide nitrogen.

Unfinished or improperly made compost can harm plant growth. If decomposition is not complete, soil nitrogen may be tied up after compost is applied, stealing it from your crops. Organic acids may also be present in compost that is not fully mature, especially if it was allowed to go ‘anaerobic’ during the composting process.

In addition, if the materials you use to make compost are contaminated with excess salts, herbicides, or trash, the final product may be unsuitable for farm or garden use. On-farm wastes tend to be relatively free of contaminants, but other waste streams may require close attention to make sure that material destined for composting is free of harmful ingredients. It is essential to work with your waste providers to explain the necessity for ‘clean’ wastes and to establish appropriate monitoring to identify potential problems.

Should I Make Compost?

The feasibility of on-farm composting depends on the availability of raw materials, a suitable site, appropriate equipment, sufficient labor, and enough ‘know-how’.

Easy access to the right mix of organic wastes, or ‘feedstocks’ is key to deciding whether or not to compost. If you produce one or more of the ingredients on your farm, you may be able to obtain the remaining feedstocks and still operate economically. But it rarely pays to haul feedstock long distances, although charging tipping fees to accept waste from other sources can help offset trucking costs. In many states a permit may be required to compost materials other than farm or yard wastes.

You will need plenty of space to build the compost piles, stockpile feedstocks, store finished compost, and maneuver equipment. Be sure to locate the composting operation downwind of residences and away from surface water and wells. Talk with your neighbors about your composting activity before you begin operations. Odors, flies, dust and noise associated with commercial composting operations can be offensive.

Equipment will be needed to move and mix material in the compost pile or windrow. A front-end loader and a truck may be all you need. Other equipment to consider may include: chipping or shredding equipment (depending upon the feedstock); a windrow turner (available in many sizes and prices, and also available for hire in some locations); screening equipment (to recycle bulking agents or remove pieces of trash); storage structures or covers to protect finished compost from weather.

If the your available space or labor for making compost is limited, keep your compost operation small so that you don’t get swamped with materials or neglect the process. If you are going to handle a lot of materials, or if you seek to make high quality compost suitable for premium bagged sales or use in a potting mix, then someone on your farm has to ‘have their finger on the pulse’ of the compost process.  In my experience, the best compost is made on farms where an individual has a keen interest in composting, along with the necessary knowledge and responsibility.

The Compost Process.

Composting can be thought of as ‘farming’ the microorganisms (bacteria, fungi, and actinomycetes) that break down organic material into more stable forms. The job of the composter is to feed these organisms a balanced diet and provide them with a comfortable environment so they can work efficiently. Composting is an art as well as a science, and although there’s a lot of flexibility in how you design and manage your compost system here are some basic guidelines.

Raw feedstocks usually have to be arranged for and collected in advance, then mixed together in a timely manner. Materials that are wet and sloppy can be challenging to store and sometimes to compost. These include food wastes, fish processing waste, lake weeds, and manures without bedding. The problem with dense, wet ingredients is they do not allow sufficient aeration and thus tend to become anaerobic. The lack of oxygen then results in offensive odors and byproducts that may be harmful to plants.

Wet materials need to be mixed with dry, high-carbon ‘bulking materials’ promptly to absorb moisture and avoid excess odors. Bulking agents include wood chips, paper, corncobs, straw, hay and leaves. To compost properly, the bulk density of the mix should be below 1,000 lb per cubic yard to allow for good aeration and the maintenance of oxygen levels at 3 to 5% in the pile. Bulking agents that are slow to decay, such as wood chips or corncobs, can be screened out and re-used in future compost piles.

Moisture content between 40% and 60% is a good target range. The compost mixture should feel about as moist as a wrung-out sponge. You may need to protect the compost piles from excessive rainfall by covering, especially later in the process. Early on, some moisture will be driven off from the mixture by the heat of the composting process. During dry weather, the mixture may need water added to maintain moisture and microbial activity.

A carbon to nitrogen ratio (C:N) around  30:1 is considered best for the mixture of wastes to be composted. (The composting resources listed later contain estimated C:N ratios for many raw materials.) Higher C:N ratios slow down decomposition, because low nitrogen limits microbial activity. Lower C:N ratios cause excess nitrogen to be volatilized as ammonia, producing odors and wasting nitrogen. If odors are a major concern, consider an initial mixture with a C:N ratio closer to 40:1, and cover the pile with a layer of finished compost or shredded leaves.

Some microbial inoculants are marketed to improve the compost process. While these are unlikely to be harmful, they are not often necessary. They may improve decomposition of relatively sterile wastes, such as cooked food wastes. However, most agricultural wastes contain sufficient microbial populations to initiate composting.

Pile temperature is a good indicator of the compost process. During the first few days of composting, pile temperature should increase to between 105 degrees and 140 degrees F or higher. These temperatures may be maintained for several days or more, until the microorganisms use up the available ‘food’ or until moisture conditions become less than optimal. Mixing the pile then brings more undecomposed food in contact with the microbes, replenishing their energy supply. Mixing also helps to redistribute moisture throughout the pile.

After the readily decomposable material is depleted, the pile will no longer heat after remixing. The temperature will drop to ambient since only slow decomposition will continue. The material should have a pleasant earthy odor and a friable texture similar to a good potting soil. It is recommended that compost be allowed to cure for several months more prior to use in order to allow for additional breakdown of more resistant constituents. Typical field application rates of finished compost for vegetable production are 10 to 15 tons/acre.

Two good resources available to help the farmer with composting are: On Farm Composting Handbook and Field Guide to On-Farm Composting, both available from the Natural Resources and Agricultural Engineering Service, Riley Robb Hall, Ithaca, NY 14853, (607) 255-7654, or www.nraes.org, as well as from many Extension offices.

Published: February 2004
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