Composting is a deliberate activity by gardeners aimed at accelerating what occurs naturally—rotting, or the decomposition of organic matter. As previously mentioned, a typical mineral soil consists of about 5 percent organic matter. Organic matter affects both the physical and chemical properties of soil. It improves the aeration and moisture retention of the soil and, through gradual decomposition, releases both major and minor nutrient elements into the soil for plant use. Composting in effect is organic matter recycling. In the soil, compost acts like a source of slow-release fertilizer, in addition to its desirable influence on the physical characteristics of soil. Compost An organic soil amendment consisting of highly decomposed plant organic matter. 22.14.1 PRINCIPLES OF COMPOSTING To be successful at composting, one needs to understand the underlying science of the biological processes involved. Composting involves both biotic and abiotic factors, the essential ones being decomposers (microorganisms), organic material (plant or animal), environmental factors for growth (of the decomposers), and time (over which decomposition occurs). 22.14.2 DECOMPOSERS Decomposers are the agents that convert organic matter into compost through the process of decomposition. Types There are two groups of decomposers that inhabit the soil—microorganisms and macroorganisms. Decomposers Microorganisms that break down organic matter. Microorganisms The major microorganisms (or microbes) involved in decomposition are bacteria, fungi, and actinomycetes. Macroorganisms The major macroorganisms include earthworms, grubs, and insects. These groups of organisms should be provided the appropriate environmental factors for their growth and development in order to have large enough populations to work effectively in the compost pile. Microbes have four basic requirements for growth: 1. Source of energy. Microbes obtain energy from the carbon inorganic materials. Plant materials differ in their carbon content, and thus proper materials must be selected for the compost pile. 2. Source of protein. The protein source for microbes is the nitrogen from materials such as blood meal, manure, and green vegetation. Protein is required in only small amounts. 3. Oxygen. Aerobic microbes, which use oxygen for respiration, are more effective and efficient decomposers than anaerobic microbes, which do not use oxygen for respiration. As such, a compost pile should be well aerated. When a pile is poorly aerated and thus dominated by anaerobic bacteria, the decomposition process produces foul odors. 4. Moisture. Moisture is required by organisms for metabolism, but excess moisture in the compost pile fills up the air spaces and creates anaerobic conditions. An active compost heap is, in effect, an environment teeming with a wide variety of microorganisms that operate in succession, depending on the temperature in the heap. The temperature in the heap changes because the by-product of a metabolic reaction is heat. It is important to note that bacteria operate over a broad spectrum of temperature conditions. Some prefer cool conditions while others prefer warm conditions. A group of microbes called psychrophiles can operate at temperatures below freezing (2.22°C or 28°F) but work best around 12.8°C (55°F). They dominate a compost heap when the temperature is cool (at the initiation of the pile), but soon the by-products of their 22.14 Composting 661
662 Chapter 22 Organic Farming metabolic activities cause the temperature to increase. A temperature range of between 21.1 and 32.2°C (70 and 90°F) ushers in another group of microbes called mesophiles. Mesophiles are the workhorses of composting. However, at 37.8°C (100°F), they are replaced by thermophiles, heat-loving bacteria that work to raise the temperature of the compost heap to a much higher level, reaching a peak of about 71.1°C (160°F). After bacteria have operated on the organic matter, cellulose, lignin, and other hardto-metabolize substances are left behind in the pile. Fungi and actinomycetes are able to decompose these substances. Their presence in the heap is indicated by the occurrence of whitish strands or cobweblike structures. Larger microorganisms such as earthworms are important in the compost heap. They feed on organic matter and excrete materials rich in nutrients for plant growth. Earthworms abound in soils that have high microbial activity. 22.14.3 COMPOSTABLE MATERIAL The quality of compost depends in part on the materials included in the compost heap. The secret to quality is variety. Avoid including too much of one type of material. Good compostable materials include the following: 1. Grass clippings from mowing a lawn. A mulching mower may be used to spread fine clippings on the soil. When clippings are bagged, they may be used as a good source of plant material for composting, provided a few cautions are observed. Do not use clippings from a lawn that has been recently sprayed with pesticides. A fresh pile of grass clippings has a tendency to form a slimy and soggy product with a foul odor. It is best to spread grass clippings in thin layers or to mix them in with dry leaves. 2. Household garbage that is void of fats and oil. Greasy materials are hard for microbes to metabolize. 3. Leaves. During the fall season, leaves drop from trees. Dry leaves may be gathered for use in composting. Leaves decompose slowly and need some help to accelerate their breakdown. Instead of using full-size leaves, they should be chopped before adding them to the compost pile. Leaves should be added in thin layers. 4. Sawdust. Sawdust from softwoods, pine, and cedar decompose more quickly than those from hardwood (e.g., birch and oak). When including sawdust, it should be sprinkled lightly and in layers like the other materials. 5. Straw or hay. Old (highly weathered), not fresh, hay makes a good compost material. Straw or hay should be chopped before adding it to the pile. 6. Ash. Wood ash from the fireplace contains potash and is a good material to include in a compost heap. 22.14.4 MATERIALS TO AVOID Some materials are undesirable in a compost heap because they either are not biodegradable or produce toxic factors that are harmful to microbes. These materials include: 1. Diseased plants. All pathogens may not be killed by the heat generated, even at the peak temperature of about 71.1°C (160°F). 2. All nonbiodegradable material (e.g., plastics, synthetic cloths, and styrofoam). 3. Pesticides. Pesticides should not be used under any circumstance because they destroy the organisms that are the agents of decomposition. 4. Pet litter. Other materials should be used with caution. For example, the plant remains from corn harvest, including cobs and husks, are hard to decompose. If they must be added, they should first be chopped into small pieces.