COMPOSTING: The Basics
Hadi Bux Leghari
Composting is the transformation of organic material (plant matter, farmyard manure, poultry manure, sugarcane filter cake etc.) through decomposition into a soil-like material called compost.
Microorganisms (bacteria and fungi) help in transforming the material into compost. Composting is a natural form of recycling, which continually occurs in nature. An ancient practice, composting is mentioned in the Bible several times and can be traced to Marcus Cato, a farmer and scientist who lived in Rome 2,000 years ago. Cato viewed compost as the fundamental soil enhancer, essential for maintaining fertile and productive agricultural land. He stated that all food and animal wastes should be composted before being added to the soil. By the 19th century in America, most farmers and agricultural writers knew about composting.
Compost added to gardens improves soil structure, texture, aeration, and water retention. When mixed with compost, clay soils are lightened, and sandy soils retain water better. Mixing compost with soil also contributes to erosion control, soil fertility, proper pH balance, and healthy root development in plants.
The standard means of disposal for most yard and food waste include land filling and incineration. These practices are not as environmentally or economically sound as composting. Yard waste, which is
landfilled, breaks down very slowly due to the lack of oxygen. As it decomposes, it produces methane gas and acidic
leachate, which are both environmental problems
Land filling organic wastes also takes up landfill space needed for other wastes. Incinerating moist organic waste is inefficient and results in poor combustion, which disrupts the energy generation of the facility and increases the pollutants that need to be removed by the pollution-control devices. Composting these wastes is a more effective and usually less expensive means of managing organic wastes. It can be done successfully on either a large or small scale, but the technique and equipment used differ.
Decomposition occurs naturally anywhere plants grow. When a plant dies, its remains are attacked by microorganisms and invertebrates in the soil, and it is decomposed to humus. This is how nutrients are recycled in an ecosystem. This natural decomposition can be encouraged by creating ideal conditions. The microorganisms and invertebrates fundamental to the composting process require oxygen and water to successfully decompose the material. The end products of the process are soil-enriching compost, carbon dioxide, water, and heat.
Composting is a dynamic process, which will occur quickly or slowly, depending on the process used and the skill with which it is executed. A neglected pile of organic waste will inevitably decompose, but slowly. This has been referred to as "passive composting," because little maintenance is performed. Fast or "active" composting can be completed in two to six weeks. This method requires three key activities; 1) "aeration," by turning the compost pile, 2) moisture, and 3) the proper carbon to nitrogen
(C:N) ratio. Attention to these elements will raise the temperature to around 130=-140=, and ensure rapid decomposition.
The success with which the organic substances are composted depends on the organic material and the decomposer organisms involved. Some organic materials are broken down more easily than others. Different decomposers thrive on different materials as well as at different temperature ranges. Some microbes require oxygen, and others do not; those that require oxygen are preferable for composting.
A more diverse microbial community makes for a more efficient composting process. If the environment in the compost pile becomes inhospitable to a particular type of decomposer, it will die, become dormant, or move to a different part of the compost pile. The transforming conditions of the compost pile create a continually evolving ecosystem inside the pile. Factors Affecting The Composting Process
All organic material will eventually decompose. The speed at which it decomposes depends on these factors:
Carbon to nitrogen ratio of the material.
Amount of surface area exposed.
Aeration, or oxygen in the pile.
Temperatures reached in compost pile.
Carbon and nitrogen are the two fundamental elements in composting, and their ratio
(C:N) is significant. The bacteria and fungi in compost digest or "oxidize" carbon as an energy source and ingest nitrogen for protein synthesis. Carbon can be considered the "food" and nitrogen the digestive enzymes.
The bulk of the organic matter should be carbon with just enough nitrogen to aid the decomposition process. The ratio should be roughly 30 parts carbon to 1 part nitrogen (30:1) by weight. Adding 3-4 pounds of nitrogen material for every 100 pounds of carbon should be satisfactory for efficient and rapid composting. The composting process slows if there is not enough nitrogen, and too much nitrogen may cause the generation of ammonia gas which can create unpleasant odors. Leaves are a good source of carbon; fresh grass, manures and blood meal are sources of nitrogen.
Decomposition by microorganisms in the compost pile takes place when the particle surfaces are in contact with air. Increasing the surface area of the material to be composted can be done by chopping, shredding, mowing, or breaking up the material. The increased surface area means that the microorganisms are able to digest more material, multiply more quickly, and generate more heat. It is not necessary to increase the surface area when composting, but doing so speeds up the process. Insects and earthworms also break down materials into smaller particles that bacteria and fungi can digest.
The decomposition occurring in the compost pile takes up all the available oxygen. Aeration is the replacement of oxygen to the center of the compost pile where it is lacking. Efficient decomposition can only occur if sufficient oxygen is present. This is called aerobic decomposition. It can happen naturally by wind, or when air warmed by the compost process rises through the pile and causes fresh air to be drawn in from the surroundings. Composting systems or structures should incorporate adequate ventilation.
Turning the compost pile is an effective means of adding oxygen and brings newly added material into contact with microbes. It can be done with a pitchfork or a shovel, or a special tool called an "aerator," designed specifically for that purpose. If the compost pile is not aerated, it may produce an odor symptomatic of anaerobic decomposition.
Microorganisms can only use organic molecules if they are dissolved in water, so the compost pile should have a moisture content of 40-60 percent. If the moisture content falls below 40 percent the microbial activity will slow down or become dormant. If the moisture content exceeds 60 percent, aeration is hindered, nutrients are leached out, decomposition slows, and the odor from anaerobic decomposition is emitted. The "squeeze test" is a good way to determine the moisture content of the composting materials. Squeezing a handful of material should have the moisture content of a well wrung sponge. A pile that is too wet can be turned or can be corrected by adding dry materials.
Microorganisms generate heat as they decompose organic material. A compost pile with temperatures between 90= and 140=F (32=-60=C) is composting efficiently. Temperatures higher than 140=F (60=C) inhibit the activity of many of the most important and active organisms in the pile. Given the high temperatures required for rapid composting, the process will inevitably slow during the winter months in cold climates. Compost piles often steam in cold weather. Some microorganisms like cool temperatures and will continue the decomposition process, though at a slower pace.
Microorganisms do some "dirty" jobs:
Many kinds of soil bacteria and fungi decompose organic matter into crumbly humus that does all those great things for the soil. The compounds produced by decomposition are also beneficial.
Most of the nitrogen, phosphorus, and sulfur in fresh plant residues are tied up in the unavailable organic form, which plants cannot use. Soil microbes change these tied-up nutrients into available inorganic (mineral) forms that plants can use.
Several kinds of bacteria "fix," or capture, nitrogen from the air and convert it to a form that plants can use. The most important type is a rhizobia bacterium that lives in small nodules on the roots of legumes. Legumes are plants that produce their seeds in pods, such as beans, peas, and peanuts. The rhizobia have a symbiotic, or mutually beneficial, relationship with legumes. The bacteria live off sugars provided by the plant and supply their host with nitrogen.
Mychorrhizae are a kind of mushroom fungi commonly found in most soils and infest the roots of many plants and trees. They cause no harm. They enhance the host's uptake of plant nutrients, especially phosphorus. They also improve water uptake, lessen the toxicity of salinity, and stimulate the growth of other beneficial microbes like rhizobia.
Organic Matter, A Soil's Best Friend:
Organic Matter in the soil includes plant and animal residues at various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by plant roots and soil microorganisms. Most cultivated topsoils contain about 2-4% organic matter by weight. Despite its small proportion, organic matter has a remarkable beneficial effect on soil behavior and crop yields. Organic matter in the soil is frequently in the form of humus, partially decomposed organic matter that has become dark and crumbly and continues decomposing at a slow rate. Humus benefits the soil in many ways:
It can improve overall physical condition (tilth), especially in clayey soils.
It can help reduce soil erosion by wind and water because it acts as a "glue" to bind soil particles together into "crumbs," called aggregates, which improve water intake rates and lessen runoff. Aggregates are resistant to being moved by wind or flowing water.
It stores and supplies nutrients, especially nitrogen, phosphorus, and sulfur. These are slowly released for use by plant roots as organic matter decomposes. It is estimated that for each one percent of organic matter in the topsoil, over 500 pounds per acre of maize can be produced without additional fertilizer.
It increases the water-holding capacity of sandy soils. Its high negative charge helps prevent positively-charged nutrients from leaching. Per equal weight, humus has 30-40 times the negative charge of many clays and can account for the major part of a soil's nutrient-holding ability. In addition, negative charge improves a soil's buffering capacity, or its ability to resist changes in pH. It can reduce the incidence of some soil-borne diseases and stimulate growth of beneficial soil bacteria, fungi, and earthworms.