What's Hot, What's Not?

Every fall, homeowners have the same problem: dead leaves. Trees shed leaves and taint once perfected lawns. To get rid of these leaves, people rake the leaves and assemble them into large piles that trucks come and remove. If these piles are left long enough, they will heat up. On a cold day, steam can be seen rising from these piles of leaves. But why does it do this?

Decomposition is the breaking down of a substance into parts that it was made of. Microorganisms break down organic matter while composting and making carbon dioxide, water, heat, and humus. Composting usually has three phases: 1) the mesophilic (moderate temperature) stage which lasts two days, 2) the thermophilic (high temperature) stage which lasts for four to six days, and 3) the cooling down, maturing phase, which can last up to several months (Columbia University Press, 2000.)

Mesophilic microorganisms carry out stage one. The mesophilic microorganisms break down the stable, readily degradable compounds. The heat they create makes the compost temperature rise quickly. Stage two (the high temperatures) accelerates the breakdown of proteins, fats, and complex carbohydrates, at temperatures of 55 degrees and higher. Many microorganisms that are human or plant pathogens are destroyed. Little holes are necessary to aerate this stage. Stage three has the supply of high-energy compounds that become exhausted. The compost temperature gradually decreases and mesophilic microorganisms take over for, the final phase of "curing" (maturation of remaining organic matter (Southwestern, 2003).)

There are three main gases in compost piles: carbon, nitrogen and oxygen.

Carbon provides the energy source and is the "building block" that is 50% of mass microbial cells. Brown and woody materials are very high in carbon. Carbon is more readily available for microbial use if an object has a large surface area. Nitrogen is an important component of the proteins, nucleic acid, amino acids, enzymes and co-entyns necessary for cell growth and function. Any materials that are green and moist are high in nitrogen. The nitrogen cycle is the continuous movement of nitrogen from the atmosphere, to plants, and back to the atmosphere (or directly into plants) again. Oxygen is essential for a compost pile to work. As microorganisms oxidize carbon for energy, oxygen is used up and carbon dioxide is produced. Without enough oxygen, the process will become anaerobic and produce bad smells. Oxygen concentrations greater than 10% are considered best for maintaining aerobic composting. Some compost systems can be kept up enough oxygen passively (through natural diffusion and convention) and some systems require active aeration (Michigan Sate University, 2000.)

The temperature at any time depends on how much heat is being produced by microorganisms. The temperature is balanced by how much heat is lost through conduction, convection, and radiation.

Conduction is a medium for heat and electricity and is why the edges of a compost pile aren't as hot as the middle. The heat spreads from the middle and fades out by the edges. Convection is the transfer of heat through the air. When the compost gets hot, the heat rises very slowly. Radiation is electromagnetic waves that carry out the heat of the pile. The warmth made in a compost pile radiates out into the cooler surrounding air. The smaller the pile, the greater the surface area-to-volume ratio and therefore the larger the degree of heat loss to conduction and radiation. Insulation helps to reduce these losses (Raloff, 1993.)

There are five main microorganisms that live in a compost pile. They are bacteria, actinomycetes, fungi, protozoa, and rotifers. Bacteria are the smallest living organisms and the most plentiful in compost (they take up 80-90% of the billions of microorganisms