Page 68 - Introduction to Agriculture by: Aqleem Abbas

Introduction to Agriculture Notes prepared by: Aqleem Abbas

A broad range of organic matter, including manure from plant-eating animals, grass clippings, and dead leaves or garden plants,
provides a veritable feast for microorganisms. For optimal decomposition, the combined starting materials should have an
appropriate carbon to nitrogen ratio, preferably 30 parts carbon to 1 part nitrogen. Leaves, straw, and paper, called brown
materials, have a high carbon to nitrogen ratio, about 300 to 1, while grass clippings, kitchen scraps, and manure, called green
materials, have a low carbon to nitrogen ratio, about 15 to 1. For the best mix, green materials should be added in abundance;
brown materials should be used more sparingly. Materials that should not be used to make compost include manure from meat-
eating animals, because it may contain disease-causing organisms that can harm humans who eat plants grown in the compost.
Meat should be avoided since it may attract rodents. Fatty foods such as cheese also should not be added to the compost pile, as
they are hard for most microorganisms to digest.

The starting materials are heaped into a pile—in a home garden, the pile is typically about a meter high and a meter wide (about
three feet high and three feet wide); on farms, composting is done on a larger scale. The pile may sit loose on the ground or it
may be enclosed using a variety of materials, including wire fencing, wood boards, cinder blocks, or widely stacked bricks.

MANAGING COMPOST

A variety of techniques may be used to increase the rate of compost decomposition. One technique is to cut the starting materials
into 10- to 15-cm (4- to 6-in) pieces to increase the surface area on which the microorganisms act. Increased surface area
accelerates decomposition, much like a large ice chunk melts faster if broken up into small pieces. The microorganisms in the
compost pile also thrive when oxygen and moisture are present. Fluffing the compost pile every week or so with a pitchfork or
other tool introduces oxygen into the pile, and sprinkling water on the pile when it dries out provides the necessary moisture.

In a well-managed compost pile, the microorganisms eat and reproduce rapidly, and heat is released as a byproduct of their
intense biochemical activity. The heat in the pile kills most plant diseases and weed seeds that may have been present on the
starting materials. The increased heat may also kill the microorganisms doing the decomposing as well, especially those at the
center of the pile where temperatures may climb to 90° C (200° F). Mixing the materials well about once a week prevents lethal
temperature increases by distributing the heat evenly throughout the pile.

The time it takes microorganisms to decompose the starting materials in compost varies. Factors include the size of the pile, the
techniques used to manage the pile, and the nature of the starting materials—green materials decompose readily, while brown
materials take longer to break down. In an actively managed compost pile, microorganisms use up their food supply and become
less active after about six weeks. Then the pile slowly cools, signaling the near-final stages of decomposition. If the materials in a
compost pile are relatively large, if the pile is not kept moist, and if oxygen is not introduced, microorganism activity is slow and
the pile does not heat up. Depending upon the climate, it may take months or years for decomposition to occur.

No matter how long decomposition takes, when in its final stage, the compost pile is about half its original size and resembles
dark soil. The material in the pile is now called humus—although the terms humus and compost sometimes are used
interchangeably. Humus is the highly beneficial material that is added to the garden soil. Once in or on the soil, it continues to
decompose at a very slow rate, releasing ammonia, carbon dioxide, and salts of calcium, phosphorus, and other elements that are
beneficial for plant growth.