Horticulture Principles and Practices

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
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