Compost Primer
Operational Path
Part 2: sweet spots
The overall objective of building and maintaining active (thermal) compost piles is to generate a bioamendment that supports thriving holobionts. If you manage environmental conditions in your system, the chemistry, physical habitat, and dynamic ecology will tend to favor the growth of organisms that partner effectively with plants (allies).
If our (high-quality) compost has an abundance (biomass) and a diversity (species richness) of organisms, our local biological community will be better able to compete with potentially problematic organisms (such as pathogens). But how do we select for the organisms we want in our compost?
For most environmental variables (nutrient concentrations, pH, moisture, oxygen, etc), there is a range (or gradient) that most organisms will tolerate or prefer.
Nitrogen Example: Many organisms become increasingly growth-limited when nitrogen concentrations are extremely low (say, 5 parts per million or less). Substantial nitrogen concentrations (say 200 ppm or more) can be toxic or disruptive to many organisms.
pH Example: Many plants and microbes prefer a pH between 5.5 and 7.0 (slightly acidic). Some microbes do best if the pH is below 5.5, and others do best if the pH is above 7.0. Generally, a slightly acidic pH will support the widest population of different organisms at the same time.
Calcium (Example Mineral): Many filamentous fungi require plenty of calcium. While soils often contain sufficient Ca, it’s possible that materials in a compost pile will not. This might limit fungal growth. If crushed eggshells or other sources of Ca are added, fungi may be able to expand their population.
Oxygen/Moisture Example: Some environmental factors push and pull on each other. For example, if a pile becomes saturated with water, the spaces between materials can fill up and block diffusion of air/oxygen into the pile’s center. This can shut down the activities of oxygen-loving microbes. If moisture is below 50% of a pile’s weight (use a squeeze test), organisms can be limited by the lack of available water even if there is plenty of oxygen.
In the End…
So, if we maintain optimal ranges for key variables in our pile, we can set the stage for a robust and rich biological community to expand and flourish during active composting and maturation. What key environmental ranges must we maintain in a compost system?