Composting is a biological process where microorganisms decompose organic materials into a dark, stable soil amendment. This process requires a precise balance of four components: carbon-rich materials (browns), nitrogen-rich materials (greens), adequate moisture, and oxygen. A common question is whether native soil should be included to facilitate the breakdown of materials. While soil is the eventual goal, its role in the active decomposition phase is often misunderstood.
Soil’s Contribution to Active Composting
Soil is not a required ingredient for successful composting. Organic materials already harbor the bacteria and fungi needed for decomposition, and these native microorganisms rapidly colonize the pile once moisture and temperature conditions are met.
Adding a small amount of finished compost or native soil serves as a microbial inoculant, effectively kick-starting the process. This introduces a diverse population of decomposers, speeding up the initial colonization phase, which is helpful when starting a new pile of non-colonized materials like shredded paper.
The benefit is primarily one of speed. However, microorganisms will multiply exponentially even without this initial boost once the pile reaches the appropriate conditions, meaning a small addition is beneficial but not necessary for an active composting system.
Issues Arising from Too Much Soil
Introducing a large volume of soil into an active compost pile interferes with the aerobic process. Soil contains dense mineral particles like sand, silt, and clay, which reduces the crucial pore space necessary for air circulation.
Excessive soil causes compaction, restricting oxygen flow and quickly leading to anaerobic conditions. When oxygen is depleted, decomposition shifts to anaerobic bacteria, resulting in a slower process and the production of foul-smelling compounds like methane and sulfurous gases. The dense material also holds water excessively, creating waterlogged sections that suffocate aerobic microbes.
Too much soil also inhibits the thermophilic stage of composting. Mineral soil acts as a thermal sink, reducing the insulating properties of the organic matter. This prevents the pile from reaching the high temperatures (131 to 160 degrees Fahrenheit) needed to kill weed seeds and pathogens efficiently.
Using Soil for Curing and Storage
Soil finds its purpose when integrated with compost after the active decomposition phase is complete. This stage is known as curing or maturation, where the nearly finished compost stabilizes its nutrient profile. Mixing the finished compost with a small amount of native soil during this period helps stabilize remaining organic acids and nutrients, preparing the material for direct garden use.
Specialized Composting Methods
Soil is also integral to specialized composting methods, distinct from a traditional hot pile. In vermicomposting (worm composting), a layer of soil or mineral grit is often added to the bedding. The mineral particles act as gizzard stones for the earthworms, helping them grind down food scraps.
In trench composting, which involves burying food scraps directly in the garden, the soil acts as the primary medium for decomposition and stabilization. This process relies on the immediate incorporation of organic matter into the existing soil structure.