A compost starter is a commercially packaged product or natural material used to accelerate the biological process of decomposition in a compost pile. Composting relies on the intense activity of microorganisms to break down organic matter into nutrient-rich humus. By introducing a high concentration of these microbes and necessary chemical elements, a starter helps initiate the rapid decay phase. These components are designed to overcome common limitations, ensuring a quicker path to finished compost.
Biological Components
The living organisms within commercial compost starters are primarily bacteria and fungi, which are the engine of decomposition. Bacteria are the most numerous inhabitants, making up the largest percentage of the microbial biomass present. They are responsible for the majority of material breakdown and the generation of heat during the initial stages of composting.
A special group of filamentous bacteria called actinomycetes is also included in many starters. These organisms resemble fungi but function as bacteria, playing a significant role in degrading complex organic structures such as cellulose, lignin, and chitin. Fungi, including molds and yeasts, are also present and can attack organic residues that are too dry, acidic, or low in nitrogen for bacterial colonies to thrive.
These microbes are selected for their ability to thrive in the various thermal phases of composting. Thermophilic organisms, which prefer high heat, are particularly useful for generating the high temperatures needed to rapidly break down material and eliminate pathogens. Introducing a concentrated population of these beneficial organisms bypasses the natural, slower process of waiting for them to colonize a new pile.
Nutrient and Mineral Accelerants
Beyond the living microbes, a compost starter contains non-living chemical components that fuel and regulate biological activity. Nitrogen is a primary accelerant, often supplied in concentrated forms like urea or ammonium sulfate. This readily available nitrogen acts as a direct food source for the bacteria and fungi, enabling them to multiply rapidly and produce the necessary enzymes for decomposition.
The presence of nitrogen is required to maintain the ideal carbon-to-nitrogen ratio for microbial growth, typically around 30:1. If the pile is too rich in carbon materials, the added nitrogen corrects the imbalance and prevents the process from stalling. Some commercial starters also include mineral additives like gypsum (calcium sulfate) or lime (calcium carbonate); gypsum can help reduce the loss of nitrogen to the atmosphere.
These mineral components also help optimize the chemical environment of the pile. Lime is a common pH buffer that can neutralize overly acidic conditions, which inhibit the activity of many beneficial bacteria. Certain starters also include trace minerals, such as iron and manganese ions, which act as cofactors to accelerate the enzymatic reactions of the microbes.
Natural and DIY Compost Starters
The function of a commercial starter can be replicated using readily available natural materials that contain high concentrations of microbes or nutrients. Finished compost itself is an excellent starter, as a handful contains billions of the appropriate bacteria, fungi, and actinomycetes. Using a few shovelfuls of finished compost to inoculate a new pile instantly introduces a diverse and active microbial community.
Aged manure, especially from sources like cattle or poultry, serves as a powerful starter due to its high nitrogen content and existing microbial populations. Manure is a rich source of concentrated nitrogen that feeds the new pile’s organisms, mirroring the function of synthetic nitrogen additives. Certain plant materials, such as comfrey and stinging nettles, are also used as activators because they naturally accumulate high levels of nitrogen and other minerals.
Simple household items can also function as accelerators by providing easily digestible sugars or nitrogen. A small amount of beer or molasses, for example, provides a quick source of carbohydrates that can trigger an initial burst of microbial activity and heat. These natural alternatives essentially supply the same biological and chemical components found in commercial products.
Application and Necessity
The use of a compost starter is most beneficial when a pile is built with materials that are difficult to decompose or lack sufficient nitrogen. A starter is particularly useful when creating a large pile consisting mainly of high-carbon materials, such as fall leaves or wood chips. They are also helpful in cold weather, where sluggish natural microbial activity can be jumpstarted by the concentrated inoculum to generate heat.
To apply a starter, the product, whether commercial or natural, is typically mixed into the pile’s layers as it is built or sprinkled over the top and watered in. Proper application ensures the microbes and nutrients are distributed throughout the material, maximizing their contact with the organic matter. Starters are applied at the beginning of the composting process to ensure the rapid colonization of the new materials.
While starters can significantly accelerate decomposition, they are not always strictly necessary for a successful pile. A well-constructed pile with the correct balance of carbon and nitrogen, adequate moisture, and sufficient aeration will eventually decompose on its own. However, for those seeking to shorten the time to finished compost, especially in challenging conditions like a very dry or nitrogen-poor mixture, a starter can be a valuable tool to ensure a faster and more reliable outcome.