A compost starter, also called an activator, is a substance added to a new or slow-moving organic pile to accelerate the natural decomposition process. Starters introduce or stimulate necessary microbial populations and balance the material’s chemical composition. They provide the organisms responsible for breakdown with the energy and building blocks required for rapid reproduction. The goal is to move the pile quickly into the high-temperature phase, drastically reducing the time required to create finished soil amendment.
The Biological Needs of a Fast Compost Pile
Microorganisms, primarily bacteria and fungi, drive the fast breakdown of organic matter, and they require four elements to thrive: carbon, nitrogen, oxygen, and moisture. Carbon serves as the energy source for these microbes, while nitrogen is necessary for protein creation and reproduction. For decomposition to proceed rapidly, the carbon-to-nitrogen (C:N) ratio needs to be around 25-to-1 to 30-to-1.
If the C:N ratio is too high, the process will slow down significantly because there is not enough nitrogen to fuel microbial growth. Conversely, if the ratio is too low, excess nitrogen is lost to the atmosphere as ammonia gas, often resulting in an unpleasant odor. A starter is chosen to address an imbalance, usually by supplying concentrated nitrogen or beneficial microbes. Moisture is also important, as the pile needs to be consistently damp, similar to a wrung-out sponge, to support microbial metabolism.
DIY Starters: High-Nitrogen Accelerants
The simplest way to accelerate a carbon-rich pile, such as one made primarily of dry leaves or wood chips, is to introduce high-nitrogen materials. These “greens” immediately lower the C:N ratio, providing the necessary fuel for the existing microbial community.
High-Nitrogen Accelerants
- Fresh animal manure (chicken, cow, or horse) is effective, with C:N ratios as low as 5-to-1. A thin layer (one to two inches thick) provides an immediate nitrogen boost.
- Human urine is a highly available nitrogen source, which can be diluted 1-to-10 with water for fresh application.
- Coffee grounds are excellent, offering a reliable C:N ratio of approximately 20-to-1, making them a balanced accelerator.
- Fresh grass clippings and leaves from plants like comfrey are potent nitrogen sources that break down quickly to stimulate heat generation.
DIY Starters: Introducing Microbial Activity
To jumpstart a pile lacking decomposers, focus on directly importing active organisms. The most straightforward method is using finished compost or simple garden soil, as both contain the diverse bacteria and fungi needed for decomposition. A thin, one-inch layer of finished compost or soil can be sprinkled over new layers of organic material.
Another effective method is brewing a sugar and yeast solution to create a powerful microbial inoculum. This starter is made by dissolving one tablespoon of active dry yeast and one tablespoon of sugar or molasses in four cups of lukewarm water. The sugar acts as immediate food for the yeast, causing rapid reproduction and foaming, which indicates it is ready to apply. The yeast releases enzymes and byproducts that initially break down tough organic matter, creating an environment favorable for the native thermophilic bacteria.
Integrating the Starter into the New Compost Pile
Proper application ensures maximum contact with organic materials, leading to the fastest possible breakdown. The pile should begin with a coarse base layer of twigs or wood chips (six to eight inches deep) to promote air circulation. Once the base is established, the compost materials and the starter should be applied in thin, alternating layers, often called lasagna layering.
A typical layering sequence involves a layer of carbon-rich material (“browns”), followed by a thin layer of nitrogen-rich material (“greens”) or a high-nitrogen starter, and then a light dusting of soil or finished compost. Liquid starters, such as the yeast solution or diluted urine, should be poured evenly over the layers to ensure the solution penetrates the material. Immediate watering until the entire pile is uniformly damp activates the microbes and jumpstarts the process. Within 24 to 48 hours, successful starter integration results in a noticeable increase in the internal temperature of the pile.