A compost activator is a high-nitrogen material that can provide the boost of nitrogen your compost pile needs to start composting. An activator could be especially helpful to start decomposition in a pile of brown compost, like shredded leaves, which may be low in nitrogen. Adding a material that has a high percentage of nitrogen will decrease the carbon to nitrogen ratio in the compost pile, producing a nitrogen level preferred by the bacteria most at work during the compost process.
Compost activators are usually dissolved in water and then sprinkled onto the pile as it is “layered,” or built. Organic compost activators include high-nitrogen manures, blood meal (used in some organic fertilizers) and fish meal. Compost activators can also be synthetic, like high-nitrogen fertilizers. Some compost resources, especially those following organic gardening guidelines, caution against using synthetic high-nitrogen fertilizer as an activator. One reason given is that synthetic fertilizers lack beneficial proteins found in organic nitrogen sources. The choice to include synthetic fertilizers in the compost pile mainly reflects your own personal preferences.
Commercially produced compost activators and additives are available from many garden supply sources. In addition to nitrogen, these additives may contain bacteria, fungi and enzymes – all which are commonly found in topsoil and finished compost. Although these may be beneficial to your compost pile, beneficial bacteria, fungi and enzymes are often already found in the topsoil or the finished compost layered into your pile. It’s usually the nitrogen contained in commercial compost additives that really helps the pile start heating up.
Here’s how to determine how much of a compost activator your pile needs. A compost pile which has a large amount of high-carbon material, like shredded leaves, will benefit from adding 0.15 pounds (or 2.4 ounces) of actual nitrogen to every three bushels (four cubic feet) of organic matter. Divide 2.4 by the percent nitrogen in your material to determine the number of ounces needed to add. For example, you would need to add 24 ounces of fish meal with 10% nitrogen content (2.4/0.1) to add 2.4 ounces of actual nitrogen.
“Composting to Reduce the Waste Stream,” Northeast Regional Agricultural Engineering Service, 1991.
“Materials for Composting.” University of Illinois Extension. http://web.extension.illinois.edu/homecompost/materials.cfm