The Use of High Carbon Materials as Bulking Agents

Quality compost usually consists of three components  – the primary substrate, amendments, and bulking agents. The primary substrate is the waste of interest to be decomposed (e.g. food waste or animal mortalities). Amendments are mixed with the primary substrate to balance the compost’s carbon-to-nitrogen ratio (C:N between 25:1 to 30:1) and regulate moisture. Bulking agents provide the compost structure to improve aerobic decomposition conditions. Bulking agents often contain a relatively high carbon content and have decay-resistant properties (e.g. elevated lignin content). The purpose of this article is to discuss why bulking agents are beneficial for hot composting and suggest ways to include bulking agents while composting.

Bulking agents and their benefits

Commonly used bulking agents include straw, wood chips and shavings, leaf litter, and cardboard. These types of organic materials add structure to the composting mass and create air pockets. A compost with many air pockets (enhanced porosity) permits ventilation throughout the compost mass, allowing oxygen to come in and carbon dioxide and water vapour to move out. Turning and mixing does add air to a hot compost. However, added ventilation from bulking agents improves the effectiveness of compost turning, and supports airflow when the compost mass is stationary. In addition, increasing the oxygen levels of a compost supports aerobic microbial activity and growth, which raises the compost temperature. Enhancing both microbial activity and temperatures speeds up the composting process. 

Without a bulking agent, a compost can become compact. Compaction hinders airflow, creating an anaerobic environment that holds onto moisture. Composting in an anaerobic environment results in a compost that is slow to decay, smelly, and wet. Moreover, composting within an anaerobic environment causes nitrogen losses through ammonia volatilization and nitrous oxide emissions.

Selecting bulking agents

Selecting bulking agents

Most often, composting bulking agents are selected for being economical, readily available, and locally accessible. It is very important that bulking agents selected are dry to absorb the excess moisture (and not add moisture) within a compost. It is also recommended that bulking agents have a particle size between 5 to 25 mm (0.2 – 1 inches) to achieve ventilation and adequate surface area for microorganisms to access carbon. Classic bulking agents include wood chips, wood shavings, and chopped materials such as straw, leaf litter, cardboard and crop residues (e.g. pruning waste, stalks, grain husks, and corn cobs). Other materials have also been tried as bulking agents, such as dried cattle manure, pinecones, nut shells, and spent mushroom substrate.  If possible it is best to add a variety of organic materials that can be considered an amendment and/or bulking agent.  Each type of organic materials has a differing moisture, C:N, and lignan content, along with other characteristics that can influence the compost environment and impact microbial activity.  

Sawdust is often considered a bulking agent as well as an amendment, though sawdust is not as nearly as good as a bulking agent as other organic materials. The combination of sawdust characteristics: tiny particle size, very high carbon content, and high decay resistance –  can result in a woody compost. Similarly, shredded paper is suitable for water absorption and as a carbon amendment. Yet, shredded paper is not the best option as a bulking agent –  as it does not have the rigidity to create air pockets.

Wood chips are a suitable bulking agent, but they are very slow to decay and become unrecognizable in a compost system – due to their high carbon (e.g. 400:1) and lignin content. Wood chips that are not decomposed can be sorted from the finished compost and added to a new compost batch. Other options are to acquire semi-decomposed wood chips or use chips derived from ramial wood (e.g. Alder or willow brush). The lower C:N ratio (e.g. ~134:1) makes ramial wood chips more readily compostable than other wood chip types. Straw has shown to be an excellent bulking agent as it has a slightly high C:N ratio (50-80:1) – permitting decay at a moderate rate; also, straw has a high capacity for water absorption. Leaf litter and shredded cardboard are also sufficient as bulking agents.

Procuring bulking agents

Collecting enough high-carbon amendments and bulking agents can be challenging for some composter operators depending on the site location and access to organic materials. A couple of managing strategies for sourcing carbon are establishing partnerships with businesses such as farms and landscape companies or building relationships within communities. (e.g. organized leaf litter and plain cardboard pick up). Furthermore, investing in ways to prepare and store carbon materials are worth considering. For example, procuring machinery that shreds, chips and/or chops organic material would ensure ideal particle size for bulking agents. As well as securing a dry storage space for carbon ensures that amendments and bulking agent components are readily available to be added to compost with the primary substrate.