Bulking Agents

Understanding how bulking agent functions in the compost process allows us to find the optimal material for any composting operation. This article explores: What is bulking agent? Why is it important to the compost process? What bulking agent should you use? And what makes a good compost recipe?

Bulking agent is a carbon-rich material that adds structure, or bulk, to your compost pile. Classic examples include: wood chips, wood shavings, sawdust, dry leaves, shredded landscape waste, shredded paper, shredded cardboard and animal bedding.

Composting is one of the most basic natural processes. What comes from the earth is returned to the earth by decomposition. But composting is a very dynamic process with many variables involved. For commercial or large-scale composting, facilitating a hot composting process takes planning and forethought. What works for an equestrian center in Montana may not work for an equestrian center in Florida, and what works for composting a school’s dining hall food waste may not work for composting a restaurant’s food waste. What on paper appears to be the perfect compost recipe might not turn out so well in practice.

We often hear about customers who compost food waste that end up with compost that is too wet, despite their best efforts. On the other hand, we have customers composting agricultural byproduct who end up with compost that is too dry. Many people have difficulty getting their compost to heat up enough and some folks have issues keeping high pH levels. The way to address all of these issues is to use a proper recipe of feedstocks.

Our customers often ask us how to optimize their compost recipes to create the highest quality, fastest compost possible. One of our first questions in response is to determine what types of bulking agent they have available to use. Because bulking material is key to effective composting, we need to understand some of the key factors in the composting process: the balance between oxygen and moisture, and the ratio between carbon and nitrogen.

A Little Compost Science

Aerobic composting happens when microorganisms proliferate in your compost pile, producing enzymes that breakdown the chemicals making up the feedstocks: “The enzymes catalyze reactions in which sugars, starches, proteins, and other organic compounds are oxidized, ultimately producing carbon dioxide, water, energy, and compounds resistant to further decomposition” (Trautman).

To build a healthy, diverse community of microbes, you need to provide the microbes with air, water, food, and a place to live. The two most important sources of food for the composting microbes are carbon and nitrogen compounds. A good metaphor for the role carbon and nitrogen play in the composting process was published by Sam Angima: “the microorganisms in compost use carbon for energy and nitrogen for protein synthesis, just as we use carbohydrates for energy and protein to build and repair our bodies” (OSU). Carbon comprises about 50 percent of the mass of microbial cells. Nitrogen is a crucial component of the proteins, nucleic acids, amino acids, enzymes and coenzymes necessary for cell growth and function (Cornell Composting).

Hot aerobic composting goes through a thermophilic process, meaning that it happens at relatively high temperatures. A three stage process needs to take place. First there is a mesophilic stage in which decomposition is carried out at moderate temperatures between 68 and 113℉. As the microbes consume the nitrogen available in the compost pile, they rapidly reproduce and create heat, giving way to a second, thermophilic stage. This stage usually occurs at temperatures between 120 and 140℉. During this stage, proteins, fats, and cellulose are broken down and pathogens and weed seeds are eliminated. After the microbes deplete much of the available carbon needed for energy and growth, another mesophilic, third stage takes place. This last phase is when the compost matures. Highly resistant compounds such as lignins decompose over time, and the resistant organic mixture called humus is formed. Once your compost has matured during this final third stage and you are happy with its quality, the process is over and you can go ahead and use or sell your compost.

To have a successful hot composting process, your compost has to have a carbon to nitrogen ratio of between 20:1 and 40:1. If your feedstock is too low in nitrogen, the compost will not reach a hot enough temperature to break down your feedstocks and to kill pathogens. If your feedstock is too high in nitrogen, the excess nitrogen will be lost as ammonia gas, causing undesirable odors as well as a loss of nutrients. As carbon gets converted to carbon dioxide by the microbes, the carbon to nitrogen ratio decreases. For example, if you start off with a 30:1 carbon to nitrogen ratio, your healthy finished compost will probably have around a 10:1 ratio by the end of the composting process. Achieving a proper carbon to nitrogen balance can be a tricky thing and is achieved by balancing your nitrogen-rich and carbon-rich feedstocks.

The most common problem people have in food waste composting is not adding enough bulking agent to their pile. Imagine blending up your food scraps in a blender and letting it sit on the counter for a week. As you may have guessed, it is going to smell really bad and have a far from desirable characteristics. The same thing happens when you try to compost food scraps without adding a carbon-rich bulking agent. Food scraps are high in nitrogen and moisture and lack adequate carbon for hot composting by themselves. In addition, food scraps tend to collapse during composting and lose their ability to hold air space in the compost pile. Without oxygen, the composting process becomes anaerobic, rather than aerobic.

For composting drier material such as farm or garden waste, people sometimes have to add nitrogen-rich material to their ‘greens and browns’ to speed up the compost process and create more nutrient-dense final compost.

So What Makes a Good Bulking Agent?

The first thing to consider when choosing a bulking agent is particle size. You often want a particle size  of 1” or less in length. The rate at which microbes process carbon sources is dependent on the surface area of the particles. The greater the particle size, the harder it is for the added material to decompose and the less carbon there is available for the microbes to consume. The other factor determining the rate at which microbes process carbon sources is the lignification of the added material. Lignin is very resistant to decomposition and the higher the lignification, the less the added material will be able to break down.  

We believe‒and our customers agree‒that the best bulking agent is a free or low cost bulking agent. Examples of bulking agents we recommend are wood chips, wood shavings, leaves, chopped weeds or green waste, and chopped straw.

Some Research Findings

A recent study compared three different materials used as bulking agents in composting food scraps (Wang et al., 2016). They were leaf litter, sawdust, and wheat straw. Wheat straw functioned as the best bulking agent of the three for food scraps. It lead to a great degree of maturity by the end of the compost process. Wheat straw fostered a diverse microbial community and was effectively broken down by the composting process. Also, the carbon to nitrogen levels significantly decreased from the beginning to the end of the composting process. In short, wheat straw and food scrap compost produced a highly nutritious, well matured compost that would be a great soil amendment. Sawdust was not nearly as good as a bulking agent because its carbon to nitrogen ratio was too high, its particle size was too small, and its lignin content too high. The leaf litter performed well as a bulking agent, but not as well as the wheat straw, probably because it has a relatively low carbon to nitrogen ratio.

An interesting study compared five agro-industrial waste-based composts as growing media for lettuce and looked at the effects on yield, phenolic compounds, and vitamin C (Santos et al., 2016). The data suggest that compost amendments can help add value to lettuce by increasing its antioxidant activity as compared to other organic resources like peat. In the case of lettuce, growing evidence suggests that it is now possible, under optimized conditions, to achieve yields close to average conventional agriculture organically using compost as soil amendment. Santos et al. found that chestnut waste product compost lead to high phenolic levels in the lettuce and higher crop yields. The white grape waste product compost lead to extremely high phenolic levels in the lettuce but low crop yields. Olive, broccoli, and red grapes compost had less dramatic impacts on nutrient levels in the lettuce and in the lettuce crop yields. It was concluded that using white grape and chestnut waste products in tandem as a compost would be the most effective soil amendment for lettuce cultivation.

This is all to say that you can manipulate your compost’s nitrogen-rich materials and bulking agents to create more or less nutritious final compost that suits your particular needs.


Below is a table of carbon-rich material (bulking agents) and another table of nitrogen-rich materials good for composting. These estimates are taken from www.compostjunkie.com

Carbon-Rich Feedstocks

Material C/N Ratio
Garden Wastes 40:1
Dried Leaves 70:1
Peat 70:1
Tissue Paper 70:1
Straw 75:1
Pine Needles 80:1
Paper Towel 110:1
Paper Sludge 150:1
Shredded Newspaper 170:1
Shredded Cardboard 350:1
Woodchips 400:1
Sawdust 400:1
Small Branches/Twigs 500:1


Nitrogen-Rich Feedstocks

Materials C/N Ratio
Weeds 30:1
Freshly cut hay 25:1
Kitchen Scraps 20:1
Coffee Grounds 20:1
Seaweed 20:1
Fresh Grass Clippings 15:1
Alfalfa 12:1
Hair/Fur 10:1
Manure 10-20:1
Fish Emulsions 8:1
Blood Meal 4:1

So What is a Good Composting Recipe?

A typical recipe for hot aerobic composting is two parts bulking agent to one part high-nitrogen feedstocks (2:1) by volume. A predominant factor in determining this recipe is the moisture content of the high-nitrogen feedstocks. If the nitrogen-rich feedstocks are particularly wet, additional dry, bulking agent is necessary (e.g., 3:1). If the nitrogen-rich feedstocks are particularly dry, additional dry, less bulking agent is necessary (e.g, 1:1).

We are always hearing about new ways to solve the bulking agent and compost recipe equation from our customers. We encourage everyone to ask us questions, do some research, and experiment on their own.

Works Cited

Angima, Sam. “Master Composting Program.” Oregon State University Extension Lincoln County, OR.  http://extension.oregonstate.edu/lincoln/sites/default/files/documents/n_to_Soils_-_Composting_and_the_C_to_N_ratio.pdf (accessed July 26, 2017).

Dittmar, Dave. “Compost Ingredients.” Compost Junkie  http://www.compostjunkie.com/compost-ingredients.html (accessed July 26, 2017).

Santos, Francielly T., Piebiep Guofo, Cátia Santos, Donzilia Botelho, João Fonseca, Aurea Queirós, Mônica S.S.M. Costa, and Trindade Henrique. 2016. “Comparison of five agro-industrial waste-based composts as growing media for lettuce: Effect on yield, phenolic compounds and vitamin C.” Food Chemistry 209, 293-301. Agricola, EBSCOhost (accessed July 26, 2017).

Trautmann, Nancy M., Tom Richard, and Marianne E. Krasny. 1996. “Cornell Composting: Compost Chemistry.” Department of Crop and Soil Sciences http://compost.css.cornell.edu/chemistry.html and http://cwmi.css.cornell.edu/chapter1.pdf Cornell Waste Management Institute (accessed July 26, 2017).

Wang, Xiaojuan, Gao Hua, Gu Jie, Qin Qingjun, and Zhang Wenwei. 2016. “Effects of different bulking agents on the maturity, enzymatic activity, and microbial community functional diversity of kitchen waste compost.” Environmental Technology 37, no. 20: 2555-2563. Agricola, EBSCOhost (accessed July 26, 2017).