From Veggie Scraps to Humus: An Ecosystem in your Compost Bin

Living in the northern hemisphere where spring showers bring May flowers and being the first post in said month, I feel it is my duty to discuss an important topic: compost. Compost is the seemingly simple product of a complex mixture of materials. Take a smorgasbord of leaves, grass, kitchen waste, coffee grounds and other organic material, mix them into a pile or in a bin, and a dark, rich soil amendment is the end result. Thus, what many people label as waste becomes something quite useful if you want to grow vegetables, flowering plants or even a healthy lawn. (You do compost, don’t you?) Certainly my bleeding hearts appreciate a top dressing of compost in the spring to keep them vigorous, so much so that I had to split them only two years after planting them. But what does the work of converting a mess of green stuff and brown stuff to a crumbling, desirable end product?

As it turns out, there are many forces that break down the starting material into humus, the stable nutrient-rich end product of composting. The process is initiated by combining organic materials, ideally in a 1:1 ratio of nitrogen-rich materials (greens like kitchen waste, grass clippings and coffee grounds) and carbon-rich materials (browns like leaves, straw or paper products) in alternating layers. Once these materials are in place, the composting process needs moisture and aeration to break down. (Anaerobic conditions are an odiferous situation no one appreciates.) If the compost pile is actively managed (e.g., turned over to add oxygen and watered to ensure the pile stays damp) and of sufficient mass (not much happens with six inches of organic material; trust me, I know), the organic materials heat up quickly in the presence of microbial activity and stay hot when more green and brown material is added. As many say “composting happens” without active intervention (or infrequent intervention like in my compost bin). However, the process occurs more slowly, and the compost pile or bin does not get as hot.

Forces at work in the compost pile include microorganisms, fungi, insects, worms and other creatures that reduce the volume of waste materials to the precious humus. The primary consumers and heavy lifters are bacteria, including many from genus Bacillus (1). These bacteria are found in the soil and migrate to the all-you-can-eat meal offered by the compost pile. The work is started with psychrophiles, aerobic bacteria that thrive in lower temperatures (~55 degrees Fahrenheit) and are still active even at 0 degrees Fahrenheit. As biological activity and ambient temperature increase, mesophiles become active in the 70–90 degrees Fahrenheit range. This is where the majority of the organic material break down occurs (e.g., carbohydrates and proteins are converted to their components) and is compatible with other organisms at work including worms and insects. However, heat is released in this process and can increase the temperature further when thermophiles take over. These microbes take over the process at ~100 degrees Fahrenheit and continue until the temperature stabilizes at 160 degrees Fahrenheit. Thermophiles burn through organic material at these high temperatures for 3–5 days before their work is completed, the temperature moderates and the mesophiles take over. If the compost pile is turned, the thermophiles continue working until their energy supply is depleted (; Accessed May 3, 2010).

When the pile is dominated by mesophiles, other organisms like fungi, actinomycetes, earthworms, sow bugs, millipedes and other insects also feed on the organic residue. Fungi digest the carbon materials, while earthworms help to aerate and mix the pile and add nutrients to the mix in the form of castings. Actinomycetes give compost its earthy smell and appear as cobweb-looking gray fluff. Many insects also feed directly on the organic material and contribute by adding minerals (e.g., ants) or adding themselves to the compost mix.

However, there are also secondary and tertiary consumers busy at the compost pile. Secondary consumers like nemotodes and protozoans feed on bacteria and fungal spores present while tertiary consumers like centipedes and beetles feed on the insect larvae, newly hatched earthworms and other insects (; Accessed May 3, 2010). That compost pile really is a complex ecosystem of its own. And I was really impressed with how much the organic material reduced when I added water and stirred the bin.

My three-year-old bleeding hearts fed by compost.

What does the gorgeous humus that results from all this biological activity do? It adds tilth to the soil, supplies nutrients for vigorous plant growth and can help prevent disease and pest attacks. So start a compost pile and enjoy watching biology at work in your own backyard.

Do you compost?

1. Ryckeboer J, Mergaert J, Coosemans J, Deprins K, & Swings J (2003). Microbiological aspects of biowaste during composting in a monitored compost bin. Journal of applied microbiology, 94 (1), 127-37 PMID: 12492933

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Sara Klink

Scientific Communication Specialist at Promega Corporation
Sara is a native Wisconsinite who grew up on a fifth-generation dairy farm and decided she wanted to be a scientist at age 12. She was educated at the University of Wisconsin—Parkside, where she earned a B.S. in Biology and a Master’s degree in Molecular Biology before earning her second Master’s degree in Oncology at the University of Wisconsin—Madison. She has worked for Promega Corporation for more than 10 years, first as a Technical Services Scientist, currently as a Scientific Communication Specialist. Sara is camera shy but may succumb to peer pressure and post an image.

One thought on “From Veggie Scraps to Humus: An Ecosystem in your Compost Bin

  1. Dear Sara Klink,

    Thanks for this blog. Just add fine sized bone powder or rock phosphate mineral to the compost and see the magic.

    Hope you will try.


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