Hot Box Compost: Modular Composting System that Heats, Waters, and Fertilizes Grow Bed for Off-Season Production

Initial Concept

Imagine a cube measuring 4 ft on each side. On top of the cube is a rich bed in which edible greens are growing. The soil is watered and fertilized from below, plus carbon dioxide seeps up from the soil. The soil is warm, even though this box is in an uninsulated greenhouse. Inside the box is a cubic yard of compost, waiting for use in the spring. This is the central idea of hot box composting.

Although compost-heated greenhouses are nothing new, they require dedicated infrastructure and space in a greenhouse. This project seeks to test a scalable, modular, and mobile composting box that any market gardener could build to grow greens during the shoulder and off seasons, when these products fetch a higher price.

The project will run a series of controlled tests to determine the best practices with this system, including determining the optimum aeration of the compost; ideal carbon-to-nitrogen ratio, moisture content, and free air space in compost inputs; measuring nitrite and nitrate outputs; how to best control moisture, heat, and carbon dioxide output; and best crops to use based on available resources.

2021 Project Summary

The project began in the spring of 2021 with the construction of twelve compost boxes. This was complicated by the spike in commodity prices at that time, especially building materials. By modifying the boxes slightly, we were able to build the boxes with fewer 2-x-4s. We were within an hour of the budgeted 48 hours of construction for the boxes.

We assembled a polytunnel donated to us from a neighbor to house our boxes for the summer test. The boxes took considerably more time to move than expected, although this will not be an issue for any market-gardeners who adopt this strategy as they are likely to have access to a forklift or skidder or tractor with forks.

We filled and cycled the boxes through three 1-month cycles of filling and emptying with compostable materials at various levels of C:N. While we got reasonable composting through the trials, they required turning and adjustment throughout each cycle. Although we recorded and tested for different compounds (CO2, temp, Ammonia, Methane, Nitrites, etc.) we did not get a good picture of predictable information.

We varied feedstock sizes and found that materials of the consistency of grass clippings worked well (smaller was too fine and compacted, restricting air flow; larger was not compact enough and temperatures didn't rise properly).

Overall the summer cycles were not satisfactory in providing us with a good methodology for filling and monitoring the boxes for maximum composting efficiency or nutrient, water, and heat production.

Nevertheless, we moved the polytunnel and six boxes to a sunny location for the winter trials. After a few weeks of loading the boxes with horse manure and bedding from a nearby stable, it was clear that we were having trouble getting the materials to start composting at expected temperatures. We know that larger systems of this type can function in freezing temperatures and that more troubleshooting would be necessary than expected.

At this point, we asked for and received an extension of our project so that we could have two winters to run these tests, using the winter of 2021-22 to find a way to get the boxes up and running before testing various configurations in the winter of 2022–23.

2022–23 Project Expectations

In addition to continuing the trials to identify feedstock and management practices that optimize compost box performance, we plan to share our project with market-farmers through our budgeted field days (one summer, one winter) and online outreach (videos, literature).