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

Progress report for FNC21-1280

Project Type: Farmer/Rancher
Funds awarded in 2021: $13,747.00
Projected End Date: 12/31/2023
Grant Recipient: Low Technology Institute
Region: North Central
State: Wisconsin
Project Coordinator:
Scott Johnson
Low Technology Institute
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Project Information

Description of operation:

We operate an experimental growing operation on 1.5 ac in southern Wisconsin (5a) in coordination with the Low Technology Institute ( We grow typical garden vegetables as well as small-scale cereals (wheat, rye, corn, barley) and others (flax, peas). We primarily research market-garden production solutions that involve no or minimal fossil-fuel inputs.


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).

Project Objectives:
  1. Build twelve hot box composting units and evaluate the optimum configuration of variables of over the spring and summer of 2021 and 2022.

  2. Test six units each at twoone hoop-house location over the winter of 2021–22 and two locations over the winter of 2022–23 and evaluate performance through qualitative and quantitative recording.

  3. Disseminate findings through field days, presentations to industry groups, write-ups in popular and technical publications, and digital podcasts and videos.

[Jan. 2022 updates underlined and struck through]


Click linked name(s) to expand/collapse or show everyone's info
  • Dr. Scott Johnson, Scott Johnson - Producer
  • Terry Parisi - Producer


Materials and methods:

This project will make use of twelve “hot boxes,” each consisting of a compost chamber below and a grow bed on top. The cubes measure ca. 4 ft on a side are built on standard pallets and sized to contain 1 cu. yd. of composting space, six made of a stud-frame with a plywood exterior and a corrugated plastic interior (see figure) and six made of solid wooden sides, both mounted on wooden pallets. The top and one side will open to fill and empty the chamber. The chamber uses aeration to feed oxygen to the compost, meaning the pile doesn’t need to be turned to decompose. The air intake is a fan-fed, perforated, 4-in PVC pipe embedded in woodchips in the box bottom. The exhaust gasses exit the compost through ninesix perforated, 1-in PVC pipes pushed into the top of the compost. Above the compost chamber, the exhaust flows through a screen supporting a bed of woodchips, which form the lowest layer of the grow bed. The boxes contain temperature and humidity sensors in the compost and beds. Extra grow boxes will be used as control comparisons.

A series of tests will be carried out between the competing designs (stud-wall vs. solid wall) over the summers of 2021 and '22 and the best configuration will be tested in the winters of 2022 and '23. A test consists of filling the chamber and letting it run through a composting cycle (ca. 1 month). Each test will look at a single variable, holding everything else as constant as possible. Test include optimum carbon-to-nitrogen ratio (20:1, 30:1, and 40:1), feedstock material and preparation, air flow (from unforced to low and high amounts of forced air), moisture content, free air space, and recycled compost. Throughout we will record the temperature, humidity, and gas production in the chamber and the soil moisture and temperature. We will also record labor time, yield, and growth (vs. control beds).


[Jan 2022 updates underlined]

Research results and discussion:

Results have been recorded but not analyzed for the summer 2021 cycles, including temperature, carbon dioxide, and other gasses.

Current results are not indicative as the learning curve to achieve optimum composting cycles hasn't been overcome in time to have consistent results.

Participation Summary
1 Farmers participating in research

Educational & Outreach Activities

1 Published press articles, newsletters
1 Other educational activities: Sharing our progress across our social media platforms.

Participation Summary:

Education/outreach description:

The bulk of our outreach will come in 2022 and 2023.

We are planning a field day in the summer and again in the winter to show interested growers the set-up during different seasons.

We have been filming the entire process to create an instructional video when the field trials are finished.

We will be writing a technical document to outline the practices we found to work best (and those to avoid) as well as other shorter outputs (brochure/info page, presentations, etc.)

Our local paper, the Edgerton Trading Outpost, carried a short story and photo of the project being set up, but they have  no online presence to share.

Learning Outcomes

1 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Lessons Learned:

Although we are only at an interim point in the study, a few lessons are already apparent:

Small aerated static piles are harder to start and maintain, as the small amount of material must be more closely monitored to maintain optimum conditions.

Compost feedstock size is an important factor in effective system function, as small materials compact and prevent airflow, while large materials allow too much. Grass-clipping size for plant material appears to be about right.

The boxes are heavy to move and also to open with the grow bed supported on top of the box. A modification is needed to make them more user friendly for those without significant strength to open them.

We abandoned the high-flow settings, which we had planned to test throughout, as the high through-put of air cooled the piles too much and composting activity ceased prematurely.

Project Outcomes

Success stories:

Too early to report on this.


Too early to report on this beyond the initial lessons learned (mentioned above).

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.