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

View the project final report

Commodities

Practices

• Crop Production: season extension
• Soil Management: composting

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.

A long time ago (the 1980s) in a land far, far away (Cape Cod), the New Alchemy Institute (NAI) researched ways to live with less reliance on fossil fuels and outside inputs. They pioneered bioshelters and aquaponics before these were well known in alternative sustainability circles. They also created a composting greenhouse, which tested the premise that all of the products of compost could be used to grow plants through the winter.

Greenhouses (more technically “hothouses”) have been heated by compost for hundreds of years, as mentioned in a 1693 English translation of a French gardening manual, for example (de la Quintine 1693). At the time, they would have recognized that the moisture from the compost was also beneficial for the plants, but they would not have known that the carbon dioxide off-gassing from the pile was also helping their growth until at least the 1800s. Eventually the advent of cheap and ready fossil fuels supplanted the more labor-intensive compost heat.

At the New Alchemy Institute, they recognized that they could create a system to both compost large volumes of manure and capture all the beneficial products of that process to grow plants through the winter. The heart of their system was what is called today an aerated static pile (ASP). This type of composting set-up consists of a sealed bin with a perforated pipe below it (technically known as an “in-vessel bioreactor”). A blower pushes fresh air into the pile from the pipe (i.e., aerates it), feeding the composting microorganisms with fresh oxygen without needing to turn the compost (i.e., a static pile). Meso- and thermophilic (heat-loving) bacteria use that oxygen to break down organic compounds to create heat, carbon dioxide, water vapor, and nitrogen (although it is a more complex biological process, it is essentially: various CHO and N compounds + O₂ → nitrogen compounds (NH₃, NH₄⁺, NO₂⁻, NO₃⁻) + CO₂ + H₂O + heat). With the fresh air coming in, the off-gasses must go out, and they do so through vents into woodchip beds. The shredded wood creates a biofilter by both providing carbon and an environment for bacteria that convert harmful nitrites and ammonia into plant-absorbable nitrates.

The greenhouse at NAI was custom built for this system. The 12-×-48-ft hoop house was oriented from east to west and had a 25-cubic-yard composting chamber along its north side. On top of the wood-framed, sealed chamber was a grow bed, consisting of 18 inches of woodchips under sandy loam supported by hardware cloth. As the compost chamber was filled with manure from nearby stables, blower motors pushed 12,000 cubic feet of air through the system each day (running 15 minutes every 6 hours). Much of the off-gasses went into the bed above the chamber, but another grow bed on the south half of the greenhouse was fed with underground distribution pipes and dedicated blowers. In the winters, they grew leafy greens and started seeds in this greenhouse and filled the space with cucurbits, peppers, and other heat-loving crops in the summer. The entire setup and details for their research project can be found in their third research report on their website: https://newalchemists.net/portfolio/compost-greenhouse/.

Although compost-heated greenhouses are nothing new, they require dedicated infrastructure and space in a greenhouse. This project sought 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 ran 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 methane, ammonia, nitrites, and nitrates outputs; how to best control moisture, heat, and carbon dioxide output; and best crops to use based on available resources.

References Cited:

de la Quintine, Jean. The Compleat Gard’ner, or Directions for Cultivating and Right Ordering of Fruit-Gardens and Kitchen Gardens, trans. John Evelyn (London, 1693), 2:193.

1. Build twelve hot box composting units and evaluate the optimum configuration of variables of over the spring and summer of 2021 and winter of '22–'23. As these units proved inefficient, a new, bladder-based system was tested in the summer of '23 and winter of '23–'24.

2. Test the bladder system at an on-farm location 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, which will be ongoing after this project ends.