Research Project No. 7 — Hot Box Compost: Modular Composting System that Heats, Waters, and Fertilizes Grow Bed for Off-Season Production

This proposal was recently submitted for funding and constitutes our seventh research project. See all our research projects here.

This project tests the performance of a modular, scalable compost box that uses a composting chamber to support an attached growing bed with nitrogen, moisture, carbon dioxide, and heat to grow greens in the shoulder and off seasons; includes a series of controlled tests to determine best practices.


Imagine a cube measuring 4 ft on each side. On top of the cube is a rich bed in which 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 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.


Scott Johnson is a market-scale grower. He produces potatoes, tomatoes, and other typical produce as well as hand-harvested heritage wheat and rye. He keeps poultry and a commercial bee apiary involved in research to breed stronger colonies. He grows on his three-quarter-acre property in Cooksville, Wisconsin, as well as on another half acre on an adjacent property. Plants are grown in managed beds, fields, a 100-×-30-ft hoop house and a heated 25-×-15-ft in-ground greenhouse for year-round production.

Johnson directs the nonprofit Low Technology Institute, which is chartered to develop ways to better feed ourselves when fossil fuels are no longer available. With SARE support, he has carried out and disseminated research on market-scale potato-growing methods. The organization will provide logistical support to this project, including online hosting of results and other generated media.

Parisi Family Farm is located in the Town of Dunn, it is 90 acres of mixed use including a 23 acre pasture for rotational grazing, a 7 acre pollinator habitat, 3 acres of vegetables, herbs and berry/fruit and nut trees and 7 acres of alfalfa field for hay for goat feed as well as 40 acres of woods. We have been certified organic for a decade, growing a variety of vegetables from asparagus to zucchini and selling them at various farmer’s markets, CSA shares and our farm stand. The farm is ran by a mother and son, Terry is a retired teacher and Franco has a horticulture and art degree from the University of Wisconsin.

Project Objectives

  1. Build ten hot box composting units and evaluate the optimum configuration of variables of over the spring and summer of 2021.
  2. Test five units each at two hoop-house locations over the winter of 2021–22 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.

Measuring Benefits and Impacts

This project will have measurable impacts in primary and secondary fields.

The primary economic impact will be to improve income and profitability as well as market opportunities. Market farming is a competitive business as most vendors have the same produce available at the same time due to seasonal constraints. By growing greens in a hot-box system, farmers will have the first produce in the spring and the last produce in the fall without having to pay for expensive greenhouse heating. The hot box is a one-time, up-front cost and can be used for years. To quantify this, input costs of materials and labor will be compared with yield and market value.

The primary production and efficiency impact will be linking together systems that most sustainable market farmers are already using to eliminate or reduce purchased inputs. Specifically, by using compost to heat and feed crops, greenhouse heating fuel is reduced or eliminated. Additionally, the finished compost will reduce the amount of bought-in fertilizer needed in the subsequent growing season. We will quantify this by calculating the market cost of the generated heat and finished compost.

One secondary impact is environmental sustainability. Traditional, open-air composting emits significant amounts of ammonia, methane, and carbon dioxide. These emissions are increased if the pile is not turned regularly and becomes overheated and/or anerobic. By not only producing a well-aerated pile but also converting nitrites into nitrates and feeding them directly into a soil substrate, this method has the potential to sequester greenhouse gasses otherwise emitted during composting. This sequestration will be measured with emissions data recorders.

Finally, this method of growing increases the availability of greens not only for market farmers but also for individuals with a bit of sunlit, protected space to grow spinach or other greens to supplement their winter diets.

Activities and Timeline

April 2021Purchase materials and build 10 hot boxes.Johnson
May–August 2021Run series of tests to determine best configuration of air intake, carbon-to-nitrogen ratios, etc.Johnson
September 2021Distribute hot boxes to two test locationsJohnson and Parisi
September 2021–May 2022Run series of performance tests throughout shoulder and off seasons; host field days.Johnson and Parisi
May–December 2022Analyze data, write articles, prepare and give presentations and other outreach.Johnson

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 4 ft on a side, 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 nine 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 summer and the best configuration will be tested in the winter. 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, including 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).

Figure 1: Diagram of box builds.


April 2021–December 2022

The project, its goals, and its methods will be shared online through the Low Technology Institute’s website (, Facebook page (, Twitter account (, and Instagram profile ( Monthly updates sharing the data will be shared through these channels.

September 2021–May 2022

Market-farmers and other interested growers will be invited out to visit a field day at either or both of the test locations to view the hot boxes, preferably on a day when they are being unloaded and reloaded. This will be advertised online and through local industry groups, such as Fair Share CSA Coalition ( and the Seed to Kitchen Collaborative (

May–December 2022

A detailed report will be made publicly available through the above-mentioned channels. Video and podcast episodes summarizing the study and its findings will be produced and shared online. Appropriately detailed summary articles will be prepared and submitted to general audience and professional publications such as the following:

Organic Growers’ Publications

Acres USA (
Organic Farming (
Growing for Market (
Greenhorns (

Wide-Audience Publications

Mother Earth News (
Organic Life (

General Agricultural Publications

Modern Farmer (
Farm World (
Farm Journal (

The sample size of this study may not be robust enough for an academic article, but the data will be freely available for any researcher to use in further research. Presentations of the study and results will be made at local or regional meetings for market farmers or other small-scale growers. In addition, a short, one-page graphical summary (similar to a social media “meme”) of the study and its results will be shared online.

All disseminated information will be targeted at three audiences: market farmers, specialty growers, and large-scale personal gardeners. All designs and data are available free of charge.

Previous Research Review

This project combines well-known methods in a novel configuration. We need not reinvent the wheel, but we do need to fine-tune the variables for this use. Compost has been used for hundreds of years to heat growing spaces in the cooler seasons. The USDA developed the aerated static pile (ASP) system in the 1970s. The New Alchemy Institute (NAI) tested a large ASP integrated into grow beds in a greenhouse for winter growing in the 1980s. And while our knowledge of composting biochemistry and ASP has advanced since then, to my knowledge no series controlled tests have been carried out on small, modular units. The difficulty here is creating an adaptable system that can accept a variety of compost materials—we’re likely to come up with a variety of “recipes” for different materials.

This project is directly inspired by the New Alchemy Institute’s large-scale greenhouse design, first tested in 1983. Their 12-×-48-ft greenhouse had a 25-yd³ compost bay that produced over “100 tons of compost and tens of thousands of seedlings in its first full year of operation” (, pg. 1). The design pushed air through the compost, exhausting carbon dioxide, ammonia, heat, and moisture into a layer of woodchips below the growing medium, which held bacteria that converted the ammonia to ammonium and nitrates and diffused the “waste” products into the grow beds, fueling strong plant growth (and reducing greenhouse gas emissions) This filtering is seen in modern ASP beds when finished compost is layered on top of the active pile to control odors. Even 37 years ago, much of the complex biochemistry was understood (see NAI 1986 for a start), and this entire project is meant to adapt NAI’s elegant, self-feeding system for smaller, modular use. We will likely rely on their plant variety suggestions to standardize our results.

SARE has funded six projects that use compost heat. Four were large-scale, permanent installations that used liquid-filled pipes to exchange heat with greenhouses (FW15-057, FNE13-777, FNC10-835, and FNE09-675). One study used pig manure compost to heat farrowing pens (FNC02-438). The closest SARE-funded comparison is FNE12-739, which grew seedlings and plants on top of compost piles in a winter greenhouse with promising outcomes. Our project differs in a few key ways: our set-up more effectively traps and uses the nitrogen, carbon dioxide, and moisture created by the compost; it offers a mobile unit that can be moved out of the way during the regular growing season; and most importantly, it will use a series of controlled tests to determine the best conditions for this set up with continuous data collection.

An internet search of small ASP systems returned no systems this small or mobile, let alone one that is designed to heat and feed plants in a grow bed. Other grants have funded the design of a variety of small-scale ASP systems, such as for smaller horse stables ( or general compost creation (such as the Johnson-Su Bioreactor, A similar ASP composter was developed in New York Public Schools, but the developers do not appear to have carried out controlled tests nor used the exhaust gasses and heat for growing plants (

As part of the research and design of the hot box, we will be continuing to refine or designs with input from composting industry professionals (e.g., Peter Moon [], We are in also in contact with NAI and Bruce Fulford (, who led the NAI greenhouse project and has since continued his work with composting systems. As such, the design may change and improve over the winter of 2020–21.

Contribution to Sustainable Agriculture

This project is economically, environmentally, and socially sustainable. This composting system not only provides greens for sale during the shoulder and off seasons (fetching a higher price), but it more efficiently turns bio-waste into usable compost more quickly, reducing the amount of bought-in fertilizer for a growing operation. Additionally, by producing their own heat, these boxes will reduce or obviate the need for purchased heating fuels. A market farmer could build on this system to create a mobile or permanent ASP design for their needs. The small scale of this design allows them to test the system with a low-cost investment during the off season, when time is available.

The hot boxes are more environmentally friendly than other, single-purpose composting systems. Because composting generally releases carbon dioxide and other greenhouse gasses into the atmosphere. One solution in the composting industry is to add a “biofilter” (a layer of finished compost and/or woodchips) over the file to reduce these emissions. The system here will not only do that but also direct these gasses into the grow beds for uptake by leafy green growth. The production of compost and reduction of fossil fuel use, mentioned above, are also environmental benefits.

Finally, this system helps not only the farmers that adopt it, but also their customers. The farmers have a chance to “stack” jobs they’re already doing separately (composting and growing) and get more out of it (salable product, not just compost). The work is largely done in the “slow” seasons, when farmers have more time available. Customers will have access to local, off-season produce that did not require the use of excessive fuel for heating or transport.


Materials and Supplies

12 Boxes (6 type I, 6 type II): $4,032

The materials for twelve boxes, six of each type, breaks down as:

$2,922 = 6 × $487 for each Type I — “Overengineered”
17 2x4x8’ wood
2.5 2x4x8’ synthetic lumber
2 1x2x8’ wood
2.5 4x8x1/2” plywood
2.5 4×8 corr. poly
1 4×4 1⁄2″ screen
4’ 4” pvc
1 4” pvc cap
30′ 1” pvc
9 1” pvc cap
1 lb 16d nails
1 lb 1 1⁄2” cab. screws
2 Pair Heavy hinges

$1,110 = 6 × $185 for each Type II — “Simple”
3 – 2x4x8’ wood
2 – 1x2x8’ wood
2.5 – 4x8x3/4” plywood
1 – 4×4 1⁄2″ screen
4’ – 4” pvc
1 – 4” pvc cap
30′ – 1” pvc
9 – 1” pvc cap
1 lb – 16d nails
1 lb – 1 1⁄2” cab. screws
2 Pair – Heavy hinges

Aeration Equipment: $860

Aeration system, requiring blowers, piping, and timers.

2 – 1000 cfm blowers ca. $200/ea
30’ – 6” PVC plus fittings $200
2 – Timers $30/ea

Testing Equipment: $1,200

To monitor and record the conditions produced during the tests.

24 – Temp/Hum. monitors (Elitech RC-4HC, $240/10)
1 – CO2 probe $160
1 – Ammonia probe: $160
1 – Methane probe, $100
3 – Nitrate testing kit $20/ea

Free Items

Pallets: From a local business.
Feedstock: Woodchips from our neighbor, who is an arborist. Grass clippings and leaves
from our neighborhood. Straw from our own fields. Animal manure (chicken droppings)
from our coop.
Seeds: From our own supplies.

Other Direct Costs

Consultants: $400

A few hundred dollars in consulting fees will save us wasted time, effort, and money by
avoiding mistakes they’ve already learned from.

New Alchemy Institute & Affiliated Scientists
Each, $200 upfront for four hours consulting, plus $50/hr thereafter

Other Composting Consulting
Each, $100 upfront for two hours consulting, plus $50/hr thereafter

Field Day: $100

We will hold a field day to showcase the boxes to growers in the winter of 2021–22.

$50 for refreshments and printed material for attendees.
$50 for advertising and outreach.


Build 12 Boxes: $960

Box building hourly rate includes time getting and preparing materials, as well as wear
and tear on equipment in woodshop.

$960 = 12 boxes × 4 hr/box × $20/hr

Summer Tests: $1,280

A series of tests will be carried out over the summer to determine optimum performance of different variables (including C:N ratios, aeration amounts, feedstock and particle size, free air space, pile moisture/temperature.

Each month, a pair of type I and a pair of type II boxes will be assigned to “no flow,” “low flow,” and “high flow” aeration. One of each pair will have high free airspace and large particle size and the other will have low airspace and small particle size. Four grow beds without compost will be grown next to the boxes as “control” beds.

Month 1: 20:1 C:N ratio across boxes.
Month 2: 30:1 C:N ratio across boxes.
Month 3: 40:1 C:N ratio across boxes.
Month 4: Best four performers from each month will be tested against one another.

Feedstock includes: leaves, grass clippings, straw, manure, and woodchips.

$640 = Filling, unfilling boxes, 4 times (May–Aug) × 8 hr × $20/hr
$640 = Weekly monitoring (temperature, ammonia, methane, nitrate/nitrite), plant care,
16 weeks (May–Aug) × 2 hr × $20/hr

Winter Tests: $3,200

The best performing configurations will be tested over the winter months in two

Johnson and Parisi each (i.e., ×2)
$800 = Winter filling/unfilling 5 times (Oct–Feb) × 8 hr × $20/hr
$800 = Weekly monitoring, plant care, 20 weeks × 2 hr × $20/hr

Administration, Analysis, Etc.: $1,600

Johnson will be investing time to collect, compile, analyze, and present the data.

$200 = 10 hr Administration (data collection, field day organization, etc.) × $20/hr
$200 = 10 hr Analysis (data compiling and analysis) × $20/hr
$1,200 = 60 hr Write Up and Presentation Prep (two articles and presentations for
general and professional audiences) × $20/hr

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