Compost Thermal Subsidies in Commercial Passive Solar Greenhouse Design

Final Report for FW99-063

Project Type: Farmer/Rancher
Funds awarded in 1999: $1,750.00
Projected End Date: 12/31/2004
Region: Western
State: Washington
Principal Investigator:
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Project Information


The participants want to see if a large berm of hot compost in the north side of a greenhouse will provide enough heat to grow standard cool-season crops like salad greens and brassicas.

Members of the Cold Springs Conservancy, a nonprofit educational land trust of about 4 acres, want to research CO2 levels, soil and air temperatures and plant production and mortality rates of the compost-heated greenhouse during the cold season - mid November to mid April. They will also evaluate the incidence of damping-off disease.

The plan is go design a model that can be used by producers in the Pacific Northwest and other areas that don't receive enough sunlight in the winter to effectively heat a greenhouse. The process will include solar-citing education from the technical advisor, Greg Acker, and the building of two greenhouses, each 80 feet long, 20 feet wide and 15 feet high.

Participants will build and turn the compost pile - manure and vegetable waste - starting in early October and plant out various salad greens by late October. They'll monitor temperature daily and ventilate if needed. Each week, ammonia and carbon readings will be taken along with compost pile heat levels. Weekly compost samples will be sent to a soil lab for analysis. By late November, if the temperature in the compost pile drops, the pile will be freshened by adding more manure, urine and water.

The results of the study, skewed by several unexpected events, have been sufficient to suggest some validity in establishing a compost pile inside a greenhouse, perhaps underneath benches or underneath a layer of soil with tomatoes growing on top.

Originally planned for two years with a greenhouse built from scratch, the project was altered to save time. The prefabricated greenhouse ordered showed up late, and when it was finally constructed and the experiment begun a wind gust tore off the plastic. In the short time it was sealed, however, the compost actively decomposed and plant growth was luxurious. It is unknown whether the enhanced growth inside the greenhouse was from the compost heat and carbon or simply the effect of the hoop house.

The greenhouse worked in the second year, but the anticipated compost material - manure from a dairy - wasn't delivered until November, too late for a proper test of the composting technique.

Still, project coordinator Rebecca Thistlethwaite says some valuable lessons emerged from the failures:

1. Timeliness and preparation of the compost is essential.
2. Plant transplants, not seeds, in the hoop houses.
3. Salad greens work best in the hoop-house environment.
4. Hoop houses help keep out rabbits and ground squirrels, which, in this area, devour most growing produce.
5. On the other hand, the structure harbored thriving winter populations of squash bugs that attacked early-planted zucchini.

Thistlethwaite observes that the compost pile did give off considerable heat. For the short time the plastic remained on the hoop houses the first year, a tropical atmosphere was created inside. In the second year, tomatoes and basil were planted two months earlier in the hoop house than in the outdoor planting in late February. Despite slow growth inside the hoop house, the plants did grow and were harvested a month earlier than the outdoor planting. Also in the second year, a foot-deep layer of finished compost was spread inside the greenhouse, and the plants, says Thistlethwaite, responded beautifully. The soil was workable and the weeds easy to pull.

The project indicates that the heat generated by the compost process could help provide heat for early vegetable growth in a greenhouse.

There have been no reports that others have tried to replicate this project.

Thistlethwaite suggests that further research on the subject is needed, especially on the effects of the carbon and nitrogen gases that are released from the decomposing organic matter in the compost pile.

The findings have mainly been disseminated through the SARE reports.


Participation Summary

Research Outcomes

No research outcomes
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.