There is a very substantial bonus associated with composting and treating an organic waste stream properly. I have composted in the greenhouse for the last 11 years, and have always appreciated what I thought were the benefits of the operation. For me, the primary benefits were three: waste stream reduction, thermal mass for climate control, and carbon dioxide generation for enhanced growth. The greenhouse is fairly heavily instrumented and automated, so I have lots of data to look at, but it was not until recently that I decided to measure two interesting variables: the amount of CO2 generated by my compost, and the reduction in dry mass due to the composting process. I compost in both large beds (5'-6' wide, 16-40' long, 2'tall), and in rubbermaid tubs of about 27 gallon capacity. Most of the compost has worms introduced to enhance the process. The resulting worm compost is a useful part of my potting soil mix, or a good top dressing for plants. I find that I need about 28 pounds of fresh composting materials per 100 sq.ft of the greenhouse to maintain a daytime atmosphere of the greenhouse at 600-900ppm CO2, about 2-3x atmospheric levels. I have plenty of carbon rich sources to compost, but do not often have suitable N sources. This summer I tried using horse manure, a N source I can tap for many tons if I wanted. Much to my chagrin I introduced stable flies to the greenhouse. I am still in the last phases of getting rid of these little devils. Stable flies are not attracted to standard fly traps; they are blood feeders, requiring a couple bits of blood to reproduce. They are the notorious ankle biters: they disturb horses and cattle so much by their biting that the animals will lose weight. They disturbed me so much that I needed to wear long pants and shirts for months in the greenhouse to avoid their bites. I never use pesticides in the greenhouse, so I've had to eliminate the flies by removing all the composting material that could harbor their larvae. I think I am almost done. The moral of the story for me is that this is one place where I will use a synthetic fertilizer instead of an organic. I'll probably go back to using urea, a common choice for some commercial worm farmers to fatten the worms and enhance the worm feed stock. I made a semi-careful attempt to determine the reduction in dry mass associated with composting. I believe that in my rubbermaid containers with worms I experience about a 27% reduction in dry mass over a period of about 35 days. The rotting is not done at that point, and if I wait another couple months the mass has reduced quite a bit more, but I don't have numbers. It is only in the first month or so that the compost produces lots of CO2, so I try to keep new containers starting regularly. I assume that most of the dry mass loss is CO2 generation from the C in the raw materials. There may be some N loss as well, but I do not believe it is significant. I hope this is helpful to others who may have wondered about efficacy of greenhouse composting. By the way, my numbers are reasonably consistent with the results of one of the seminal studies in this area: J. Plant Nutr. Soil Sci. 2009, 172, 418�424 Carbon dioxide enrichment by composting in greenhouses and its effect on vegetable production Chongwei Jin, Shaoting Du, Yue Wang, Jason Condon, Xianyong Lin, and Yongsong Zhang if you want more info.
Renais