We’re always looking for more compost. While we often have “waste” from our garden and property during the spring, summer, and fall, the winter is a tough time to make compost. But we’d love to build compost in our greenhouse during the cold season: it adds heat and CO2. Compost is derived of carbon and nitrogen, ideally at a 30–25:1 ratio. One thing we do have access to during the winter months is woodchips (600:1::carbon:nitrogen by weight) and urine (8:1::carbon:nitrogen by weight). Can these two main ingredients provide us with ready compost feedstocks in the winter? Leave your suggestions in the comments below, or email us.
Last Update: 03 Feb 21
Expected Completion: Spring 2022
Can woodchips and urine be composted nearly exclusively in a winter greenhouse context to provide finished compost, heat, and carbon dioxide, and if so, what are the optimal operating parameters?
We have a few suggested facts that can help us narrow down our trials. For each kg of urine, we are adding 5.0 g of carbon and 6.6 g of nitrogen (according to the average of a NASA study: Table IV). For each kg of woodchips, we are adding 562 g of carbon and 1.1 g of nitrogen (source). Therefore, the following C:N ratios should be expected for each ratio of urine to woodchips:
|Woodchips (kg)||Urine (kg)||Carbon (g)||Nitrogen (g)||Ratio (__:N)
But then I came upon this study suggesting another ratio, closer to 1 kg urine to 2 kg woodchips and completely different carbon and nitrogen ratios (woodchips: 386 g C and 3 g N/kg; urine: 2.2 g C and 10.7 g N/L), giving the following, contradictory table:
|Woodchips (kg)||Urine (kg)||Carbon (g)||Nitrogen (g)||Ratio (__:N)|
This suggests that I’m going to have to try a wide range of ratios to determine an appropriate zone for successful composting. In my bias for symmetry, a 1:1 ratio would be golden (pun intended).
Since woodchips are easier to measure by cubic foot, we’ll use the coefficient of 10.76 kg/ft3 (this is a dry weight, even though our chips may be damp — the extra moisture will not affect the composting).
We’ll also add some finished compost to jump start the microbiome. For each kg of finished compost, this adds about 200 g of carbon and 20 g of nitrogen. But we’ll be adding such a small amount (<5 percent), that it won’t greatly affect our calculated ratio, which will vary by woodchips and urine anyway.
Composting woodchips with urine is possible and even an effective source of heat, carbon dioxide, and compost for a winter greenhouse.
The test will take place in the spring of 2021. It will consist of three piles of 15 ft3 of woodchips, which each weighing approximately 161.4 kg. Approximately 5 kg of finished compost (3 percent of woodchip weight) will be added. Each pile will receive a different amount of urine: 80 kg (1:0.5::woodchips:urine), 160 kg (1:1), and 240 kg (1:1.5). If these three ratios do not return a satisfactory result, three more trials with higher urine content will be attempted: 320 kg (1:2), 400 kg (1:2.5), and 480 kg (1:3), but I cannot imagine 15 ft3 of woodchips (about half a yard) absorbing two 55-gal drums of urine. Once a satisfactory result is achieved, a further trial that brackets the successful ratio will be carried out to further refine the result. For example, if a 1:1 ratio returned the best result, another trial with 120 kg (1:0.75), 160 kg (1:1), and 200 kg (1:1.25) will be completed.
Each pile will be turned as dictated by temperature: the ideal composting temperature range is 135–160°F, and accordingly when a pile has dropped to 140°F or below, it will be turned in order to add oxygen and revitalize the composting process.
Each trial’s conditions will be recorded, including pile ingredients and ratios, ambient temperature (daily), pile temperature (daily), pH (daily), moisture (daily), turnings, length of composting process, and final compost volume.
Data will be compiled and appropriate statistical analyses will be carried out and reported.
A video and write-up of the data will be produced.
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