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.
Stage: Conception
Last Update: 03 Feb 21
Expected Completion: Spring 2022
Problem Statement
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?
Research
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) |
| 1.00 | 2.00 | 572.00 | 14.30 | 40.00 |
| 1.00 | 2.25 | 573.25 | 15.95 | 35.94 |
| 1.00 | 2.50 | 574.50 | 17.60 | 32.64 |
| 1.00 | 2.75 | 575.75 | 19.25 | 29.91 |
| 1.00 | 3.00 | 577.00 | 20.90 | 27.61 |
| 1.00 | 3.25 | 578.25 | 22.55 | 25.64 |
| 1.00 | 3.50 | 579.50 | 24.20 | 23.95 |
| 1.00 | 3.75 | 580.75 | 25.85 | 22.47 |
| 1.00 | 4.00 | 582.00 | 27.50 | 21.16 |
| 1.00 | 4.25 | 583.25 | 29.15 | 20.01 |
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) |
| 1.00 | 0.50 | 387.10 | 8.35 | 46.36 |
| 1.00 | 0.75 | 387.65 | 11.03 | 35.16 |
| 1.00 | 1.00 | 388.20 | 13.70 | 28.34 |
| 1.00 | 1.25 | 388.75 | 16.38 | 23.74 |
| 1.00 | 1.50 | 389.30 | 19.05 | 20.44 |
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.
Hypothesis
Composting woodchips with urine is possible and even an effective source of heat, carbon dioxide, and compost for a winter greenhouse.
Test
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.
Results
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.
Analysis
Data will be compiled and appropriate statistical analyses will be carried out and reported.
Reporting
A video and write-up of the data will be produced.
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I’m keen to see the results of this!
I’m using wood pellet cat litter, and my cats are generally using it.
I came here to see if it was safe to allow it to degrade in the garden.
It’s not breaking down as quickly as I’d like so I’ll take you advise and mix a bit of composted stuff with it.
Same, I have had a hard time learning about the right ratio and conditions. Would love to see what this yields.
This looks interesting.
I do however think the size of the trial piles may be a little small to produce reliable results.
I suggest 1 cubic yard each would be better, especially if contained as a cube, say by using old pallets lined with cardboard.
Just started checking out your site. Wondered if you are considering how fresh the urine needs to be to retain the nitrogen? Urea starts volatilizing pretty quickly, so much of the nitrogen is lost if not combined and fed on by microbes shortly after being produced.
Urine is also a good source of phosphorus, which does stay bioavailable in the urine.
Good point! Many times I see people storing urine in a sealed container for 6 months before using it. I wonder if the nitrogen is retained in the sealed container, or if it becomes ammonia and volatizes as soon as the cap is open. Definitely something that should be looked into. If it hasn’t already. There is surprisingly little work done on urine and it’s use as fertilizer, even though it has been used for centuries as such. Most of the studies have been done since the Harber Bosch process came along,, and so didn’t study urine.
You can lacto ferment urine to keep the nitrogen from volatilising as ammonia
I have seen alot of research saying lactic acid and temperature extends the time urine breaks down
Another suggestion is to use ground charcoal to adsorb the urine. The charcoal will capture the urea and prevent the nitrogen from volatilizing. Once saturated, the urine/charcoal mixture is applied directly to the soil as a fertilizer/soil amendment. In my shop is a 5 gallon bucket filled with ground charcoal. A plastic shield acts as a backsplash and it is used as a urinal. A sign above the urinal reads “ Urinate in this bucket, It is filled with charcoal that adsorbs the odors and nutrients. „This valuable material will then be used as fertilizer while the charcoal will sequester 6 pounds of CO2 for thousands of years.“
Have the results been published? Very interested to see them. Also, given how variable urine composition is, why did you opt use weight:weight ratios as opposed to testing the urine for its nitrogen content and making (what would seem to me to be) more accurate C:N ratios? I suppose that’s a bit more high tech… Did you pull all the urine from a master batch so each group has the same composition? That would at least keep things consistent even if exact composition was untested.
New to the site, looking forward to checking out other content and hearing back!
Hi Zane,
No, I need to do an update. I haven’t had success getting the system to heat up and compost well.
I don’t test the urine because the percent is within a smallish range, plus the C:N ratio isn’t so picky that the natural range of urine is going to mean a huge difference.
I collect the urine in 3-gallon containers, so they are averaged over say a two week collection period. I am working on a 55-gallon system, which would make things even more consistent.
I will update this some day and will make a video on our YouTube page, too. 🙂
Thanks for checking in and if you have any luck, let me know what worked for you.
Wood shavings, sawdust etc absorbs alot of urine. It’s used in animal bedding for that purpose. Your likely going to need a lot of urine and to make sure the pile itself has enough moisture.
So what is the end result? Was performed in 2021 here we are in nov 2022. Very interested in the final results. I know that either wheat bales, alfalfa bales, oat bales, sawdust, wood shavings etc can absolutely be composted with just urine. I’m curious as to which ratio was most effective.
Yes you can slow down the breakdown of urine by adding lactic acid. Such as used in the brewery industry or the juice off of sour kraut. Also found in some cheese such as feta. Or you can buy the cultures. Brewery lactic acid is easy and not too expensive same for the sour kraut.