18th May 2023
Imagine you had a simple kitchen ingredient in your cupboard that could make your compost hotter, enhance its nutritional value, AND reduce harm to the environment.
If a new Japanese study is right, the cooking oil you use every day could be that ingredient!
The researchers had already identified that adding cooking oil to cattle manure could reduce ammonia – a harmful gas that is released in the composting process.
However, that study had taken place in laboratory conditions and without aeration.
In this second study, the researchers wanted to see if the results were repeated in real-world scenarios
To do so, they tested the addition of cooking oil on aerated piles, measuring the impact on ammonia emissions, temperature, and compost content.
A second pile, with manure but without added cooking oil, was used as a control.
The researchers found that the pile with added cooking oil:
- Generally had lower emissions
- Lost around 21.5% less nitrogen (a key nutrient for plants)
- Had higher temperatures than the pile without oil
The pile with oil did lose more moisture, which is unsurprising given the increased heat it achieved.
Could cooking oil help improve the composting process?
The researchers were mainly interested in reducing harmful emissions.
However, the study also has some other interesting implications for us composters!
As a hotter pile turns organic material into compost more quickly, it should also help us to speed up our compost heaps.
However, Kazutaka Kuroda, one of the researchers involved in the project, wasn’t so sure.
He told us they monitored compost breakdown by measuring volatile solids.
While the organic solids were higher at the start of the process in the pile with cooking oil, they were roughly the same in both piles by the end of the process.
Because of this, Kazutaka didn’t think cooking oil would have a major impact on speed.
However, as the final compost contains more nitrogen, it should be more beneficial for our soil.
It’s also promising that this improvement in quality can be seen with quite small amounts of cooking oil.
What type of cooking oil was used?
Kazutaka told us the study used waste cooking oil collected from food factories or restaurants.
This contained many vegetable oils such as canola oil, corn oil, and soybean oil.
Could home composters use cooking oil too?
We are often told not to add cooking oil to our compost, as it can attract pests.
Still, if we followed every ‘rule’ on what NOT to add to our compost heap we’d barely compost anything!
I’d probably avoid adding cooking oil to cold compost heaps, but if you can consistently get your compost hot, cooking oil could well be worth experimenting with!
Kazutaka agreed, saying that if your compost can reach compost standards provided by the United States Department of Agriculture (over 55°C (131°F) Fahrenheit for 3 days), adding cooking oil to the compost heap will be effective.
If you do try adding it to your compost (and I’ll be trying this ;)), consider adding it under the top layer to reduce odors that could attract pests.
Alternatively, you could use a layer of sawdust or straw to act as a biofilter and capture odors.
Kazutaka also emphasized that it’s fine to use used cooking oil, as long as there are no substances in it other than fine cooking residues.
All in all, I think this is worth adding to our list of composting hacks!
Kuroda, K., Tanaka, A., Furuhashi, K., & Fukuju, N. (2023). Evaluation of ammonia emission reducing effect by adding waste cooking oil in pilot-scale composting of dairy cattle manure. Animal Bioscience. doi: 10.5713/ab.23.0027. PMID: 37170507.