Tip lists say to sprinkle coffee grounds around your vegies for free fertiliser. A trial at the University of Melbourne found the opposite: fresh grounds put straight on the soil can slow plant growth. We got part of this wrong in an earlier version of this post, so this is the corrected one.
What the trial found
Dr Stephen Livesley and Sarah Hardgrove ran a greenhouse pot trial and a field trial, as summarised by Sustainable Gardening Australia. Fresh, uncomposted spent grounds were mixed into the top 10 centimetres of soil at 2.5%, 5%, 10% and 20% by volume. The article’s conclusion is that at 2.5% and above, grounds will likely decrease the growth of all plants.
Two things people expect did not explain it:
- Acidity. Soil pH with grounds was similar to the fertilised soil, so pH was not the cause.
- Nitrogen. The popular explanation is that microbes eat the grounds and lock up the soil nitrogen. The study found nitrogen drawdown could not explain the growth loss. The grounds had a carbon to nitrogen ratio of 23, and the article says that is generally not expected to cause nitrogen tie up (the range it cites is 20 to 30).
The authors point to a toxic stress response as the most likely cause, with caffeine, tannins, polyphenols and lignin as possible culprits. They say the mechanism is unclear. In plain words: something in fresh grounds hurts plants, and nobody has pinned down exactly what.
A correction to our old post
Our earlier version blamed the problem on nitrogen being tied up in the soil. On the evidence above, that explanation is not supported. We have rewritten it, adn we would rather say so plainly than quietly change it.
What to do with your grounds instead
Compost them first. This is the safe route. The same article recommends no more than 20% grounds by volume in a compost heap. Mix them with dry brown material like leaves or shredded paper so the pile does not go soggy, and use the finished compost as normal.
Use them on weeds, not on crops. The article presents two options for slowing weeds: work fresh grounds into a fallow patch you leave for six months or more, or spread them as a thin mulch. Keep both well away from seedlings and young plants.
Don’t expect a quick soil boost. The summary reports a trend towards better water holding in poorly structured soils, with less effect in soils that were already in good shape, and notes the benefit may take time (other research it cites suggests 6 to 12 months).
What about slugs and snails?
The Livesley and Hardgrove summary does not mention slugs or snails, so this part is not from that trial. A 2002 paper in Nature by Hollingsworth and colleagues found that caffeine solutions of 1 to 2% deterred slugs and snails, while a brewed coffee is only around 0.05% caffeine by their figures. We read that through secondary summaries, not the original paper. On that reading, a ring of grounds is unlikely to stop a snail. A beer trap or a torch and a bucket at dusk will do more.
The short version
- Fresh grounds on or in the soil around crops: skip it.
- Grounds in the compost, 20% by volume at most: good.
- Grounds as a thin weed mulch away from crops: reasonable.
- Grounds as a slug barrier: don’t count on it.
Build soil that works
Good soil comes down to timing and care across the whole year, not a single trick. The Garden Year 2027 gives you a month by month plan for your climate zone, with room to write down what you added and what happened, so next season you are not waiting on memory. Not sure of your zone? Look up your postcode first.
Where our facts came from
- Sustainable Gardening Australia, summary of the Livesley and Hardgrove (University of Melbourne) coffee grounds trial: rates, pH, nitrogen, likely cause, composting and mulch advice. We read the summary, not the full paper.
- Hollingsworth and colleagues, Nature 2002, caffeine and slugs and snails: secondary summaries only, original not read.
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