Mulching with coffee grounds

Have you ever wondered what happens to used coffee grounds? Could they be a useful source of garden nutrients? There are stories around of them being packed full of nitrogen such that they could be used as a replacement for fertilizer.

A search will produce claims that used coffee grounds are the answer to everything. While that is more than a slight exaggeration, they are a useful source of organic matter and can be incorporated into soil by composting or mulching.

The best summary (see extracts below) is from Oregon State University

Some information about coffee grounds

  • Coffee grounds are about 2% nitrogen by volume.
  • Grounds are not acidic; the acid in coffee is water-soluble so the acid is mostly in the coffee.
  • Coffee grounds are close to pH neutral (between 6.5 to 6.8 pH).
  • Coffee grounds improve soil tilth or structure.
  • Coffee grounds are an excellent nitrogen source for composting. They have a C/N ratio of 20-to-1. In informal trials with OSU/Lane County Extension Service, Compost Specialists recorded sustained temperatures of 140 to 160 degrees Fahrenheit for up to two weeks when coffee grounds were 25% of the material in the compost pile by volume.
  • Anecdotal evidence suggests coffee grounds repel slugs and snails in the garden.

Another site suggested adding a layer on top of the soil and covering with leaves. As we do not want to disturb the soil that is what we have done today, 02 January 2020.

Make sure the lumps are broken up

The bed was then covered with leaves. It will be interesting to see what happens over the next 3-5 months.

Update May 2020
Using spent coffee grounds as a mulch worked really well. The soil in this bed was in good condition with a fine tilth for sowing seeds. Unfortunately the local cafes have been closed since March which means we have not been able to continue with the trial. It is certainly something we will continue when it is safe to do so.