Even among garden authorities, there are commonly-held misconceptions about coffee grounds and their use in the garden, but their origins are easily understood. Coffee is an acidic drink, so it makes sense that the coffee grounds would likewise be acidic.
Only it isn’t true.
In fact, coffee grounds are generally close to pH neutral (between 6.5 and 6.8). As it turns out, the acid that is contained in coffee is water-soluble, so the acid is essentially washed out of the grounds when coffee is made, which is why you need an acid reducer but your coffee grounds don’t.
Coffee grounds are still an excellent addition to your gardening routines. By volume they are about 2% nitrogen and they are noted as a good source of nitrogen for composting, given that they have the same basic carbon to nitrogen ratio as manure (20:1). The addition of coffee grounds to compost helps to raise and maintain a higher temperature in the compost pile for a longer period of time, which aids in killing pathogens that may be present.
Contrary to what some garden experts say, you might want to resist the urge to add the grounds directly to your garden though, unless you are using a nitrogen-rich fertilizer as well. In short, you can add coffee grounds to your compost to increase the nitrogen content, but when adding grounds directly to the soil, a nitrogen-rich fertilizer must also be added. This is because the grounds encourage microbial growth in the soil, and those tiny microbes are nitrogen-hungry wee beasties that will deplete the existing nitrogen.
Here’s what you need to know: The truth is that many experts have been mistaken for years about the use of coffee grounds in the garden. Coffee grounds are not terribly acidic and they are not a good direct soil additive without additional fertilizers. Coffee grounds are however a great addition to compost, where their nitrogen content can be fully realized and used.
So by all means, save and use your coffee grounds, but do so with the benefit of science behind you. Talk to local coffee shops and ask about collecting some of their spent grounds for your use. Your compost will love it and so will your garden – eventually.
EDIT: I wanted to add a link to this story in which the author created compost using nothing more than coffee grounds and shredded newspaper. The chemical analysis of the finished product might surprise you.