From the Viette's Views Gardening Blog
This has been the subject of much discussion in the gardening world. Some say they’re great and some say they’re good but with a word of caution.
Recently a question was posted on our Discussion Board asking about spreading coffee grounds in a raised bed …
"I have read that spent coffee grounds are a good source food for those raising worms. Would it be a good idea to add spent coffee grounds to raised vegetable beds during the winter months, knowing that the present worm population will break them down before spring planting time?"
From almost everything I have read, it seems that coffee grounds are a great addition to the garden. They can supply important nutrients to the soil; nitrogen, phosphorus, potassium, magnesium, and copper. They also improve the soil structure because as they decompose, they release humic acid which acts as a glue to bind tiny soil particles together into larger aggregates, which improves drainage and air circulation. This is especially good for clay soils.
Okay, so coffee grounds can be good for the soil. Now the question becomes, what is the best way to use these grounds that will provide the greatest benefit to your plants?
It seems that coffee grounds are most beneficial to plants after they have been broken down by microorganisms in the soil. Though potassium, magnesium, and some of the phosphorus and copper are available to plants immediately, the nitrogen in coffee grounds is not available to the plants until it has been broken down into a usable form by soil organisms.
For this reason, adding your coffee grounds to a compost pile and letting the soil critters break it down may be the most efficient way to use them. We have been composting our coffee grounds for many years, filter and all. It all breaks down.
The grounds are a great addition to a compost pile because they are high in nitrogen – around 2%. In the composting world, this means that they are considered a “green” additive; similar to grass clippings. This is great for us because our compost pile is usually a bit “carbon” heavy since we add so many leaves to the pile in the fall. Anything we can throw in that will add more “nitrogen” to the pile is welcomed. It is best to try to keep the carbon to nitrogen ratio at around 25 or 30 to 1 for a healthy compost pile.
As an added bonus, worms love coffee grounds and in a compost pile, the more worms the better! Some people that maintain worm bins add a cup or so of coffee grounds to the bin once a week.
But wait – aren’t the coffee grounds acidic? Won’t they lower the pH of my garden soil or compost? This is one of the greatest concerns that people have with putting coffee grounds in the garden or compost. It turns out that, contrary to popular belief, most spent coffee grounds are not very acidic at all. The acid in ground coffee is water soluble so most of it is leached out into the coffee that you drink – making your coffee quite acidic and the leftover grounds just slightly acidic with a pH of around 6.2 – 6.8.
Coffee grounds should not comprise more than 25% of your compost pile; but that would be a lot of coffee grounds! We average one pot of coffee a day so there is no danger of us overloading our big compost pile with coffee grounds. People who collect used grounds from coffee shops could potentially accumulate enough to cause a problem in a compost pile – they just need to be careful to add enough carbon-rich ingredients to maintain the optimal C:N ratio.
Used coffee grounds can also be broadcast right on top of the soil. If they are then tilled or scratched in, the worms and soil microorganisms can get to work on them faster to break them down and release the nutrients.