First, what makes up brewed coffee, besides caffeine? Nutrition experts agree that coffee grounds contain magnesium, phosphorus, potassium, copper, manganese and trace amounts of calcium. Nitrogen, phosphorus and potassium are the ingredients necessary for garden soil. They coincide with the figures you see on most fertilizer bags. So coffee does have what plants need to grow.
There seems to be some debate about whether or not coffee produces nitrogen. Most agree that coffee grounds contain 1.5 to 2.2% nitrogen. However, some feel that the nitrogen is not available to the soil or the plants until the grounds themselves begin to decompose.
Others say you must compost the coffee grounds with other plants to release the nitrogen in the decomposition process. They recommend adding nitrogen to the soil if using only the grounds. The rotting grounds attract nitrogen-consuming microbes, thus, the need to add nitrogen if using only coffee grounds as a fertilizer.
So to be on the safe side where fertilizing is concerned, you may be better off tossing your coffee grounds along with other compost into the garden in the fall and winter to allow them time to be of best use for spring planting.
When I emptied coffee grounds on top of the soil last fall and spring, I did not include it in a compost. I did use 8 to 10 cubic feet of a peat and compost mixture in the spring that I emptied all over my garden, which is 13 by 33 feet. I turned the soil about 6-8 inches deep before planting. I’ve never had tomatoes produce so much in my four years of gardening.
How does the acid in coffee affect things? The coffee you drink is acidic, but the grounds afterward are closer to neutral on the pH scale. Some argue that they retain their acid level until combined with compost and its microbes, which neutralize it. Either way acid is great if your soil is low in it (alkaline). In the West, where I live, soils are alkaline and getting closer to neutral or increasing the acid content is a good thing for my strawberries, blueberries, raspberries and radishes. Keep in mind that you have to use quite a bit of grounds to make any real difference in the pH scale.
Coffee grounds have other benefits aside from being a fertilizer. They boost the heat in soil better than most composting materials, including manure. These high temperatures kill seeds from weeds or vegetables that may have made their way into the compost pile. If you are thinking about raised beds, consider lining the bottom edges with coffee grounds to prevent weeds from sneaking in.
Other benefits to using coffee grounds in the garden include its ability to attract earthworms, thereby improving soil structure. It even keeps cats from pooping in your garden. (Our neighborhood stray poops along the outside of the garden instead!) If tomato blight is a concern, there are some who think the copper in coffee grounds keeps this at bay.
Coffee grounds are also good at keeping away pests for both inside plants and outside flowers, like hostas, roses, lilies, peonies or those grown from bulbs. The caffeine discourages slugs and snails. You can add about 2 cups of grounds to a 5 gallon jug of water and let it steep overnight. Use it to water plants and don’t be afraid to water the leaves, too.
So whether or not coffee grounds include enough nitrogen to be used as a fertilizer, its pest deterrent qualities and attraction to earthworms has more than sold me on its benefits. If you aren’t a coffee drinker, just ask your local coffee shop if you can have their leftover grounds. It not only helps them dispose of waste, but it also helps create a more abundant, inexpensive and organic garden.