First, what is brewed coffee made of, besides caffeine? Nutrition experts agree that ground coffee contains magnesium, phosphorus, potassium, copper, manganese, and trace amounts of calcium. Nitrogen, phosphorous and potassium are the necessary ingredients for garden soil. They match the numbers you see on most fertilizer bags. So coffee has what plants need to grow.
There seems to be some debate about whether coffee produces nitrogen or not. Most agree that coffee grounds contain 1.5-2.2% nitrogen. However, some feel that nitrogen is not available to the soil or plants until soils begin to break down.
Others say that you should compost the coffee grounds with other plants to release the nitrogen in the decomposition process. They recommend adding nitrogen to the soil if only the grounds are used. Decomposing grounds attract nitrogen-consuming microbes, so adding nitrogen is necessary if using only coffee grounds as fertilizer.
So, to be on the safe side when it comes to fertilization, it’s best to throw your coffee grounds along with other compost in the garden in the fall and winter to give them time to be of best use for spring planting.
When I dumped the coffee grounds into the ground last fall and spring, I didn’t include them in the compost. I used 8 to 10 cubic feet of a peat-compost mix in the spring that I poured all over my garden, which is 13 by 33 feet. I turned the soil about 6-8 inches deep before planting. I have never had tomatoes produce this much in my four years of gardening.
How does the acid in coffee affect things? The coffee you drink is acidic, but the later grounds are closer to neutral on the pH scale. Some argue that they retain their acid level until they combine with the compost and its microbes, which neutralize it. Either way, acid is great if your soil is low (alkaline). In the west where I live, the 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 soil to make a real difference on the pH scale.
Coffee grounds have other benefits besides being a fertilizer. They increase heat in the soil better than most composting materials, including manure. These high temperatures kill any weed or vegetable seeds that may have made their way into the compost pile. If you’re considering raised beds, consider lining the bottom edges with coffee grounds to prevent weeds from creeping in.
Other benefits of using coffee grounds in the garden include its ability to attract earthworms, thereby improving soil structure. It even prevents cats from pooping in your garden. (Instead, our neighborhood poops outside the garden!) If tomato blight is a concern, some think the copper in coffee grounds keeps it at bay.
Coffee grounds are also good for keeping pests away from both indoor plants and outdoor flowers, such as hostas, roses, lilies, peonies, or those that grow from bulbs. Caffeine discourages slugs and snails. You can add about 2 cups of ground coffee to a 5-gallon jug of water and 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 its attraction to earthworms have more than convinced me of its benefits. If you’re not a coffee drinker, just ask your local coffee shop if you can keep the leftover grounds. Not only does it help them eliminate waste, but it also helps create a more abundant, affordable, and organic garden.
