Coffee and Your Soil

If you know me then you are already well aware that two of my most favorite things are coffee and plants, but you may not know that the two go together in more ways than just what I want to have with me when I walk in my backyard. Complementary by nature, coffee is the perfect vessel for integrating essential components into your compost and soil.

Coffee and Composting

Composting is a convenient way to handle old, wet coffee grounds (as opposed to leaving them in the trash bin or allowing them to clog up your sink), and a super efficient way to make sure you are adding all the good things that your compost needs. Coffee grounds add a number of essential nutrients to compost (like nitrogen, phosphorus, and potassium), and act as a perfect complement to carbon-rich inputs like yard waste and other organic matter. 

However, you can overdo it and add too much coffee into your compost. A compost overrun with coffee grounds will become overly dense and acidic, and lack the ability to properly aerate. The most common recommendation is to not have more than 20% of your compost pile made up of coffee grounds. While coffee is a great way to boost the nutrient content in your compost, you don't necessarily have to compost it if you have too much volume or simply choose not to. 

Can you add raw coffee grounds into your garden soil?

Yes! Coffee grounds act as a slow-release fertilizer if mixed directly into your soil. There are several ways that it can be incorporated into the garden, typically either by mixing grounds directly into the soil or creating a coffee ground “tea” (which I would argue is just weak coffee, but that’s just me) to pour into the dirt. To make a tea you want to mix about 2 cups of used coffee grounds in with roughly 5 gallons of water and let the mixture sit overnight before distributing it over the soil. There is a surprising extra benefit to using this as a fertilizer, but more on that shortly…

When putting grounds directly into soil you can either use dry unused coffee grounds, or the wet remains of what you brewed this morning (sometimes referred to as “spent coffee grounds” or SCGs). The type of plants you are growing should inform your decision on whether to use dry or spent grounds. The act of brewing coffee actually changes the acidity of the grounds - raw or dry coffee is relatively acidic (pH of 5.0-5.5) which, once brewed, adjusts over to being slightly more neutral (pH between 6.5-6.8). If you are using dry coffee, I would recommend keeping it only on acid-loving plants like japanese maples, rhododendrons, azaleas, blueberries and others. 

Coffee as a slug repellent in your garden

Beyond just nutrient density, added aeration, and ability to add acidity to soil when needed, coffee has a surprise added benefit in your garden - repelling slugs and snails! Caffeine is fatally toxic to the pests and as little as 1-2% caffeine on the soil around your plants (either in tea form or plain coffee grounds sprinkled around the base) will adequately deter them. This seems to be a very effective slug management approach, as one study conducted revealed that the 1-2% caffeine solution caused 100% of slugs to leave the area and subsequently die from caffeine poisoning.

Conclusion

Coffee offers a wonderful opportunity to fuel not only yourself but also your soil. So, the next time you find yourself having one too many cups of coffee (which I may or may not be doing as I type this), you can tell yourself it’s okay because you're doing it for the sake of the garden!