By Cindy Robertson, Extension Master Gardener
Gardeners are always searching for organic and cost-effective ways to improve the health of their plants. One popular gardening myth is the belief that directly applying coffee grounds to garden soil will benefit your plants. Given that coffee grounds are so dark and rich, and that coffee provides us with so much energy and vigor, it is easy to see why even experienced gardeners might be fooled. In this article, we bust this myth and uncover the truth about using coffee grounds in your garden.
Coffee Grounds Composition
Coffee grounds contain a variety of compounds, including nitrogen, phosphorus, potassium, and trace amounts of other essential nutrients.[1] Although still a debated proposition, coffee grounds are not acidic, like the coffee we drink. Once brewed, the grounds are closer to pH neutral, between 6.5 and 6.8.[2] Despite this fact, many have been led to believe that coffee grounds can enhance soil acidity, fertility, and plant growth. While this notion holds some truth, we must consider the full picture to understand how to use coffee grounds to benefit our gardens.
Myth: Coffee Grounds as a Primary Fertilizer
The most common misconception about coffee grounds is that they can serve as a standalone fertilizer for plants. I fell for this notion and suffered the results. My household produces about a half a cup of coffee grounds every day. Wanting to put them to good use, I routinely spread three and half cups of coffee grounds around my acid-loving plants on a weekly basis, thinking it would work just like compost. Over time, I saw my once healthy plants turn yellow and drop leaves. Worse, I thought perhaps I had not added enough coffee grounds, so I continued to pile on more. This was altogether the wrong move.
I have since learned that coffee grounds, when used alone, do not provide a well-balanced nutrient profile required by plants. While they do contain nitrogen, the nitrogen in coffee grounds is in a form not readily available to plants. This means that the nitrogen needs to undergo decomposition and microbial activity before it can be released in a plant-available form. In fact, the decomposition of the coffee grounds competes for the nitrogen that would otherwise be feeding your plants, so rather than adding nitrogen to the soil, coffee grounds may actually deplete it.[3]
Researchers at the University of Melbourne determined through experimentation that even limited amounts of coffee grounds in soil had a detrimental effect on the growth of broccoli, leek, radish, violas, and sunflowers.[4] They concluded, “All horticultural plants grew poorly in response to SCG (spent coffee grounds), regardless of soil type and fertiliser [sic] addition.”[5]
Reality: Coffee Grounds Are Great for Your Compost Pile
Coffee grounds can contribute to soil health and plant growth indirectly, through composting and proper incorporation into organic matter. According to Cindy Wise, a compost program coordinator at Oregon State University Extension Service, incorporating coffee grounds into your compost pile adds valuable organic matter and contributes to the decomposition process.[6] She recommends adding grounds to your compost pile by layering one part leaves to one part fresh grass clippings to one part coffee grounds, by volume, and turning once a week for useable compost in three to six months.
Wise has observed that coffee grounds appear to help sustain high temperatures in compost piles. Higher temperatures are beneficial because they reduce potentially dangerous pathogens and destroy seeds from weeds and vegetables.[7]
Photo © Elaine Mills
Extension Master Gardener and seasoned composter, Joyce Hylton, also finds coffee grounds to be a vital part of her composting process. At the Organic Vegetable Garden, a demonstration garden managed by the Master Gardeners of Northern Virginia, coffee grounds in large amounts have made a significant difference in both the relative speed and quality of the decomposition process. Access to the used grounds from commercial coffee houses provides both volume and moisture to the compost pile, accelerating the composting process. Ms. Hylton has also noticed the beneficial impact of coffee grounds on a smaller scale, in her own garden’s compost pile, describing them as “an important ingredient.”
Other Considerations
It’s important to consider the source of your coffee grounds. If using store-bought grounds, be mindful of any additives or chemicals that may have been used during production. Organic coffee grounds are generally a safer option, ensuring that harmful substances are not introduced into your garden.
Conclusion
While adding coffee grounds directly to soil may not be the miracle solution for garden fertility, using coffee grounds in your compost pile can contribute to improved soil health and plant vitality. Remember, gardening is a science of balance, and diversifying your organic amendments will yield the best results for your garden. So go ahead and enjoy your cup of coffee, and then recycle those grounds into your compost pile—just don’t rely on them as a direct source of plant nutrition, and give them the time they need to decompose before they hit the soil. Happy gardening!
[1] Applying Spent Coffee Grounds Directly to Urban Agriculture Soils Greatly Reduces Plant Growth, Sarah J. Hardgrove, Stephen J. Livesley, Urban Forestry & Urban Greening, Vol. 18, 1 Aug. 2016, 1-8; available at https://www.sciencedirect.com/science/article/abs/pii/S1618866716300103 (hereinafter Urban Forestry and Urban Greening.)
[2] Coffee Grounds Perk Up Compost Pile With Nitrogen, Oregon State University, June 10, 2009, https://today.oregonstate.edu/archives/2008/jul/coffee-grounds-perk-compost-pile-nitrogen (hereinafter OSU.)
[3] Why You Shouldn’t Put Coffee Grounds Directly On Plants + What To Do Instead, Emma Loewe, Aug. 6, 2020, available at https://www.mindbodygreen.com/articles/coffee-grounds-for-plants-myth.
[4] Urban Forestry and Urban Greening, at 1.
[5] Id.
[6] OSU
[7] Id.