I don’t know about you, but my mornings nearly always start with coffee. I’ve been making and drinking coffee for a really long time. And all that coffee brewing ends up leaving behind a lot of coffee grounds. It is estimated that Americans drink 700 million cups of coffee a day, which translates into nearly 17 million pounds of coffee grounds per day!
There are many claims made about coffee grounds in the garden. It is said that they repel cats and insect pests, prevent weeds, kill slugs, add nitrogen, aerate the soil, and attract earthworms. Until recently, however, there wasn’t much real scientific proof behind any of those claims.
Luckily, that has changed. The Washington State University Extension Office recently published a study that tells the truth about coffee grounds in the garden.
Chemicals in coffee grounds
Coffee grounds contain a surprising 10% nitrogen after brewing. That’s some good plant nutrition right there. Coffee grounds also contain cellulose (carbohydrates), structural lignin (woody plant parts), triglycerides (oils), lipids (fats), protective phenolics (flavonoids/non-flavonoids), and essential oils. There may also be a small amount of caffeine remaining.
Breaking down coffee grounds
After adding coffee grounds to the garden, fungi and bacteria work to break them down. These bacteria, in turn, help suppress many common fungal diseases, such as fusarium wilt, cavity spot (Pythium), and white mold (Sclerotinia). The caffeine and nitrogen decompose quickly, while other ingredients take longer.
Earthworms tend to pull coffee grounds deeper into the soil. This action aerates the soil, improves soil structure, and provides nutrients to deeper roots. The slower to decompose ingredients in coffee grounds provide humic substances (a fancy way of saying humus - the organic components of soil). In this use, organic does not mean chemical-free. The word organic refers to the fact that it came from living things. As coffee grounds decompose, the carbon-to-nitrogen ratio changes from high (25:1) to the ideal (10:1). The carbon-to-nitrogen ratio is important because soil microorganisms need both carbon and nitrogen when they feed. If the ratio is high (with more carbon), they will pull nitrogen from the soil. If the ratio is lower, these microorganisms will leave nitrogen behind for the plants.
Effects of coffee grounds in soil
Coffee grounds help moderate soil temperature, the same as any other mulch. Research has shown that coffee grounds bind to pesticide residues and toxic heavy metals. This is a really good thing because it keeps those chemicals from entering your plants.
Coffee grounds also help make nitrogen, iron, phosphorus, and zinc more easily available to plant roots, especially in alkaline soils common to the Bay Area. Contrary to common belief, coffee grounds do not consistently acidify soil. The pH of decomposing coffee grounds is very unstable and can range from mildly acidic to mildly alkaline. Plus, those changes in pH only affect the immediate area and not the soil profile as a whole.
How to get the most out of coffee grounds in the garden
Coffee grounds can be applied to the soil surface as a mulch (no more than 1/2” thick) or composted as a soil amendment. Fresh coffee grounds can cause light sensitivity (phytotoxicity) to above-ground plant parts, so direct contact should be avoided. Here are a few more interesting facts about coffee grounds in the garden:
So, enjoy that cup of java and let your garden make the most of what's left behind!
You can grow a surprising amount of food in your own yard. Ask me how!