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Depending on how you use them, coffee grounds may be good for plants. Gardeners have long used coffee grounds to improve their soil or to create a pest-controlling mulch around their plants. But given recent scientific research, gardeners would be wise to assume that they’re doing more harm than good to their plants, and use coffee grounds only in the right conditions.
The Benefits of Gardening With Coffee Grounds
Coffee is the second-most traded commodity in the world, after petroleum, and research into its effects on human health is abundant and ongoing. While less abundant, research on coffee’s effect on plant growth continues to evolve. Search the internet for “plants coffee grounds” and you’ll find hundreds of gardening websites promoting the use of coffee grounds, praising their nitrogen-rich quality and ability to help plants photosynthesize. Perhaps you’ve already read that grounds also help the soil retain water, repel slugs and snails, and improve the structure of the soil, and that they are especially beneficial for acid-loving plants.
Some of this is indeed true and comes from reputable sources that have conducted their own independent research. Coffee grounds have been shown to improve water flow and soil structure. Used by itself, finely ground coffee is easily compacted, and can act as a barrier to moisture and air movement, but when coffee grounds are mixed with a variety of other types of organic material, they improve water retention and air circulation. Peer-reviewed research conducted at Washington State University recommends that coffee grounds constitute no more than 20% of total compost volume.
Coffee grounds also add nutrients to the soil. Compost specialists at the Oregon State University Extension Service concluded that coffee grounds help sustain ideal temperatures in a compost pile in order to accelerate decomposition. They also determined that coffee grounds are about 2% nitrogen by volume, which is necessary for foliage growth. Another study put the nitrogen content at 10%. Research conducted by the Soil and Plant Laboratory and others have revealed that coffee grounds will improve soil levels of iron, phosphorus, potassium, magnesium, and copper.
What Coffee Grounds Cannot Do
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Any coffee lover who can taste the difference between a dark roast and a light roast could conclude that the acidity in coffee grounds can vary. Research bears this out. The Soil and Plant Laboratory Inc. found coffee grounds to have a pH level of 6.2, meaning they are somewhat acidic. Yet research published in the Journal of Applied Microbiology has found coffee grounds to be mildly alkaline. While fresh coffee grounds are highly acidic (and can be toxic to plants), the acid in coffee acid is water-soluble, meaning most of it ends up in your cup, not in your soil. Unless you plan on measuring the acidity of your coffee grounds, amending your soil with them may do little good for your acid-loving plants and little harm to your alkaline-loving plants.
Can Coffee Harm Plants?
Despite common lore, coffee will not stunt your growth. But it can stunt your plants’ growth. Research published in Urban Forestry and Urban Greening determined that the direct application of spent coffee grounds on soil significantly reduces plant growth. One of the main culprits: caffeine. Even after brewing, levels of caffeine remain in coffee grounds that are high enough to negatively effect seed germination and early plant growth. From an evolutionary perspective, this makes sense; for the same reason that acorns are acidic, coffee beans release toxins in order to inhibit the growth of competitors. Those same toxins can also inhibit the microbial activity that makes nutrients available to plants and can deter earthworms and other underground digesters.
Just as running the same grounds through your espresso maker produces a weaker cup of coffee, over time the phytotoxicity of coffee grounds declines and their benefits increase. As the grounds mineralize, they release essential macronutrients to the soil, which attracts microbiota that, in turn, make those nutrients available to plants. Indeed, recent research published in Applied Soil Ecology found a direct correlation between increased application of spent coffee grounds, leached of their toxins, and the abundance of beneficial soil fungi and bacteria that promote plant growth. Another study found that as the toxicity level of spent coffee grounds decreased, earthworm activity increased, with no apparent detrimental effect on earthworm health.
What’s a Gardener to Do?
Before you start adding coffee grounds to your soil, you might want to get your soil tested to see what it actually needs. Most garden centers sell simple pH test kits. The extension service of your state university should also be able to provide a more comprehensive determination of the amount of essential minerals in your soil. You can also do the simple “squeeze test” to determine the soil’s composition to see what kinds of amendments you might need to make. Grab a moist handful of your garden soil and squeeze it in your fist. If the clump immediately falls apart, your soil is too sandy. Good soil will hold its shape, but crumble if you start poking at it. If it doesn’t crumble at all, your soil is has too much clay. Depending on the results of these tests, you may want to add more or less coffee grounds to your compost or mulch.
Be wise in what you do with your coffee grounds. Using them as a mulch directly on your soil can inhibit water retention and air circulation, and have a negative effect on plant germination and early growth. But the indirect use of coffee grounds can have just the opposite effect. Adding a modest amount of coffee grounds to your mulch or compost pile can improve your soil. Follow the 20% rule: Apply one part coffee grounds to four parts other organic material. As coffee lovers know, too much of a good thing is not a good thing.
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