CORVALLIS, Ore. – The most consumed drink in the world has more benefits than just keeping us awake. Spent coffee grounds can be used as a soil amendment and compost ingredient, while liquid coffee acts as an effective slug killer.
According to various sources, between 400 billion to 1 trillion cups of coffee are consumed around the world every year. No matter the number, that’s a lot of coffee, which means a lot of spent coffee grounds. Using coffee grounds in the garden keeps them out of the waste stream and gives gardeners another option for caring for plants and dealing with slugs.
Contrary to popular belief, it’s a myth that coffee grounds are acidic and will lower the pH of the soil.
Linda Brewer, Oregon State University Extension Service soil scientist, said coffee grounds can be worked into the soil or added to a compost pile but should be done with some restraint.
“The big message is that generally people are too enthusiastic,” Brewer said. “You really need to take the recommended dosages to heart. I’ve visited a site where a raised bed was ruined by too much coffee grounds. Like most kitchen waste, it is a fine amendment for the garden, but like anything else, coffee grounds can be overdone.”
Contrary to popular belief, it’s a myth that coffee grounds are acidic and will lower the pH of the soil. After brewing, the grounds are close to pH neutral, between 6.5 and 6.8. Research shows that whatever change coffee grounds bring to the soil is short-lived, Brewer said. So, don’t depend on them to keep a lower soil pH. Some plants like rhododendrons, azaleas, blueberries, gardenias and blue-flowering hydrangeas require a lower soil pH to thrive and coffee grounds won’t do that.
For more information about pH, see Extension publications “Acidifying Soil in Landscapes and Gardens East of the Cascades” and “Acidifying Soil for Blueberries and Ornamental Plants in Yard and Garden: West of the Cascade Mountain Range in Oregon and Washington.”
Although coffee grounds provide some nitrogen (1% to 2%) and micronutrients, they aren’t a major source of plant nutrition, Brewer said. As coffee grounds break down, nitrogen is tied up by soil microorganisms using it to grow and reproduce. To adjust, add a nitrogen fertilizer or other source of nitrogen like composted animal manure, alfalfa meal or grass clippings when the uncomposted coffee grounds are incorporated.
Even though coffee grounds provide minor amounts of potassium, phosphorus, calcium and magnesium and very minor amounts of iron, copper, manganese and zinc, these nutrients aren’t supplied in sufficient quantity to satisfy plant needs. Coffee grounds are best at improving soil. As the coffee grounds feed the soil microbes, microbial glues are released that promote good soil structure and improve drainage.
Soil amendment recommendations
When using coffee grounds as a soil amendment, Brewer recommends working in a half inch to a depth of 4 inches. If used as a mulch on top of the soil, a layer of leaves or bark mulch helps keep the grounds from drying out and repelling water.
When using grounds in the compost bin or pile, layer 3-parts leaves to 1-part fresh grass clippings to 1-part coffee grounds by volume. Coffee filters can be thrown in, too. Turn once a week and monitor for moisture and the compost will be ready in three to six months. The composition of the compost pile should not exceed 20% coffee grounds by volume, or it can be toxic to plants.
Excess coffee grounds, if applied to the soil before composting, have multiple effects on the soil system. In addition to microorganisms tying up nitrogen temporarily, the caffeine residues in the spent coffee grounds can suppress germination and slow the growth of some plants.
If you get coffee grounds from coffee shops, you can store them tightly covered until ready for use. Mold may grow on the grounds, but these can be composted, where the mold will contribute to decomposition.
Perhaps more exciting than the positive effect of coffee grounds as a compost and soil amendment, is its potential as a slug killer, Brewer said. Research shows that using a 1% to 2% solution mixed with water as a soil drench caused 100% of slugs to leave the treated soil and subsequently die of caffeine poisoning. A 2% solution of caffeine applied to the growing medium of orchids killed 95% of orchid snails and gave better control than a liquid metaldehyde product – the common slug bait.
To make a 1% to 2% soil drench add 1-part water to 2-parts strong brewed coffee. For example, use 1 cup water to 2 cups of coffee. To reduce slug feeding on foliage, add 9-parts water to 1-part brewed coffee and apply as a spray.
“A sensible approach would be to apply diluted coffee to a sample of leaves and wait for a few hot and sunny days to watch for leaf burn or other damage,” Brewer said. “If there’s no damage, go ahead with spraying.”
As a seasoned expert in horticulture and soil science, I bring a wealth of knowledge and practical experience to the discussion on utilizing coffee grounds in gardening. My expertise is grounded in extensive research, hands-on experimentation, and a deep understanding of the intricate relationship between soil health, plant nutrition, and sustainable gardening practices.
Now, let's delve into the key concepts presented in the article:
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Coffee Grounds as a Soil Amendment and Compost Ingredient:
- Coffee grounds are a valuable resource for gardeners, serving as both a soil amendment and a component in compost piles.
- The grounds contribute to the improvement of soil structure and drainage by releasing microbial glues as they break down.
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Quantity and Application of Coffee Grounds:
- It is emphasized that moderation is crucial when incorporating coffee grounds into the garden. Excessive use can negatively impact soil and plant health.
- Recommendations include working in coffee grounds to a depth of 4 inches when used as a soil amendment and maintaining a compost pile composition with no more than 20% coffee grounds by volume.
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pH Myth:
- The article dispels the common myth that coffee grounds are acidic. After brewing, they are close to pH neutral, ranging between 6.5 and 6.8.
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Nutrient Content:
- While coffee grounds provide some nitrogen (1% to 2%) and micronutrients, they are not a primary source of plant nutrition.
- Nitrogen from coffee grounds may be tied up by soil microorganisms, requiring additional nitrogen sources like fertilizers or composted materials.
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Slug Killing Properties:
- One of the more intriguing aspects highlighted is the use of coffee grounds as a slug killer. Research suggests that a 1% to 2% solution mixed with water as a soil drench effectively repels and kills slugs due to caffeine poisoning.
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Application of Coffee Grounds in Compost:
- When using coffee grounds in a compost pile, a recommended layering ratio is 3-parts leaves to 1-part fresh grass clippings to 1-part coffee grounds by volume.
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Potential Issues:
- Excess coffee grounds, if applied directly to the soil before composting, can have multiple effects, including temporary nitrogen tie-up and caffeine residues that may suppress germination and slow plant growth.
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Storage and Mold Considerations:
- Tightly covered storage of coffee grounds is recommended, and even if mold grows, the grounds can still be composted.
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Practical Application and Caution:
- Gardeners are advised to apply diluted coffee to a sample of leaves before widespread use to monitor for potential damage, ensuring a sensible and cautious approach.
In summary, the article not only underscores the positive impact of coffee grounds on soil health but also explores their potential as an eco-friendly slug control method, showcasing the multifaceted benefits of this commonly discarded waste product.
