CORVALLIS, Ore. — The world’s most popular drink offers more than just a caffeine kick. Spent coffee grounds can enrich soil and compost, while brewed coffee itself can help manage slugs.
“The big message is that generally people are too enthusiastic. You really need to take the recommended dosages to heart. I’ve seen raised beds ruined by too much coffee."
An estimated 400 billion to 1 trillion cups of coffee are consumed worldwide each year. That’s a lot of grounds that might otherwise end up in the waste stream. Using coffee grounds in the garden keeps them out of landfills and gives gardeners another option for improving soil and controlling pests.
Linda Brewer, a soil scientist with the Oregon State University Extension Service, said coffee grounds can be valuable for gardens and compost piles — if used in moderation.
“The big message is that generally people are too enthusiastic,” Brewer said. “You really need to take the recommended dosages to heart. I’ve seen raised beds ruined by too much coffee. Like most kitchen waste, it’s a fine amendment, but coffee grounds can be overdone.”
Myths about acidity
It’s a common misconception that coffee grounds are acidic. After brewing, they’re close to neutral — around pH 6.5 to 6.8. Any pH change in the soil is temporary, Brewer said.
That means coffee grounds won’t lower soil pH enough to benefit acid-loving plants such as rhododendrons, azaleas, blueberries, gardenias and blue-flowering hydrangeas.
For more on soil acidity, visit 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.
Coffee grounds add structure, not major nutrients
Although coffee grounds contain 1% to 2% nitrogen and small amounts of micronutrients, they aren’t a major source of plant nutrition. As the grounds break down, soil microorganisms temporarily tie up nitrogen while they grow and reproduce. To balance this, add a nitrogen-rich source such as composted manure, alfalfa meal or grass clippings when incorporating fresh coffee grounds.
Coffee grounds contribute small amounts of potassium, phosphorus, calcium and magnesium, and trace amounts of iron, copper, manganese and zinc. However, these aren’t sufficient to meet plant needs. The main benefit comes from improving soil structure and drainage. As soil microbes feed on the grounds, they release compounds that bind soil particles into stable, well-draining aggregates.
How to use coffee grounds in soil and compost
When using coffee grounds as a soil amendment, Brewer recommends working in a half inch of grounds to a depth of 4 inches. If using as mulch, cover with leaves or bark mulch to prevent the surface from drying and repelling water.
In a compost pile, mix three parts leaves to one part fresh grass clippings to one part coffee grounds by volume. Coffee filters can be composted as well. Turn the pile weekly and monitor for moisture. The compost should be ready in three to six months. Coffee grounds should make up no more than 20% of the pile’s total volume — higher amounts can be toxic to plants.
If applied directly to soil before composting, excess coffee grounds can temporarily tie up nitrogen and may inhibit seed germination or slow plant growth due to caffeine residues.
If you collect grounds from coffee shops, store them in a sealed container until use. Mold may develop, but it’s harmless in compost and contributes to decomposition.
Coffee as a natural slug control
Coffee can also deter or kill slugs and snails. Brewer said research shows that a 1% to 2% caffeine solution used as a soil drench caused all slugs to leave treated soil and die of caffeine poisoning. A 2% solution applied to orchid growing media killed 95% of snails — outperforming a common slug bait containing metaldehyde.
To make a 1% to 2% soil drench, mix one part water with two parts strong brewed coffee. For foliar sprays, dilute further — nine parts water to one part brewed coffee — to reduce slug feeding on leaves.
“A sensible approach would be to apply diluted coffee to a few sample leaves and wait a few hot, sunny days to check for leaf burn or other damage,” Brewer said. “If there’s no damage, go ahead with spraying.”
Previously titled Used appropriately, coffee grounds improve soil and kill slugs
