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Will Coffee Grounds Acidify My Garden Soil?

Every gardener has heard that adding coffee grounds to your compost speeds decomposition and adds much-needed nitrogen to the compost. Should coffee grounds be added directly to your garden soil? Will it acidify the soil or “burn” plant roots? Do coffee grounds help , blueberries, and other vegetables? Will they help and azaleas?

Coffee grounds can be an arrow in your quiver for improving your garden growth.

There are a million recommendations online (quite literally) about using coffee grounds to repel cats, boost soil nutrients, boost friendly bacteria, attract earthworms, kill slugs, prevent weeds, aerate soil and god knows what else. Some are legit, some urban myth.

Video on using Coffee Grounds in Soil

Coffee grounds as mulch

Coffee grounds by weight contain about 10% nitrogen-rich proteins, which are vital for seed germination and plant growth. 

“The carbon-to-nitrogen ratio of coffee grounds can be as low as 11:1, an ideal ratio for plant and soil nutrition. Since coffee is extracted in water, most of the hydrophobic compounds, including oils, lipids, triglycerides, and fatty acids remain in the grounds, as do insoluble carbohydrates like cellulose and various indigestible sugars. Structural lignin, protective phenolics, and the wonderful aroma-producing essential oils are also left over from the brewing process. It’s this last group of chemicals that are reported to have antioxidant and antimicrobial properties.”

According to Linda Chalker-Scott, Extension Urban Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University:


Coffee grounds are highly acidic, they will lower the pH of your soil, making it more acidic. As a consequence, they can be beneficial to acid-loving plants like blueberries, rhododendrons, and azaleas. But is this true? The science is not absolutely clear on this. In scientific studies, adding coffee grounds has shown varying results, from creating a mildly acidic finished compost of 4.6 to an alkaline compost with a pH of 8.4(!). When composted coffee grounds were added directly to garden soil, researchers found that the pH of decomposing grounds was not stable. After showing an initial increase in acidity levels, the soil pH decreased shortly thereafter.

Positive Results and Nutrients

Coffee grounds have shown substantially positive results. As with any good organic mulch, coffee grounds:

  • moderate soil temperature and moisture in the soil.
  • Bind pesticide residues and toxic heavy metals such as cadmium. As a consequence, they preventing their movement into the surrounding environment (very beneficial in keeping these pollutants out of local waterways).
  • Can increase the availability of important plant nutrients like nitrogen, phosphorous, iron and zinc.

One should note that each variety of coffee bean contains different acidity levels, which is further affected by the brewing method.

Should you add fresh coffee grounds to garden soil?

When added directly to garden soil, coffee grounds are broken down by specialized bacteria and fungi. As they decompose, humic substances are created – chemical and structural components – which are vital to the . Earthworms pull coffee grounds into the soil and use them as a food source which improves soil structure.

Though coffee grounds contain a high proportion of nitrogen, the primary ingredient for plant growth, not every plant responds well to them. In fact, fresh grounds can be poisonous to some plants (known as phytotoxicity), so never let the grounds come into direct contact with roots or stems.

Coffee grounds have been shown to suppress seed germination in many plants, and inhibit growth in some others. Scientists speculate that this is the result of certain toxic substances released from coffee grounds as they decompose, which might also be responsible for its reported effects on weed suppression. Other scientists have proposed that caffeine is the nutrient that causes this allelopathic activity. To be safe, never add coffee grounds directly to soil or use as a mulch where seeds are being germinated.

Coffee grounds control fungi and bacteria.

Coffee grounds show some remarkable properties in suppressing fungal rots and wilts. In studies under controlled conditions on cucumbers, spinach, and tomatoes, when grounds were used as part of a compost mix, they showed the ability to prevent certain pathogenic fungi and bacteria from establishing. No studies as yet show conclusive proof of their efficacy in disease suppression on other food crops, trees or shrubs.

Consider other uses for your Coffee Grounds

Coffee is an incredibly versatile plant, and coffee grounds are full of nutrients and good stuff even after a good how brew. We may not like the taste of what’s left, though! They have other uses for you to consider.

Conclusion

You can safely use coffee grounds up to 20% of total compost volume. More than that may be detrimental.  Twenty-percent total volume is sufficient for effective disease suppression and for building soil, and should not acidify your soil (pH levels vary according to type of soil, microorganisms present, rainfall, nature of compost and many other factors).

Using coffee grounds alone as a mulch is fine, but don’t lay them on too heavily, as they compact easily and may actually serve to block air and water exchange. It’s best to use coffee grounds in combination with an organic mulch, or layered underneath a mulch like wood chips to avoid compaction.

Coffee grounds will not permanently acidify garden soil as do peat moss or sulfur-based additives; they’ll only affect the acidity in the immediate area they’re added to, not the larger, surrounding areas. My best recommendation is to add coffee grounds to your compost along with dried, shredded leaves and grass clippings.

Annual Ground Cover Perennial Shrub Tree Vine