Will Coffee Grounds Make My Garden Soil Acidic?

Every gardener has heard that adding coffee grounds to your compost speeds decomposition and adds much-needed nitrogen to the compost. But what about adding coffee grounds directly to your garden soil? Will it make the soil acidic or “burn” plant roots? Do coffee grounds help tomatoes, blueberries, and other vegetables? Will they help rhododendrons and azaleas?

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. I’ve also seen articles on watering your plants with leftover coffee and its supposed benefits (highly suspect). So, I thought it was time I looked into the published science to see what the reality is on using coffee grounds in your garden.

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Should you use coffee grounds as mulch or compost?

Coffee grounds, by weight, contain about 10% nitrogen-rich proteins, which are vital for seed germination and plant growth.  According to Linda Chalker-Scott, Extension Urban Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University, “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.“

In tests, coffee grounds have shown substantially positive results as a mulch on garden soil. As with any good organic mulch, they moderate soil temperature and moisture. But coffee grounds also bind pesticide residues and toxic heavy metals such as cadmium, preventing their movement into the surrounding environment (very beneficial in keeping these pollutants out of local waterways). Coffee grounds have also been shown to increase the availability of important plant nutrients like nitrogen, phosphorous, iron, and zinc.

Do coffee grounds make soil acidic?

Common wisdom assumes that since coffee grounds are highly acidic, they will lower the pH of your soil, making it more acidic and 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 to compost has shown varying results, from creating a mildly acidic finished compost of 4.6 to a highly 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.

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 quality of your soil. Earthworms pull coffee grounds into the soil and use them as a food source, which improves soil structure.

As stated earlier, coffee grounds contain a high percentage of nitrogen, the primary ingredient for plant growth. But 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 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 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 the soil or use them as a mulch where seeds are germinated.

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Coffee grounds control fungi and bacteria.

Coffee grounds show remarkable properties in suppressing fungal rots and wilts, including Fusarium, Pythium, and Sclerotinia. In studies under controlled conditions on beans, cucumbers, spinach, and tomatoes, when coffee 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.

How to use coffee grounds safely in your garden

• 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 building soil, and should not acidify your soil (pH levels vary according to the 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 block air and water exchange. To avoid compaction, it’s best to use coffee grounds in combination with an organic mulch or layered underneath a mulch like wood chips.
• 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 pesticide-free grass clippings.

Sources: Coffee grounds—will they perk up plants? and Using Coffee Grounds in Gardens and Landscapes, Linda Chalker-Scott, Ph.D, Washington State University Extension.