Coffee grounds + eggshells for plants: Myth vs fact every home gardener should know

Coffee grounds and eggshells are two of the most popular kitchen scraps recommended for home gardening. The combination is often described as a natural, inexpensive plant booster, with coffee grounds supposedly adding nitrogen and eggshells supplying calcium. But does mixing the two really give plants a nutritional boost? The science suggests there is some truth behind the hack, but also plenty of gardening folklore. Used correctly, both can contribute to soil health, but neither is a magic fertiliser.

Myth: Coffee grounds make soil acidic

Fact: Used coffee grounds do not reliably lower soil pH.

This is one of the most persistent claims about coffee grounds. Many gardeners associate coffee with acidity and assume that leftover grounds will make the soil more acidic. But Oregon State University Extension soil scientist Linda Brewer says used coffee grounds are close to neutral after brewing, with a pH around 6.5 to 6.8. The University of Minnesota Extension similarly says coffee grounds have not been shown to consistently lower soil pH.


That means adding coffee grounds is not a reliable way to create acidic soil for plants such as blueberries, azaleas or rhododendrons. If a plant genuinely needs lower soil pH, gardeners should rely on an appropriate soil amendment and, ideally, a soil test rather than coffee leftovers.

Myth: Coffee grounds are a complete natural fertiliser

Fact: They contain nutrients, but their biggest benefit may be improving soil.

Spent coffee grounds do contain nitrogen and small quantities of other nutrients. Oregon State University Extension notes that they contain roughly 1% to 2% nitrogen, along with smaller amounts of potassium, phosphorus, calcium and magnesium. However, those nutrients are not present in sufficient quantities to meet all of a plant's nutritional needs.

Instead, coffee grounds can contribute organic material and support soil microorganisms. As microbes break them down, they can help improve soil structure and drainage. The University of Minnesota Extension also recommends coffee grounds as a useful addition to compost because they provide carbon, nitrogen and other compounds that feed soil organisms.

Myth: Eggshells instantly give plants calcium

Fact: Eggshells contain calcium, but that does not mean plants can immediately use it.

Eggshells are largely made of calcium carbonate, making them a potential source of calcium for soil. The University of California Agriculture and Natural Resources notes that eggshells are high in calcium carbonate and can be incorporated into soil.


However, simply tossing large pieces of eggshell around a plant is unlikely to produce a quick calcium boost. Eggshells break down slowly, meaning their calcium becomes available gradually.

There is also an important distinction when it comes to common gardening problems such as blossom-end rot in tomatoes. The University of Minnesota Extension says crushed eggshells do not prevent blossom-end rot. The condition is linked to the plant's ability to take up and move calcium, meaning simply adding more calcium to the soil does not necessarily solve the problem.

Myth: Coffee grounds and eggshells are a proven miracle combination

Fact: There is some scientific research on the combination, but that does not make it a universal plant food.

Interestingly, researchers have actually studied coffee grounds and poultry eggshells together. A 2022 peer-reviewed study published in Scientific Reports examined mixtures of spent coffee grounds with chicken and duck eggshells as potential soil improvers and organic-mineral fertiliser components. The researchers investigated the chemical properties of these mixtures and their potential agricultural applications.

That is useful evidence that the combination has scientific interest. But it should not be interpreted as proof that every houseplant or vegetable will grow better simply because coffee grounds and eggshells are mixed together.
The right question is not whether the ingredients are “natural”, but whether the soil and plant actually need what they provide.

So, should you use coffee grounds and eggshells?

For gardeners who want to recycle kitchen waste, both can have a place in the garden. Coffee grounds are particularly useful as a compost ingredient and can contribute organic matter and nutrients to the composting process. Oregon State University Extension recommends using them in moderation rather than applying large quantities directly to soil.

Eggshells can also be added to compost or used as a calcium-containing soil amendment when appropriate. UC Agriculture and Natural Resources recommends rinsing, drying and crushing shells before incorporating them into soil. For both materials, preparation and moderation matter more than simply dumping kitchen scraps around a plant.