Coffee Grounds and Eggshells: Garden Myths vs. Facts

This began with a simple question from a friend, "Which plants like coffee grounds and which plants like eggshells?" You may be surprised by my answers that dispel these common garden myths.

Coffee Grounds

Used coffee grounds will not acidify the soil around your blueberries, rhododendrons, and azaleas. In fact, coffee grounds are more likely to raise the soil pH well above optimal levels for these acid-loving plants.

If you need a science refresher, the pH scale measures how acidic or basic (alkaline) a substance is. The scale ranges from 0 (strongly acidic) to 14 (strongly basic), with 7 as neutral. The pH scale is logarithmic, which means that a drop of one pH unit means the soil becomes 10 times more acidic. The recommended soil pH for most garden vegetables is 6.2 to 6.8. The ideal soil pH for blueberries is 4.5 to 5.0. Blueberries do best in soil that is 100 times more acidic than your vegetable garden's soil!

Aren't coffee grounds acidic? Freshly ground coffee and brewed coffee are acidic, but spent coffee grounds are nearly neutral, with a pH of 6.8. Adding spent coffee grounds to the soil around your favorite acid-loving plants raises the pH, making essential nutrients unavailable to blueberries and other acid lovers. It does more harm than good. Instead, add the coffee grounds to your compost pile to improve that organic soil amendment. 

Eggshells

I've heard many folks say they always add broken eggshells to the bottom of the hole when transplanting tomato plants to prevent blossom-end rot. Some growers even start tomato seeds in an empty eggshell to guarantee that the seedling gets plenty of calcium. However, neither of these methods is supported by research.

Drought or uneven watering prevents a plant from transporting calcium to its growing ends. Blossom-end rot is rarely caused by a lack of calcium in the soil. Tomato roots rely on a steady flow of water to carry calcium from the soil up to the growing fruit. If the soil dries out, the plant cannot transport calcium, and damage appears on the blossom end of the fruit.

Chicken and all bird eggshells are rich in calcium carbonate, but eggshells can take decades to decompose. Archaeologists find eggshells in century-old kitchen middens. Crush the eggshells into fine bits before adding them to the compost pile. Pulverized pieces decompose to give your plants the benefits of calcium in only a few years.

Sanitize Eggshells

Eggs can carry bacteria, such as Salmonella, on their surfaces, both internally and externally. A well-managed commercial or community compost pile may reach 140 to 160°F, which kills many harmful microorganisms. However, most home compost piles never reach or maintain those high temperatures.

A simple method to sanitize eggshells is to bake them at 200 to 250°F for 10 to 20 minutes. Pouring boiling water over them will also kill pathogens.

Soil Test, Don't Guess

Don't guess about the pH and nutrient levels of the soil in your garden or landscape bed. Soil fertility test kits are available at local Penn State Extension offices. Instructions and documents are also available online at Penn State's Agricultural Analytical Services Laboratory. The report not only includes pH and essential nutrient measurements but also the quantities of amendments needed to achieve optimal conditions for the plants you want to grow.