Expert gardeners know that coffee grounds provide more than just a jump start of caffeine in the morning. A prime nitrogen source, coffee grounds improve soil structure and can further enrich your soil by attracting beneficial earthworms. But before you start tossing handfuls of coffee grounds across your garden soil, make sure you’re aware of the effects they may have on your soil pH.
The acidity of your soil plays an essential role in determining the fertility of your plants. According to Elizabeth Stell, author of “Secrets to Great Soil,” very low (acidic) or very high (alkaline) pH levels inhibit the ability of nutrients to break down in your garden soil. If these nutrients can’t break down, they won't be available for absorption by your garden plants. Ideally, your soil pH level should be between 6.0 and 7.5 to encourage vigorous growth for most plants. Selectively applied coffee grounds can help maintain this soil pH level.
Coffee grounds vary in pH value, depending upon whether you’ve brewed them. According to Science Daily, brewed coffee grounds typically possess a pH level that measures between 6.5 and 6.8, which fits into the preferred soil pH levels for most plants. Adding a 1/2-inch layer of used coffee grounds to topsoil and covering it with a 2-inch layer of dead leaves is an easy way to maintain a neutral pH value in your soil. Let it sit over the winter to compost directly on top of the soil before mixing it into the dirt right before spring planting.
Balance out slightly alkaline soil with unbrewed coffee grounds, which add more acidity to your soil than brewed. Mix a 1/2-inch layer of unused coffee grounds thoroughly with the top 4 inches of soil before planting time. Achieve the best results amending soil for plants that savor more acidic conditions, such as hydrangeas, azaleas and roses.
If you opt to compost your coffee grounds first before adding them directly to your plants as a nutrient-rich soil amendment, allow enough time for the process before you use the finished compost. With regular weekly aerating, your coffee ground compost should be ready for use in three to six months, provided you mix it with an equal amount of carbon-based materials, such as dead leaves, straw or shredded newspaper. Keep your compost about as damp as a wrung-out sponge to ensure adequate moisture levels for more rapid composting.
Some used coffee grounds may be more acidic than others, depending upon the length of brew time, the water temperature and the size of the coffee grounds. These could cause excessive acidity levels in your garden soil, which may negatively affect the health of your plants. Monitor the acidity level of your soil regularly, preferably at least once yearly, to assess the effect of the coffee grounds on the soil pH levels. Use a soil testing laboratory for most accurate results. Add lime to your soil to offset excessive acidity levels caused by coffee grounds.