If you've ever complained about the bitter taste of a strong cup of coffee, it's because it's acidic ... and that just might be what your plants are craving in the morning. If you are growing acid-loving fruits (blueberries), vegetables (radishes and parsley) or flowers (azaleas, hydrangeas and rhododendrons), then they will certainly appreciate that morning cup of joe. According to gardening experts, fresh coffee grounds are acidic and used coffee grounds are neutral.
So, don't throw those coffee grounds out just yet. If you have a compost heap (a pile of organic refuse which decomposes to produce a rich soil additive), be sure to toss the used grounds along with the used coffee filters into the compost bin. Used coffee grounds and filters are nitrogen-rich "green compost" materials, which you need to balance out the carbon-rich "brown compost" (leaves, straw, hay, etc.). Severely unbalanced compost is likely to smell and become too hot, and it won't be usable for the soil. The ideal ratio is about 2-to-1 brown-to-green material.
It's important to know what is in your soil; is it already nitrogen rich? Since used coffee grounds can raise the nitrogen level, if it is already high in nitrogen, adding more might cause problems. A 2016 study in the journal Urban Forestry & Urban Greening claimed, "Applying spent coffee grounds directly to urban agriculture soils greatly reduces plant growth" and also decreases weed growth.
Plants, like people, need a balanced diet, so be sure to test your soil before adding anything. Old coffee grounds as fertilizers lack phosphorus and calcium and do not encourage blooms and ripening, however steeping two cups of used coffee grounds in a five-gallon bucket of water for several hours will make a nitrogen-rich liquid fertilizer for your garden. According to The Spruce, a helpful home adviser site, spread your used coffee grounds out onto a newspaper lined baking sheet, and allow them to dry completely before sprinkling them around the base of acid-loving plants. Mixing an amount of used grounds into the soil will help increase drainage, water retention and aeration, and attract earthworms and other helpful microorganisms in the soil.
If you have leftover, stale, unbrewed coffee grounds in your kitchen cupboard, use them when growing acid-loving plants, but keep them away from tomatoes, asparagus and most leafy greens. Several herbs also thrive in acidic soil. Rich coffee grounds provide nitrogen and give acid-loving plants energy to promote growth. Be sure to keep those fresh coffee grounds away from areas where your pets may play; the fresh coffee grounds are highly caffeinated and could cause a dangerous caffeine toxicity within 30 to 60 minutes of ingestion.
There are inexpensive do-it-yourself kits you can buy at local nurseries or garden departments to test your soil before adding anything that will affect its pH levels. You can also bring a sample of soil to your local agricultural extension office, which can provide low-cost or free testing. While not guaranteed as accurate, there are also home tests you can do with vinegar and baking soda. Collect soil from different parts of your garden. Combine two teaspoons of soil with a half-cup of vinegar. If it fizzes, it is alkaline. Or, you can muddy two teaspoons of soil with distilled water and add a half-cup of baking soda; if it fizzes, it is acidic.
A pH soil level runs between zero and 14. Under 7 is acidic, and over 7 is alkaline. There are a few other ways you can alter your soil's pH or provide it with additional nutrients. Lower the pH using coffee grounds or white vinegar and water. Raise the pH using pulverized dried eggshells or cooled fireplace ash. Dirty fish water is full of nitrogen and other nutrients and can be used in place of any fertilizer. Bury banana peels in the soil before planting to encourage vertical plant growth. Composted or aged manure is an organic fertilizer with nutrients such as potassium, nitrogen and phosphorus.
Once you determine the pH and composition of your soil, choose one of these methods to make your garden the best it can be this spring.