By: Sandy Feather ©2008
Penn State Extension
Q. I have heard that some gardeners use coffee grounds as a plant fertilizer in their gardens. Is it possible to use coffee grounds, or liquid coffee, as a fertilizer for my houseplants?
A. It is best to compost coffee grounds prior to using them around plants, indoors or out. Fresh coffee grounds applied around plants can cause them to suffer a temporary nitrogen deficiency, until soil-dwelling microbes break the grounds down into a form where their nutrients are available to your plants. There is also a chance that a layer of coffee grounds around a plant can pack down and impede water infiltration and aeration of the soil. By composting them first, you avoid these problems.
Fresh coffee grounds support fungal growth and attract fruit flies. While these side effects may not harm your plants, they certainly create a nuisance you may wish to avoid indoors.
Materials for composting are divided into “greens” and “browns,” depending on their carbon-to-nitrogen ratio. Materials high in nitrogen are classified as greens while those high in carbon are classified as browns.
Coffee grounds have a carbon-to-nitrogen ration of 20:1, and should be considered green material in your compost pile. The Environmental Protection Agency recommends that coffee grounds not comprise more than 25 percent by volume of a compost pile.
Other examples of “greens” include animal manure (not cat or dog), fruit and vegetable peelings, and fresh grass clippings. “Browns” include autumn leaves, straw, and wood chips. Composting proceeds most efficiently when the carbon-to-nitrogen ratio of a compost pile is 30:1, but compost happens even if the ratio of a pile is not perfect. Simply add alternating layers of green and brown materials. Start with a one-to-one ratio by volume and adjust as needed. If your pile has a foul ammonia odor, it is probably too rich in greens. Add more brown materials and mix in thoroughly to help aerate the pile. If nothing seems to be happening, add greens to heat the pile up.
Although I find that coffee grounds have an analysis of 2.1-.3-.3 I could not find an analysis for the beverage coffee. The analysis is the percentage of NPK (nitrogen-phosphorus-potassium) in a container or bag of fertilizer and is represented by the three numbers prominently displayed on the label. They are always listed in that order. I could not establish the beverage’s usefulness as a fertilizer.
However, it could be a useful amendment to maintain an acidic soil pH for houseplants that prefer that, including African violets and Norfolk Island pines, as long as you use it sparingly. While the beverage is quite acidic, pH tests have revealed that coffee grounds are only mildly acidic, ranging from 6.5 to 6.9. pH is measured on an exponential scale of 0 – 14, with values below 7.0 acidic and those above alkaline.
Coffee’s acidity is removed during the brewing process, leaving the grounds with a near-neutral pH. The problem with such home remedies is that it is hard to know how much is safe to use or how often to use it to obtain the most benefit, especially if you do not know the pH of the potting mix the plants are growing in. The limited volume of soil in a pot does not have the same buffering capacity as a much larger volume of soil in the landscape; it is all too easy to overdo something that seems like it should be good for our plants.