“While coffee grounds contain a small amount of nitrogen, these kitchen scraps are not actually fertilizers—not yet,” Leslie F. Halleck, M.S., a certified professional horticulturist and author, explains. It turns out that coffee’s nitrogen, an essential nutrient that plants need to grow, is not readily available in the grounds immediately after brewing. In order to provide any real value to plants, these grounds need some time to break down in a compost pile.
“Organic matter needs to be decomposed first (through composting) to make individual nutrients they hold available to plants,” Halleck says.
Ironically enough, attempting to take a shortcut and place used grounds directly on soil can actually starve your plant of nitrogen in the long run. “As fresh organic matter begins to decompose through the decomposition cycle,” Halleck explains, “nitrogen can be immobilized by microbes in the soil, which can lead to a nitrogen deficiency in your plants.”
Soil researchers at the University of Melbourne demonstrated this in the lab when they directly applied spent coffee to the soil of broccoli, leek, radish, viola, and sunflowers. Though the grounds seemed to increase the amount of water the soil could hold, they ultimately hampered growth for all five types of plants.
Uncomposted kitchen scraps of all kinds can also create the conditions for mold as well as attract unwelcome fungus, bacteria, and fungus gnats to your plant, adds Halleck.
Jonathan Russell-Anelli, Ph.D., a senior lecturer in Cornell’s School of Integrative Plant Science, also finds fault with most recommendations to use kitchen scraps as fertilizers. “Kitchen recipes are generally based on garden folklore and can quickly damage gardens if not understood and used correctly,” he tells mbg.