Plants, they’re just like us: In addition to water and sunlight, they need food to grow—which they get in the form of soil nutrients. With this in mind, it’s tempting to just go ahead and share your kitchen scraps with them. But while they may seem innocent enough, experts say that popular “kitchen fertilisers” like coffee grounds actually do a plant more harm than good when placed directly on soil. Here’s why, and what to do with those grounds instead.

Why You Shouldn’t Put Coffee Grounds On Your Plants

“While coffee grounds contain a small amount of nitrogen, these kitchen scraps are not actually fertilisers—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 immobilised 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 mould 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 fertilisers. “Kitchen recipes are generally based on garden folklore and can quickly damage gardens if not understood and used correctly,” he says.

How To Put Coffee Grounds To Better Use

1. Add them to your compost

In addition to nitrogen, coffee grounds can add phosphorus and potassium to your compost, both of which aid in plant growth. So, by all means, add the organic material to your compost bin, and, with any luck, you’ll have a nutrient-dense soil additive a few months later. Keep in mind that the finished compost should take up no more than 10 to 20% of a houseplant’s total soil volume, according to Halleck, since any more can lead to “moisture management or pest and disease issues indoors.” With outdoor plants, there is a little more leeway.
2. Make A Compost Tea

“If you prefer to use a liquid feed versus adding whole finished compost to your potting mix, you can ‘brew’ your own compost tea,” says Halleck. “Compost tea will also contain soluble nutrients from the compost, plus some beneficial microbes.”

Russell-Anelli also vouches for this quick-release fertiliser method. Make your brew by allowing your finished compost to “steep” in water and sugar. The finished product will deliver the valuable components of compost in a liquid form that can be added directly to your plant’s soil.

The Bottom Line

While spent coffee can help support plant growth, it needs to decompose in a compost pile first. So if you’re looking for a quick way to use up your grounds, maybe just turn them into a body scrub next time?

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