Spent coffee grounds turn a waste product into an asset in the garden. It’s estimated that 6 million tons of spent coffee grounds end up in landfills per year globally (ICO).
When 6 million tons of food waste ends up in our landfills, that produces the equivalent of 2.2 million metric tons of carbon dioxide emissions - which is the same as having 470,000 cars driven for one year or the energy consumption of over 230,000 homes. (EPA 2021).
sources: EPA Cement Sector, Science Advances , EIA, EPA Green Vehicles, ICO, EPA Calculator
If you make coffee and have the option to compost it or add it into your garden, you can help decrease the amount of CO2e emissions by keeping spent coffee grounds out of the landfill.
Support the health of your soil and plants, along with the health of the planet by integrating spent coffee grounds into your garden.
in this guide, you’ll learn:
The main nutrient that coffee grounds add to the garden is nitrogen, which helps with green growth on your plants. Coffee grounds also have smaller amounts of phosphorus and potassium, along with trace amounts of minerals such as calcium, magnesium, iron, copper, zinc, and boron.
Spent coffee grounds have a NPK ratio of around 2 : .6 : .06. Their nitrogen levels are similar to the amount of nitrogen in goat manure, horse manure, or pig manure (source).
As a mulch around established plants, a thin layer of spent coffee grounds and help with water retention and temperature regulation.
It's best to be cautious when applying spent coffee grounds directly to the garden: Spent coffee grounds can hinder seed germination, so it’s not recommended to be mixed into seed starting soil, and shouldn’t be used as a mulch near areas where you are direct sowing seeds. It's also not recommended to integrate it into your soil, and you should keep it away from touching the base of plants.
However, a study by Washington State University shows that when spent coffee grounds are in the process of decomposing, the bacteria and fungi that help break the material down have been shown to help to prevent some pathogenic diseases in tomatoes and beans (WSU Extension).
When SCGs are not fully composted, they are a slow release fertilizer, as the nutrients will become available to the plants after bacteria and fungi break the material down. If you notice plants are in need of a nitrogen boost STAT, you might want to try another source.
Even though spent coffee grounds are a great source of carbon, they actually count as a “green” in your compost pile, due to their levels of nitrogen and proteins.
As we mentioned earlier, with nitrogen levels comparable to manure of other animals, coffee grounds can replace animal manure in the compost pile if you don’t have access to manure, would like to avoid potential pathogens from manure, or would prefer a vegan compost (OSU).
Coffee grounds can also help heat your compost pile up, which can also be a benefit to hot bed composting/gardening systems.
A study at the Lane County Extension, part of Oregon State University, found that if a compost pile is made up of 25% spent coffee grounds, “temperatures in the piles stayed between 135 degrees and 155 degrees for at least two weeks.” Similar amounts of animal manure kept temperatures hot for less time (OSU).
If the coffee grounds are spent (meaning, they've already been turned into delicious coffee), then their acidity level is around - pretty close to a neutral 7. Most of the acidity in the coffee gets washed out in the brewing process and ends up in the liquid beverage (OSU).
So adding spent coffee grounds won't change the pH of your soil in any significant way.
