Coffee grounds (and brewed coffee) are a source of nitrogen for plants, which is the nutrient that produces healthy green growth and strong stems. Coffee also contains calcium and magnesium -- both of which are beneficial to plant health. To use coffee as a plant fertilizer, you'll need to dilute it. It should look like weak tea -- see the photo for an example. If you aim for about 1/4 coffee and 3/4 water in your mixture (depending on how strongly you brew your coffee), that's about right, but you don't have to be fussy about it. You can use coffee fertilizer on your potted plants, houseplants, or in your vegetable garden. Coffee and coffee grounds can be acidic, but since we're diluting it so much, that's not really a problem unless you're watering the same plant with it every day.