Next time you brew a pot of coffee, don't throw out the grounds. Instead, save them to use in your garden. They're a free source of nitrogen, enriching your soil without even requiring a trip to your local nursery for supplies. Here are some time-tested ways to put those coffee grounds to use.
Coffee grounds are a nitrogen source, which means even though they are brown, they are considered a green composting material, like plant debris and grass clippings. To actively compost, most experts recommend about one part green material to two to three parts brown, like leaves. Even if you don't adhere to active composting ratios, your coffee grounds should not comprise more than 20% of your compost pile. More than that could slow the composting process and otherwise negatively impact it.
If you use coffee filters, throw those in the compost too. (Keep in mind that white filters are bleached, so if you're a strict organic gardener, you may want to forego composting them.) Coffee filters compost quickly and worms love them; they're considered a brown composting material, which can be hard to come by in the lush peak of summer.
Add Coffee Grounds to Your Worm Bin
It seems even worms like a bit of coffee now and then. Just don't give them more than one cup per week and don't give them that full cup all at once; instead, divide it up over the course of several days. Worms can't handle excessive acidity, so resist the temptation to add more.
Use Coffee Grounds as Mulch
As the organisms in the ground slowly break down the coffee grounds, they add nitrogen to the soil and improve its overall structure. Earthworms also help work the grounds into the soil, further improving its texture. A thin layer of coffee grounds not only benefits the soil, the abrasive, sharp edges and coffee's natural acidity combine to make a good slug barrier. In fact, research suggests that caffeine is toxic to slugs, making it a double whammy. Don't add a thick layer of coffee grounds, because they will compact and form a solid crust that won't allow air or water through. No more than an inch of grounds will do the trick.
When using coffee grounds for mulch, the pH of the grounds neutralizes as they decompose, so you don't have to worry about them lowering the soil's pH.
Apply Coffee Grounds as Side Dressing
Side dressing is kind of like supplemental mulching or feeding. You add just a little material at a time to boost what is already there. As with mulching, only add about 1 inch of coffee grounds at a time. You might even want to mix the grounds with your compost and side-dress with that.
Brew Liquid Fertilizer
Steep two cups of grounds in a 5-gallon bucket, for six to 12 hours. Use this liquid fertilizer to water and foliar feed your plants. The benefits of foliar feeding are the subject of much debate, but as always, let your plants be your guide. If they do not appear to be doing well after feeding, stop feeding.
Making a liquid fertilizer from steeping the used grounds is not quite the same as using leftover coffee to water your plants. Leftover coffee is more acidic and has other compounds in it that are removed from the grounds. Some acid-loving plants enjoy the occasional watering with coffee, but they take their coffee black––skip the sugar and cream.
Coffee Grounds Perk Up Compost Pile with Nitrogen. Oregon State University Website