Used coffee grounds are a great natural, chemical-free way to fertilise your garden plants and grass. Since they are acidic, they are especially good for fertilising acid-loving plants like azaleas, rhododendrons, potatoes and blueberries. They can also be used directly on grass and in clay-based alkaline soils. For non-acid-loving plants, grounds should be mixed in with compost for a more well-rounded fertiliser.
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Composition of Coffee Grounds
What makes coffee grounds so useful as a soil amendment is the amount of nutrients they contain and how they are absorbed. In a study done on the use of Starbucks coffee grounds, it was found that the grounds contained 2.28 per cent nitrogen, 0.06 per cent phosphorous and 0.6 per cent potassium. Magnesium and copper were also found in the grounds. The pH of the grounds was slightly acidic, charting in at 6.2 on the pH scale.
While the nitrogen found in the grounds needs to be broken down by soil microorganisms so that it converts to a form plants can absorb, the potassium, magnesium, calcium and copper are all readily absorbed. Over time, the grounds essentially act as a slow-release fertiliser with some of the nutrients available immediately and some available over a period of time.
Fertilising Plants and Shrubs
For acid-loving plants, used coffee grounds can be mixed directly into the top 6 to 8 inches of soil. With shrubs, grounds can be placed around the bases of shrubs and trees. Leftover brewed coffee can also be applied in the same way. For the non-acid-loving members of your garden, mix the used grounds in with other compost before applying.
Fertilising grass with coffee grounds is a much bigger endeavour than merely hitting a few plants and shrubs with leftovers from your morning brew. If you've got the grinds, though, there are a couple of ways you can go about spreading them on your grass. You can take a shovel and literally fling them around the lawn. Or, you can get a bucket and cut holes on the bottom and shake the grinds onto your lawn. Either way, you will need to get a broom and sweep any piles so that the grinds are evenly distributed and are not thickly covering patches of grass that need sun. As far as how much to use, a spread rate of one cubic yard per 1,000 square feet of lawn is sufficient. Lawns and plants can be fertilised with grounds every month or two.
Other Benefits to Fertilizing with Coffee Grounds
In addition to being a great natural fertiliser, coffee grounds have other benefits in the garden. Not only are they a natural slug repellent, but they also attract earthworms, who love to eat them. This leads to worm castings, which are one of the best natural soil amendments.
On an olfactory note, fertilising with coffee will also give you the most pleasant smelling yard for about a week or two. Much more pleasant than manure.
Where to Get Large Volumes of Grounds
If you are thinking of fertilising your garden with coffee grounds, you are either going to have to drink a heck of a lot of coffee in a short period of time or collect them from local coffee shops and restaurants. Starbucks has been known to offer bags of grinds free upon asking as part of their corporate recycling protocol. Grounds are usually emptied during the day into large white garbage bags. These bags are then emptied into one huge bin at the end of the day. Check with your local Starbucks or coffee shop and ask about taking grounds for your garden. Chances are they will be more than happy to have one less bag of trash to worry about.
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