Coffee grounds were once thought to be good only for acid-loving plants, such as tomatoes, blueberries, and some fruit trees.
However, a study conducted by Wood’s End Research Laboratory in Maine found that when mixed with other yard waste .. compost … the ph was neutralized. So basically raw straight coffee grinds are not a great idea, the acidity varies, in some cases, it is too high even for acid-loving plants.
When added to compost, it wont significantly increase the acidity.
About a half-pound of damp coffee grounds in a bucket of water, mixed with crushed eggshells sitting in the sun and air for a few days and you have a good liquid fertilizer.
You can also mix coffee grinds right into the compost heap.
Coffee Grinds as Melon Fertilizer?
Old coffee grinds really do enhance the flavor of many crops, cantaloupe, tomatoes, melons. No study to prove it, but it is said you get bigger melons if you use coffee grinds.
As they break down, the grinds will release nitrogen into the soil. Other nutrients in Coffee Grinds are phosphorus, potassium, magnesium, and copper.
Coffee In Compost Pile
“Coffee grounds, as an organic material, can be added to your compost pile. Worms like coffee grounds, so you may want to put a layer of coffee on the bottom of your pile to attract worms.” Cornell University
They also serve well in Worm Composting