Well, since you asked, and I’m coincidentally enjoying my morning java: I tried an experiment years ago, planting seedlings in spent coffee grounds. just to see what happened. A white, fuzzy mold grew in the pot, probably harmless, but not good for seedlings. This was straight coffee grounds. The moldy grounds also became water repellant; also not good! Mixed with soil or compost, the rich nutrients in coffee get diluted and a plethora of soil microbes will start breaking it down. Coffee grounds are acidic, and best used composted or diluted/mixed into a larger volume of soil or compost. Microbes can stop decomposing if the environment gets too acidic for them, e.g. Tollund Man, an original “Bogman”, mummified in an acidic swamp.
Here’s an excerpt from a good article, from UC Master Gardeners:
“The best article I came across was from Sunset Magazine – they actually sent a batch of Starbucks grounds to a soil lab – Soil and Plant Laboratory Inc., in Bellevue, WA. The findings were encouraging – amending soil with up to 35% grounds will improve soil structure over the short and long term. They should be tilled into the soil to a depth of 6 to 8 inches and will improve the availability of phosphorus, potassium, magnesium and copper – negating the need for additional sources of these nutrients. In addition, each cubic yard of coffee grounds provides 10 pounds of nitrogen (0.09% available). The grounds will provide nitrogen in a slow release fashion for use by plants over the long term. It is an excellent soil amendment and is recommended to be used at a rate of 25 to 35% by volume to improve soil structure. The slightly acid properties of the grounds are also a welcome addition, especially here in the west where soil tends more towards the alkaline.”
This would indicate that in our region, where soils tend to be acidic, one would need to add a bit of limestone, such as powdered dolomitic lime, unless one were growing blueberries, azaleas or other acid-lovers.
My suggestion would be to either mix it with compost or mix it with 75% soil and don’t plant in it for a few weeks, let the microbes act on it and allow decomposers to multiply first. Those organisms help to release the nutrients from the grounds, often in chemical forms the plants can take up.
Coffee, being a seed inside a berry, is apt to be nutrient dense, like most seeds. As my Plant Anatomy and Physiology professor once said: “A seed is a little plant, in a box, with its lunch.”