What is the best way to prepare and add used coffee grounds as fertilizer (not in compost piles).
Oakland County Michigan
This is a difficult question to answer. Scientific research is being done on ways to utilize coffee grounds in an industrial setting but in the case of a home owner putting spent grounds around plants in the garden, I could find very little information. However, in a non scientific setting, people have been putting grounds on plants for a long time.
As an Expert is first and foremost dedicated to giving people information based on science. Therefore it's hard for me to come up with a definitive answer.
Woods Hole Laboratories in Woods Hole MA has done a lot of study of coffee grounds in compost settings. Here is what they found. First coffee grounds per se do not contain a significent amount of nutrients... some but not a lot. Therefore, they would not be an effective substitute for fertilizer.
Secondly, they are veryacid on the acid/akaline scale(around a 5). Some plants do well in acidic soil, some do not. The study recommends caution in applying spent grounds directly to plants for this reason.
However, the article did not state the amount of grounds being discussed. As I mentioned earlier, they were studying effects in a composting situation and suggest adding lime and ground leaves to the compost to soften the ph impact.
What does this mean for you?
Do you know the ph of your soil? Do you have plants that specifically enjoy an acidic soil.... azaleas, blueberries, rhodendrons , ferns. etc.
Coffee grounds, being short on nutrients, do add organic material to the soil. That is a good thing.
One article recommended rinsing the grounds before applying to lessen the acidic content.
It is possible to acidify the soil too much. My recommendation for you is to sprinkle coffee grounds around your plants (rinsed or unrinsed) being careful not to add too much to any one area. Don't let the grounds clump together. You should be ok.