Every week I get about a five gallon bucket full of spent coffee grounds from various friends and neighbors. That may sound like a lot, but when it's spread across 21 1/2 acres, it's not all that much. But I'm glad to get those grounds and I'm not complaining.
Initially I added the grounds to the compost bins. And truthfully, I feel that is excellent way to use them. But I've also done a bit of experimenting just to see what would happen.....
1) Sprinkle grounds over the soil prior to tilling as one part of the numerous soil amendments to be tilled in between crops. I can't say that this makes the plants grow better or worse, because any difference is too small to notice. But I've noticed one huge difference. The tilth of the repeated coffee grounds treated garden beds is far better than the beds that haven't gotten the multiple applications of coffee grounds. The coffee ground treated soil is more crumblier (is that a word?), has a nicer feel to it. The change didn't happen quickly, but rather, over the years. But it's a noticeable change.
2) Till in a one inch thick layer of coffee grounds. This was an experiment just to see if coffee grounds made any change in the crop. I think that the bean plants were a tad less productive, but it could have been due to some other factors. Since I wasn't sure, I opted for the next experiment.
3) Till in a 3 inch thick layer of coffee grounds. They were tilled in for around 6"-8" deep. Now this time the difference in the plants was noticeable. The bean plants were smaller, had less leaves, looked to be a bit nitrogen deficient, and produced a smaller crop. Well, that was no big surprise. The grounds were fresh, not composted. Therefore as they decomposed in the soil, they robbed nitrogen in the process. That resulted in less nitrogen available for the bean plants. So I made note of the results but wasn't astonished. But one other side note -- since then, that garden bed soil has had significantly improved tilth.
Here are my observations and conclusions.....which very well might be off base since I'm no ag research biologist. Using coffee grounds via the compost piles works just fine. They don't seem detrimental to the crops and are another source of organic material for decomposing into plant nutrients. Tilling a light layer into the soil between crops seems to work fine too. But tilling in thick layers of non-composted coffee grounds results in nitrogen binding, just what would happen if I tilled in a lot of sawdust, dry grass clippings, tree leaves. So I conclude it's best to use non-composted grounds sparingly or compost them before using. One observation that took me by surprise was the effect on soil tilth. The grounds definitely improved my soil, making it feel lighter in my hands and more crumbly to the touch. I actually liked the effect, and therefore will continue to add coffee grounds for now on.
(Above- that some bed right after I used the hand pick to roughly mix in the grounds. I next added some coral sand and rabbit manure, chopped it in lightly, then seeded beans.)
Just a few more words for other gardeners who might start using coffee grounds.
1- I see that dried grounds tend to shed water. So I don't allow the grounds to stay on the soil surface where they would dry out, becoming hydrophobic. After a light rain they form a surface crust, shedding water even more so.
2- I take care not to over do the amount of grounds added to the soil at any one time. 1/2" seems fine from what I've observed. Adding a bit of some other nitrogen source along with the grounds may be a good idea. I routinely add a bit of rabbit or composted chicken manure to most of my garden beds between crops anyway as part of my bed preparation routine.
3- The coffee grounds I'm using are slightly acidic, but not overly so. Not a problem for me. When I use them in the compost piles, they do not change the pH of the finished compost. But then again, the grounds make up less than 10% of my compost mix. Plus I don't see them making a significant effect on the soil pH in the garden beds.