I get used coffee grounds from two local coffee shops, and I compost them before putting them on the garden. Read more on the composting forum on this web site

Here is a link that might be useful: Composting Forum

Using undecomposed organic materials in container gardening is controversial, so be prepared to hear differing opinions. I do use coffee grounds in my containers as a surface mulch. There's only so much you can add due to container volume, and I would advise not digging it in due to root damage. Even if you use it, you should still fertilize with a soluble fertilizer.

I hill it at the base of my veg plants. The insects dont like the feel of it which keeps them off the plants and it always boosts my plant growth and slow releases as you water.

but I understand there is a danger with nitrogen getting "washed out" from the soil (I mixed pro-mix in a 3-1 ratio with mushroom compost) whenever the plants are watered.

First, please understand that ALL nutrients wash out of containers with watering. it isn't just nitrogen.

Second, using un-decomposed organic materials is controversial because organic materials have to be acted on by the soil bacteria for their nutrients to be available to the plants. That takes soil bacteria being present in the container and it isn't in most containers because they are filled with soil-less potting mix. And TIME. Nothing organic is quick-acting.

Since you added compost to your containers you likely have at least some soil bacteria in there, some of the "micro-herd". But time will still be required. You can reduce some of the time required by adding your organic materials in a liquid form. Adding them in a liquid form also eliminates the "room in the container" and the "disturbing the roots" issues.

So if you want to use the coffee grounds use them as a liquid. But keep in mind that you will have to find something else to add for the P and the K and the needed micro-nutrients.

Dave

Thanks, everyone. Great info, Dave! I'll let the grounds steep in the water I use on my plants but find some additional sources of fertilizer as well.

I do not use compost teas of any kind, I mix the UCG in the soil.
But many persons use the compost tea/coffee, so it should work for you.
To stop the washing out of nutrient, use a lid, water tray to caught the water. I don't water my container, I put the water in the tray & let the plant wick the water up into the container. There is less washing & no lost, because you do not over water & the tray holds any washed nutrients.

I had completely run out of browns for the compost pile. UCG are very GREEN and today wifey brought home four large trash bags full of shredded paper from work. Shredded paper is very BROWN. Yipeeee!