I saved my own used K cups and asked my co-workers to save theirs over the course of a couple months. Then I gathered all of them, numbered them, and made a spreadsheet to keep track of the different types of coffee.
Then I ripped the foil off all the cups and dumped the coffee grounds into a plastic container. I had to be careful not to save any coffee grounds that contained mold.
I decided to use 3 different types of soil mixes for the tomato seeds: 1) Miracle Gro Gardening Soil, 2) coffee grounds 3) a combination of gardening soil and coffee grounds. The spreadsheet really came in handy here.