You can plant acorns directly in the ground without the seedling tray. Still, if you insist upon using the tray, you need to add fertilizer, which you get from cows where their dung is then put into fertilizer storage and then turned into fertilizer. The cows need to be milked before they will produce dung though.
In other words, there are several steps to get the whole process going. Using the seedling trays gives you some extra seedlings compared to simply planting acorns in the ground, so it is worth the hassle.
I'd suggest when you are starting though, just build a general storage container to hold acorns that one bot collects and then another one takes out of that container to plant.