Could they just be leftover slow-release fertilizer balls?

Most ferns form rhizomes, which are basically horizontal stems under the soil. They also reproduce via spores but those are smaller than the naked eye can see. Spores develop directly into little plants on the surface of the soil, so the little "balls" are probably not associated with the ferns.

I have noticed these little bulb like thingies stuck to the root ball area. They are definitely something to do with the roots, but I tried planting them and no fern emerged...