dubious muckheap treasures

I understand the presence of plastic toy horses in the allotment muckheap. The manure's delivered from local stables; I've been the kind of small child who carries wee toy horses around with me everywhere as a comfort object.

I'm significantly more confused by the small ?terracotta orbs, lots of small bubble-esque indents on their surfaces, which keep showing up. I've collected most of a ramekin of them at this point; I have absolutely no idea what they think they're for. They almost look like baking beads.

I pretty much just find the chafer grubs charming, now that I know what they are.