A reader recently asked us an interesting question about worms on the sidewalk. The subject matter isn’t strange – lots of people wonder why worms come out on the sidewalk – but his focus was somewhat unusual. He wasn’t curious about the presence of the worms, but rather about the “black balls of dirt” that came along with them onto the sidewalk. The reader asked “what” he was finding, which seems fairly simple – black balls of dirt (or possibly worm castings) – so we suppose we should expand the scope of the question, covering the matter of why he is finding dirt on the sidewalk along with worms.
In general, sidewalks are not good environments for worms. (Here and elsewhere, we’ll primarily take the definition of “worm” to mean earthworms, although some of what we say has applicability to other types of worms.) Worms must remain moist to breathe and are susceptible to drying out, so they tend to stay in the soil, which provides both moisture and protection from the sun. It is for this reason that you generally only find worms outside after it rains. The reader mentioned nothing about rain, but his email only contained one short question (or actually only a fragment of a question), so clearly his message didn’t contain all the relevant information. It therefore seems like a reasonable assumption that the reader found the worms on the sidewalk after it rained.
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And if the reader did in fact find the worms after it had rained, this gives rise to one possible explanation for why dirt was on the sidewalk: it was probably splashed onto (or flowed onto) the sidewalk as a result of the rain. This is a common enough occurrence after rain, which explains why it is often necessary to sweep once the rain has dried. As for why the reader specifically found “black balls of dirt,” we suppose this has to do with the soil surrounding the sidewalk. Lots of soil is black and essentially clumped into small balls – just think about picking up a scoop of garden soil, which is not like refined sugar, but rather consists of clumps of dirt of various sizes (at least largely). This is especially true when the soil is moist, as would obviously be the case after it rains.
And even if the rain itself didn’t cause the presence of the soil on the sidewalk, the worms themselves could have. Once again, worms spend most of their lives in the soil, so the fact that they could drag some of it onto the sidewalk is not at all unexpected. In fact, worms are covered in “slime,” basically a nitrogen-containing secretion, which is slightly sticky. This slime can hold soil together, creating small balls of dirt called “aggregates,” and if stuck to a worm, they could end up on the sidewalk if the earthworm crawls there.
All that has been said so far is based on the assumption that our reader is actually finding clumps of dirt or soil. However, it is also possible that he is finding worm castings (worm poop). Worm castings look a lot like soil and can often clump up into little balls. If our reader is finding worm castings, it is a bit of shame they are on the sidewalk, as they make excellent fertilizer.
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So, the reader perhaps found soil on the sidewalk because rain or worms or both brought it there. The soil is in balls because soil clumps up, and in fact worms even help this clumping process because of the slime they secrete (and also because of their activity in the soil). Alternatively, the reader is not finding soil or dirt at all, but rather worm castings. We hope that answers our reader’s question.