OK, I received this reply from the extension office and I think its correct:
This question is asked often. Bumps in the lawn that make it uneven, hard to work on and plain unsightly, could, sometimes, actually be a god thing. From the enclosed pictures it appears that you have an army of earthworms and night crawlers providingâ€biological aeration†for your lawn. These little acorn-like hardened structures are called ‘castings’ with a tiny hall in the middle that allow air to get deeper down. Thinning of the lawn can also be a sign, among else, of more compacted soil. Earthworms too need oxygen to survive and take some (for many of us) unexpected steps to overcome air deprivation… they chew through the sod and thatch, come to the surface for a breath of fresh air. You may see more activity in spring and fall with increased rain events. The excess moisture will cause them to move towards the surface to get more air. Believe it or not, these bumps are an indication of improved soil texture…below each one of those castings there is about ¼†diameter hole/tunnel left behind. Now, the question is what to do about it? Your lawn would benefit from dethatching whether you use the dethatching rake or have the core-aeration done by lawn maintenance firms. After that application, over-seeding is strongly recommended followed by regular fertilization program to keep your lawn strong and thick so the bumps will not be much of a problem.
I am planning to rent an aerator/coring/machine and I am also interested in the organic program you mentioned earlier. I welcome your advise on how to proceed. Thank you
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