I have sandy loam soil on my property. We get regular ant colonies in various areas of our backyard where they will colonize a patch, perhaps 2'x2' or slightly larger, and dig their colony underneath.

This area always has rich grass, and the weeds are shortly completely eliminated. The grass doesn't necessarily come in thicker in that area - it generally remains thin like the rest of my lawn, but all the 'infill' of weeds is elminated.

Has anyone else seen this? Why / how does an ant colony have this effect?

My immediate thought is soil aeration... but if that is the case, the weeds would die because of the grass choking them out. But the grass isn't necessarily improved.

On the downside, once the ants move on in the fall, the colony tends to cave in, and that beautiful weed-free patch of grass collapses a beautiful, weed-free sinkhole 2-6" deep that needs to be filled and re-seeded come spring. On the flip side, I'm slowly renovating my lawn one ant colony at a time!