The soil in this stretch of the Connecticut River valley packs down hard by the end of a summer — clay beneath the loam, and a season of feet and mower wheels on top of it. Dave's Lawn Aeration Service comes through in the cool weeks of September, when the mornings are damp and the ground gives, pulling cores and leaving them to crumble back in. A week later the lawn drinks differently, roots find room, and the color comes back deep instead of pale. It is one of those jobs nobody watches being done and everybody notices afterward. Across Springfield, from the river terraces to the flat ground out east, autumn is the window and the work follows it.