Another claim made for earthworms is that they "aerate" the soil. Compared with the aeration effected by a single plowing, this is insignificant. Even in a lawn, earthworm aeration is more imagined than real. Worm burrows are of small diameter and quite short in length (at least in summer) and practically always plugged with a wad of grass. Since the burrows are open only at one end and are lined with a thin coating of lime, the amount of air that can move in and out is infinitesimal. This "aeration" is not the same as the movement of gaseous vapors between particles of a well-aerated soil, where gas dispersion takes place in all directions.
The slight aeration that earthworms do provide is more than offset by the harm done in spreading infection. Here we are not concerned with the role of the earthworm as a carrier of diseases to animals, but with its part in spreading organisms of plant maladies. I have seen a valuable planting of delphinium, for example, destroyed by mustard seed fungus when earthworms pulled infected plant material into their burrows between the plants. In an adjoining planting of delphiniums the soil had been treated with arsenate of lead for Japanese beetle control (which, of course, destroyed earthworms in the process) . Here not a plant was infected.
Earthworms rely on lime for proper functioning of their digestive apparatus, and this limits them to soils that are neutral or alkaline. They are never found in strongly acid soils.
A LAWN EVIL
Another mark against the earthworm is its destruction of smooth lawn surfaces. Its ability to ruin putting greens is well known. The day-by-day deposit of castings is bad enough but can usually be partially overcome by regular mowing. The real trouble occurs in winter, particularly if snow falls early and remains on the surface most of the winter. Protected under a blanket of snow, soil remains unfrozen and worms are able to work night and day without interruption. The height of their mounds under snow may reach one to two inches but it is difficult to judge because, as snow melts, the mounds usually are broken and spread over an area of several inches.
Earth thus brought to the surface leaves the lawn so uneven that the owner is compelled to roll it and thereby compact the soil, which more than offsets any slight improvement in aeration effected by the worms. Where owners complain of rough, uneven lawns and "heaving" of grasses, I often recommend treatment with chlordane, calcium arsenate or dieldrin to rid the soil of earthworms (and grubs) so that the need for rolling will not recur.