Moldboard plows, which invert the topsoil layer where most worms feed and many live, are the most damaging. Chiseling or disking is less disruptive, especially if it is done minimally. Not all farm chemicals are bad for earthworms, but many are sure to reduce their populations. Probable the worst group is fumigants, which are a fiasco for earthworm populations. Fungicides are also generally not toxic to earthworms, although they can decrease the amount of plant residue that ends up in the soil for earthworm consumption, thereby reducing their food supply.
Toxicity varies in other groups of chemicals.
Organochlorine compounds, which continue to persist in various forms in many farm soils, are generally highly toxic, especially chlordane, heptachlor, and toxaphene. Organophosphorous compounds such as malathion and chlorpyrifos are probably not toxic at normal field application rates, while others like phorate are very toxic to earthworms. The carbamates also vary, with many being highly toxic. Ammonium-based fertilizers can also discourage earthworms because they acidify the soil. On the other hand, earthworms benefit indirectly form the increased amount of vegetation caused by higher fertility.