Earthworm populations will vary enormously according to climate and native soil fertility.  Earthworms need moisture; few if any will be found in deserts.  Highly mineralized soils that produce a lot of biomass will naturally have more worms than infertile soils lacking humus.  Dr. Hopp surveyed worm populations in various farm soils.  The table below shows what a gardener might expect to find in their own garden by contrasting samples from rich and poor soils.  The data also suggest a guideline for how high worm populations might be usefully increased by adding organic matter.  The worms were counted at their seasonal population peak by carefully examining a section of soil exactly one foot square by seven inches deep.  If you plan to take a census in your own garden, keep in mind that earthworm counts will be highest in spring.

Earthworms are inhibited by acid soils and/or soils deficient in calcium.  Far larger populations of worms live in soils that weathered out of underlying limestone rocks.  In one experiment, earthworm counts in a pasture went up from 51,000 per acre in acid soil to 441,000 per acre two years after lime and a non-acidifying chemical fertilizer was spread.  Rodale and Howard loudly and repeatedly contended that chemical fertilizers decimate earthworm populations.  Swept up in what I view as a self-righteous crusade against chemical agriculture, they included all fertilizers in this category for tactical reasons.

Location Worms per sq. ft.  Worms per acre
Marcellus, NY 38 1,600,000
Ithica, NY 4 190,000
Frederick, MD 50 2,200,000
Beltsville, MD 8 350,000
Zanesville, OH 37 1,600,000
Coshocton, OH 5 220,000
Mayaquez, P.R.* 6 260,000

Because of the high rate of bacterial decomposition, few earthworms are found in tropical soils unless they are continuously ammended with substantial quantities of organic matter.

Howard especially denigrated sulfate of ammonia and single superphosphate as earthworm poisons.  Both of these chemical fertilizers are made with sulfuric acid and have a powerful acidifying reaction when they dissolve in soil.  Rodale correctly pointed out that golf course groundskeepers use repeated applications of ammonium sulfate to eliminate earthworms from putting greens. (Small mounds of worm casts made by nightcrawlers ruin the greens’ perfectly smooth surface so these worms are the bane of greenskeepers.) However, ammonium sulfate does not eliminate or reduce worms when the soil contains large amounts of chalk or other forms of calcium that counteract acidity.

The truth of the matter is that worms eat decaying organic matter and any soil amendment that increases plant growth without acidifying soil will increase earthworm food supply and thus worm population.  Using lime as an antidote to acid-based fertilizers prevents making the soil inhospitable to earthworms.  And many chemical fertilizers do not provoke acid reactions.  The organic movement loses this round-but not the battle.  And certainly not the war.