The earthworms' role has long been ignored. They were thought to be good only for the mechanical function of making the soil lighter. Notwithstanding, some research workers have demonstrated that annelids modify the chemical composition of the earth.
In his "Treatise on Microbiology of the Soil"* Pochon, of the Institut Pasteur, gives various experimental results. The earthworms increase the quantity of limestone in the soil. Their glands excrete CO3Ca so that the pH of soil containing earthworms increases. Earthworms are most abundant in neutral or slightly acidic soil and can be found in good soil by the hundreds of thousands per acre. Some authors declare that each worm ingests l/l0th of a gram of earth per second, which is three tons per year. Darwin gives a higher figure, but one should be careful with such calculations since earthworms have resting periods in winter and in dry seasons. Other believable figures, which resulted from observations made in England, indicate that a field of earthworms rejects an average of 57 tons/ha/year (23 tons per acre per year): the equivalent of four spreadings of farm manure per year. But this constitutes only the amount of earth rejected on the soil's surface. One cannot deduce from this the exact weight of earth having passed through the digestive tube of each worm. Compared to the surrounding soil, these rejected excrements were five times richer in nitrogen, two times richer in calcium, two-and-a-half times richer in magnesium, seven times richer in phosphorus, and eleven times richer in potassium.
[*Dunod Publ. (Paris 1954).]
taken from: Louis Kervran: Biological Transmutations, chapter 11