At age 28, Charles Darwin presented “On the Formation of Mould” to the Geological Society of London.  This lecture illustrated the amazing churning effect of the earthworm on soil.  Darwin observed some chunks of lime that had been left on the surface of a meadow.  A few years later they were found several inches below the surface.  Darwin said this was the work of earthworms, depositing castings that “sooner or later spread out and cover any object left on the surface.”  In a later book, Darwin said,

“The plow is one of the most ancient and most valuable of man’s inventions; but long before he existed the land was in fact regularly plowed and still continues to be thus plowed by earthworms.  It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.”

Earthworms also prevent runoff.  They increase percolation of water into fine-textured soils by making a complex system of interconnected channels or tunnels throughout the topsoil.  In one study, soil lacking worms had an absorption rate of 0.2 inches of rainfall per minute.  Earthworms were added and allowed to work over that soil sample for one month.  Then, infiltration rates increased to 0.9 inches of rainfall per minute.  Much of what we know about earthworms is due to Dr. Henry Hopp who worked for the United States Department of Agriculture during the 1940s.  Dr. Hopp’s interesting booklet, What Every Gardener Should Know About Earthworms. is still in print.  In one Hopp research project, some very run-down clay soil was placed in six large flowerpots.  Nothing was done to a pair of control pots, fertilizer was blended in and grass sod grown on two others, while mulch was spread over two more.  Then worms were added to one of each pair of pots.  In short order all of the worms added to the unimproved pot were dead.  There was nothing in that soil to feed them.  The sod alone increased percolation but where the sod or mulch fed a worm population, infiltration of water was far better.

Amendment to clay soil Percolation rate in inches per minute
                         Without worms With worms
None 0.0 0.0
Grass and fertilizer 0.2 0.8
Mulch 0.0 1.5

Most people who honestly consider these facts conclude that the earthworm’s activities are a major factor in soil productivity.  Study after scientific study has shown that the quality and yield of pastures is directly related to their earthworm count.  So it seems only reasonable to evaluate soil management practices by their effect on earthworm counts.