Category: Soils and Growing

Humus and Worm Castings

Gary Kline

Humus and Humility – Recently we sent out an article that I (GK) wrote entitled “Understanding Humus”.  It was fairly long, so probably not many read it.  However, a few who did gave it what has to be described as high praise.  Try as I might, being self-effacing, at some point, becomes suspect.  Fortunately, I continue to make enough mistakes to remain authentically imperfect and unavoidably modest.  An example is in the Humus article.

Castings Correction – In the section about composition of worm castings, I stated that worms accumulate high levels of minerals; and what I should have said, or explained, is that they transform unavailable minerals into available nutrient minerals as soil particles pass through their intestines.  More than two thousand years ago Aristotle proclaimed that earthworms were the intestines of the earth.  Smart guy.

Classical Content – Gardeners have long known that the presence of earthworms in their soil is a sign of good soil health, and their absence a sign of something being wrong.  There is an often quoted reference to the mineral content of castings compared to the mineral content of surrounding soil (aka, dirt).  Thus, castings are said to contain 5 times the nitrogen, 7 times the phosphorus, 11 times the potassium [and 1.5 (?) times the calcium, and 3 times the magnesium] as ordinary topsoil.  How can that happen?  The extra minerals can’t come out of thin air, nor, seemingly, out of thin soil.  Transmutation of elements would be a stretch.

Enigma Explained – Initially, I figured the worm must somehow extract and concentrate the minerals as ingested soil moves through its body and is then excreted.  Here’s what Sir Albert Howard, in Soil and Health (1947), had to say about the process and the make-up of castings:

“The casts are manufactured in the alimentary canal of the earthworm from dead vegetable matter, and particles of soil.  In this passage the food of these creatures is neutralized by constant additions of carbonate of lime from the 3 pairs of calciferous glands near the gizzard, where it is finely ground prior to digestion.  The casts which are left contain everything the crop needs – – – nitrates, phosphates, and potash [NPK] in abundance, and also in just the condition [soluble?] in which the plants can make use of them.”

Further Explanation – Howard goes on to cite that often-mentioned analysis as follows:  “Recent investigations in the United States show that the fresh casts of earthworms are 5 times richer in available nitrogen, 7 times richer in available phosphates and 11 times richer in available potash than in the upper 6 inches of soil.”  It’s that word “available”, in contrast with previously non-available, which explains the higher content of nutrient minerals in the castings coming out, versus the “soil” and organic matter going in.  As near as I can tell, these numbers came from a single USDA study done in Connecticut in July, 1944.

Restoration Tool – Very large portions of the earth’s soil have been eroded and exhausted over the past 5,000 years all around the world.  In recent centuries the rate has been stepped-up many times the rate of natural soil formation.  The earthworm potentially is a major tool (if properly fed) for restoring and rebuilding those soils.  Here’s further explanation from Dr. Thomas J. Barrett’s 1947 and 1959 book Harnessing the Earthworm:

“In the chemical and mechanical laboratory of the earthworm’s intestines are combined all the processes of topsoil-building.  The earthworm swallows great quantities of mineral earth with all that it contains of vegetable and animal remains, bacteria and microscopic life of soil.  – – – Finally, it is ejected in and on the surface of the earth as castings – earthworm manure – humus, a crumbly, finely-conditioned topsoil, richly endowed with all the elements of plant nutrition in water-soluble form.”

In Summation – I’m not sure that all (or most) topsoil is from worm castings which are, in turn, humus.  Recall that this was the claim of Barrett and Darwin.  Darwin calculated that worms deposit 10 tons of castings per acre each year.  Sir Albert Howard figured it was 25 tons.  In some parts of the world these deposits have been measured to far exceed those amounts.  Whether or not topsoil is castings and castings are humus, or that humus requires soil and clay for its formation, the creation of 25 tons per acre or more of new topsoil per year is nothing to sneeze at.  This represents a too-often overlooked means of restoring billions (yes, billions) of acres of ground lost to the plow, it’s modern successors, and a century of reckless chemicalized agriculture that are soon going to be necessary to the survival of civilization.  In ancient Egypt it was a crime to kill earthworms.  Let’s not forget the lowly earthworm.  It’s the least we can do.    GLK

© 2013 Gary L. Kline

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