Although the literature is abundant about the earthworm, Lumbricus terrestris, some evidence of popular fallacy exists over the true scope of earthworm activity.
L. terreeiris is only one of several worms that comprise that complex soil animal life population.
Where earthworms flourish, they comprise 50 to 75 percent of the total animal weight in the soil.
In such a situation, they dominate the soil fauna. Nothing is known of their average age in the soil, but they have lived as long as 60 years in captivity.
Characteristics of Earthworms
Earthworms vary in size from 25 cm., which may weigh from 2 to 7 grams, to the smaller size of 2.5 ern, which may weigh as little as 0.05 grams.
According to J. E. Russell, author of Soil Conditions and Plant Growth, all species appear to obtain a part of their food from the soil surface; others collect their food on the surface and drag it down into their burrows.
Little is known about their food preferences, but they do as a whole cat cow dung, and in forests, they eat elm, ash, and birch litter more freely than oak and beech.
Effects on Soil
Earthworms have two principal effects on the soil: aeration and mixing of the soil constituents.
They pass large quantities of material through their bodies which they eject on the surface as worm casts.
They honeycomb the soil with their burrows, and the burrows help to aerate the soil and allow water permeation.
Organic Matter Is Vital
A major finding of a study at Rothamsted Experiment Station in England was that the amount of organic matter present governs the number of earthworms in a given weight of soil.
Where earthworms are active, organic matter is distributed throughout. Earthworms can survive only in soils well-provided with organic matter, such as forests, pastures, and arable sails.
Earthworms are very sensitive to soil acidity, and the population is greatest at or near neutrality (pH 7.0); worms need a continual supply of calcium which they convert into calcium carbonate and excrete from special glands in the digestive tract.
This helps to explain their intolerance of acid soils. Generally, they are absent in soil with a pH below 4.5, as they need lime.
According to Russell: “Earthworms improve aeration and at the same time the water-holding power of the soil crumbs, for the outstanding action of the worm is to ingest soil particles along with organic matter, grind up organic matter with soil particles in its body in the presence of calcium carbonate, secreted by the digestive glands, and excrete this calcium mixture of organic matter and soil as a black-brown loam. This loam is well-suited for plant growth. On the lighter soils, it has higher water-holding power than the soil, and on the heavier soils, it provides good structure.”
The worm is of limited importance in arable soils, which get cultivation or “mixing.” Arable conditions discourage the growth of earthworms.
This is possibly due to the relative scarcity of suitable food and, more probably, to the disturbed condition within the soil caused by cultivation.
The worm is of great importance, though, in undisturbed forest and pasture soils, with a pH above 4.5.
Darwin’s Observations
It is not until one gets into the intriguing classic of Charles Darwin that the vast concept of soil fauna is revealed.
This treatise relates to earthworms that bring earth up to the soil’s surface as castings.
As early as 1881, Darwin had determined that earthworms were omnivorous and that they swallow an enormous quantity of earth, out of which they may extract any digestible matter. In addition to soil, the worm consumes fresh or half-decayed leaves.
Calciferous
The most interesting features of the digestive system are the so-called “calciferous” glands, which Darwin claimed contain carbonate of lime, as extremely fine granular matter.
The excretion of the calciferous glands partially counteracts the acid generated by half-decayed leaves and other organic matter.
The calcium carbonate granules may also serve to triturate food, thus acting as millstones in the intestine.
At any rate, these glands help explain a worm’s calcium requirements and the reason for its intolerance of acid soil.
After swallowing earth, whether for making its burrow or for rood, some worms come to the surface to empty the body.
Darwin says, “I have watched worms during the act of ejection, and when the earth was in a very liquid state, it was ejected in little spurts and by slow peristaltic movement when not so liquid.”
Some worms, rather than eject their castings on the ground’s surface, leave them in any subterranean cavity adjacent to their burrow.
From the records of many investigators, collected from all parts of the world, Charles Darwin reported that from seven to 18 tons of fine earth per acre were ejected as castings.
Even after familiarity with the functions of these soil animals, we must temper our enthusiasm for the earthworm with some knowledge of their limitations.
Plant Nutrients
For instance, Russell concludes that it cannot be proven that the earthworm can make organic matter (other than by the decay of dead worms), nor is it clear that he can convert plant litter into humus.
The earthworm, rather, uses humus in the interest of soil fertility. And, for him to do so, the humus must be in the ground initially.
In so far as humus is related to the availability of plant nutrients through cation exchange reactions in the soil, the chief value of the earthworm is in the comminution of the organic matter and its intimate mixing with the soil particles.
In other words, the humus must be there to support the earthworm since we know that high-humus-containing soils sustain high earthworm populations.
It becomes apparent that worms introduced into near-barren soils concerning humus will not long survive such an environment.
On the other hand, worms will increase in soil into which humus has been incorporated. Nothing is more demonstrative of this than the figures reported for the earthworm population at the Rothamsted Station.
Fertilizer Is a Factor
Nor is organic matter the only material that will accelerate earthworm production.
Again from the Rothamsted experiments, it was found that research plots that were fertilized had about the same population of worms as those which had not been fertilized, but the worms in the former plots weighed more.
They were larger and fatter. Other workers found that the addition of superphosphate to the soil not only increased the bacterial flora.
It also increased the organic matter through accelerated decomposition and promoted an increase in the earthworm population.
Without offending the avid organic gardener, it must be remembered that earthworms will not by themselves be the cure-all for the soil.
The earthworm has, however, a distinct function in the overall fertility picture. It tends to simplify the nutrient availability problem, but the organic matter and the plant nutrients must be present in the soil to start with.
With those factors present, the earthworm can work under optimum conditions of good tilth and fertility.
Selected References
(1) Russell, J. E. — Soil Conditions and Plant Growth. 8th ed., London 1950, pp. 179, 180, 183.
(2) Darwin, Charles — The Formation of Vegetable Mould Through the Action of Worms with Obserrations on Their Habits. London 1881, pp. 57, 65.
(3) “Science vs. Witchcraft,” Fertilizer Review Vol. 27, No. 1, Feb.-Mar. 1951, p. 5.
44659 by D. H. Williams