Henry Hopp - What Every Gardener Should Know About Earthworms

Dr. Henry Hopp

With the exception of brief sections by Dr. Douglas Taff on earthworms effects on soil nutrients, this report is by Henry Hopp, formerly with the United States Department of Agriculture and without doubt the worlds leading authority on earthworms. The Department set him to the study of earthworms because of the many inquiries they were getting about the characteristics and values of these creatures. He digested all the previous work in this field, and then embarked on some experiments of his own.

We went to Dr. Hopp to get an authoritative account, in terms of the layman, on the work he and others have done.

The facts about worms are important for anyone who does composting or wishes to have better soil for farm or garden. You will find that his studies have made Dr. Hopp an enthusiast about earthworms, albeit with a scientists caution about claims that cannot be experimentally proved. In reading his account it should be kept in mind that what the worms do in the soil essentially is what they also do in the compost pit. You will wish to encourage the maximum earthworm population in either, after reading Dr. Hopps absorbing report.

Every farmer and gardener is aware that earthworms occur widely in agricultural soil. They are especially frequent in the richer soils, although they also occur in ordinary garden or farmland. The association of earthworms with productive soil causes people to wonder if earthworms play a positive role in soil productivity or merely prefer to live in the better soils without contributing to their productivity.

This is not just an academic question. It is one that can vitally affect the productivity of many millions of acres of agricultural soil where farming or gardening is being carried on without regard to what it may be doing to the earthworm population.

There are over 3,000 species of earthworms in the world, but only a very few that are important in the tillable soils of this country. Some common ones are shown in the accompanying illustration shown opposite.

The night crawler, Lumbricus terrestris, is the largest of our earthworms. It is more common in the northern states. Heavy organic fertilization seems to favor its development in meadows and lawns.

Allolobophora caliginosa, with its variant known as the form trapezoides, is described as the common field worm. It occurs throughout the humid area of the country. It is more common than the night crawler particularly on the southern states. In the same locality, this species may prevail where the fertility level is too low for the night crawler.

On soils of extremely low fertility, neither of these species prospers. In rundown bromesedge fields around Washington, D.C., for example, the main kind is the small slim worm, Diplocardia verrucosa. It has no English name. There may be quite a few of them in the soil, but its holes and casts are so small that it has only a minor effect on soil properties.

Another species found quite widely in agricultural soils is the green worm, Allolobophora chlorotica. It is a rather short but stout worm of typical greenish color. Only because of its prevalence is this worm deserving of comment; it is actually quite inactive. Very often it is found curled up in a semi-dormant condition while the other worms are active.

Some of the more common kinds of earthworms: from left the nightcrawler, the field worm, the Diplocardia, the green worm, the manure worm, and the rubellus worm. The first four occur in farm or garden soil. The other two occur mostly in compost, but occasionally in garden soil where a large quantity of refuse has been added. Worms are pictured hereths their actual size.

Two other kinds worth mentioning are especially common in compost piles. One is the so-called manure worm, Eisenia foetida. It is known also as the branding or red wriggler, the latter because of its squirming reactions when handled. This species can be told by the transverse rings of yellow and maroon which alternate the length of its body. The other is the stouter Lumbricus rubellus. It is a deep maroon color and does not have the yellow bands of the mature worm. Both these earthworms invade refuse, although the former is more prevalent in manure piles. Neither occurs commonly in agricultural land; although they will come in where large amounts of refuse are added to the soil.

In addition to these recognized species of earthworms, a so-called hybrid is being sold by commercial growers. It is claimed by some to be a cross between the manure worm and one of the field worms. This claim is groundless so far as we can determine. Shipments of these worms that we have examined proved to be identical taxonomically with Eisenia foetida. Their rates of cast production are also similar.

All of the important earthworms are exotic, having been introduced from Europe, probably in soil brought along with plants. However, earthworms have become widely distributed throughout the country. By now, their distribution is largely a reflection of natural variations in climate and soil from place to place.

They are more prevalent in the humid sections of the East than in the arid West. But even in desert regions they sometimes occur along water courses and in irrigated land.

Where reasonable moisture conditions prevail, their occurrence is determined primarily by soil variations. They are more common in soils derived from limestone or otherwise rich in plant nutrients, than in shale or outwash soils. Soil texture influences the earthworm population. Sandy soil contains fewer earthworms than clay soil. This is fortunate because sandy soil is likely to have good structure naturally, whereas clay soil packs together and becomes too hard for crop growth unless agencies like earthworms are present to keep the soil granulated.

Another cause of variation in their distribution is the quantity and quality of organic matter found in the soil. Earthworms require both organic debris and mineral soil for food. Organic compounds satisfy their need for carbon as well as nitrogen. Carbon is normally supplied in the form of starches or sugars while nitrogen is furnished by amines and proteins. Neither nitrate nor ammonium, which are common nitrogenous compounds used to fertilize plants, can satisfy an earthworms nitrogen budget. Proteins and amines represent only a small fraction of the total organic matter in soil while at the same time protein comprises 72% of an earthworms dry weight. Obviously for any earthworm population to survive, there must be a quantity of available nitrogen to be ingested, digested and resynthesized into worm protein. When physical parameters such as soil texture, moisture, and temperature are suitable for optimal growth, the availability of a nitrogenous food supply becomes the ultimate factor limiting population.

In a deciduous forest, where a constant leaf fall takes place every autumn, the earthworm population is regulated by the quantity of leaves which fall and their nitrogen content. The same factor regulates the populations found in prairies and grasslands. The earthworm population will rise above its steady state limit only when organics are continuously imported into an ecosystem. Each time a gardener works compost into his soil he is raising the soils carrying capacity and ultimately the earthworm population.