Sulzer Technical Review Issue 2 / 2016

How the earthworm improves the soil

July 13, 2016 | Herbert Cerutti

Nature has ways of converting vegetable and mineral raw materials into high-quality products. The inconspicuous earthworm provides striking example — it is a minirefinery in the soil.

Earthworm in the grass

The dew worm or common earthworm, which is native to Europe, is just one of more than 3000 earthworm species in the world. About 30 centimeters long, the powerful earthworm digs vertical burrows many meters down into loamy, sandy soils. To collect organic material, it comes to the surface in the dark because its pale body is poorly protected against UV radiation. It also has to keep its skin moist, which is why it prefers to move about in rainy weather to look for a partner or to change its burrow.

Modern investigations have confirmed the usefulness of earthworms: in healthy meadowland, up to 500 earthworms work per square meter of ground and produce up to 60 kilograms of humus a year. Earthworms perform high-quality recycling and refining of plant and animal residues in their gut.

Microbes aid digestion

The worms cannot chew, nor do they have enzymes with which to dissolve the cellulose and the lignin of the plant cells and to access their nutritious content. Instead, they suck on leaves or stalks lying on the ground, pull them into their burrows, and let bacteria and fungi break down the cells there.

Only after the preparatory work of these micro-helpers do the worms suck in the food, including the nutrient-rich microorganisms. Along with the food they ingest, the earthworms consume mineral particles from the soil, which then act as an abrasive powder in the gizzard and thereby grind the food into a fine pulp. The food is chemically decomposed in the intestine, allowing the worm to absorb the nutrients into its bloodstream through the intestinal wall. It deposits everything undigested at the entrance of the burrow as excrement.

High-quality fertilizer as the end product

The earthworm can only extract five to fifteen percent of the nutrients from the food: most of the nutrients thereby return to the surface in the feces. There, bacteria, insect larvae, and springtails immediately pounce on the nourishing remains and release more nutrients. This is then an invitation for the earthworm to take in its own, refined excrement once again. In this way, the same nutritious pulp passes through the earthworm’s gut repeatedly, continually supplemented by new minerals, plant remains, and microorganisms. The earthworm excrement becomes a high-quality mixture of organic and mineral substances in a form that plants can readily absorb — substances that have been refined in multiple working steps.

Increase in yield thanks to earthworms

The importance of this soil-biological potpourri for the growth of plants has been shown by studies in Holland, where earthworms were introduced into newly reclaimed land. Thanks to the help of the earthworms, the winter wheat yield doubled, and the clover yield increased tenfold compared with new fields without earthworms.

Sulzer Technical Review


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