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What Is Healthy Soil, and How Can Terra Preta Contribute to It?

By Caroline Pfützner

This is an excerpt from the book The Modern Grower’s Guide to Terra Preta by Caroline Pfützner, published by Acres U.S.A. This excerpt is reprinted with permission from the publisher.

It is often assumed that terra preta is a very special soil that immediately causes unbelievable plant growth. This presumption is completely wrong, starting with the fact that terra preta isn’t a particular soil at all, but rather a soil type. Its special properties are primarily due to its high charcoal content. The effects of this are so diverse that they exert a positive influence on practically every single process in the soil. To help you better understand the wonders of terra preta, the next section will describe the various processes taking place in the soil; their seamless interplay is what makes plant growth possible.

Complex biological, chemical, and physical processes are constantly taking place in healthy soil. Soils are composed of approximately 50 percent mineral components like sand, silt, and clay, and about 20 percent air and water. The remaining 5 to 10 percent consists of plant roots, soil life, and humus (page 23). In the topsoil—the uppermost thirty or so centimeters various different small and microscopic organisms work to help maintain the circle of life: their work breaks down dead material into nutrients, which can then be reincorporated into living creatures. They are responsible for supplying the plants with nutrients, loosening the soil, and building humus.

If the soil is improperly cared for, activity in the soil will decrease and the fragile interactions between organisms will be thrown out of balance. Black earth is extremely conducive to soil life and diversity, allowing it to react more flexibly to outside influences. Among other things, this leads to a significant increase in microorganisms and mycorrhizae, and even “large” earthworms will happily ingest black earth compost and transport the biochar contained within it through their wormholes into deeper layers.

two hands holding dark soil
Many people are completely unaware of the importance of soil. It literally forms the basis for all life; without it there would be no plants, animals, or humans.


The significance of the life in the soil was recognized over a hundred years ago by the Austrian biologist Raoul Heinrich Francé, who introduced the term edaphon (from the Greek edaphos, “ground”) to collectively describe organisms in the soil. The term encompasses both plants (soil flora) and animals (soil fauna). The soil flora include bacteria, fungi, algae, and lichen, while the soil fauna include nematodes, springtails, mites, woodlice, earthworms, voles, moles, and many others.

The Earthworm

One of the most well-known inhabitants of the soil is the earthworm. There are forty-six different species in Germany alone, and more than three thousand worldwide. Earthworms are blind, deaf, and dumb, yet they still make an extraordinary contribution to soil fertility: their deep wormholes provide the soil with good aeration and structure, which increases its water absorption capacity. Earthworms also improve the soil’s nutrient supply by carrying minerals and trace elements from lower soil layers to the area around the plant roots. Their most important characteristic, however, is their ability to break down dead organic material in a way that makes the nutrients contained within it available to plants again. They accomplish this by eating about half of their body weight every twenty-four hours and then excreting a large portion of it as valuable humus, which, in addition to its high nutrient concentration, also contains many clay-humus complexes.

closeup of earthworm in dark soil
When people talk about earthworms, they are generally referring to the common earthworm (Lumbricus terrestris).

The number of earthworms within a given soil says a lot about its condition. Ten square meters of healthy soil should contain about two to three kilograms of earthworms—i.e., about three to four thousand individual worms—enough to process 250 kilograms of soil.

Deep plowing, however, is significantly reducing their numbers, as are pesticides. In 2015, it was shown that earthworm activity and rate of reproduction has been seriously reduced by the weed killer glyphosate. Nowadays, they can only be found in limited numbers— if at all—in damaged soils.

The best way to attract earthworms is to garden as naturally as possible, without assaults on nature. This means working the soil gently, using green manure, and mulching. Introducing black earth into the soil will also quickly cause the earthworm population to rise significantly.

Underground Cows

A single handful of fertile soil contains more microorganisms than there are humans on the entire planet. One hectare (2.5 acres) contains fifteen metric tons (16.5 tons) of them, which equals the weight of twenty to thirty cows. A 500-square-meter garden thus contains at least one “underground cow” converting dead biomass into humus.

To read about mycorrhizal fungi, humus and more from Chapter 1 of The Modern Grower’s Guide to Terra Preta, find the book at the online Acres U.S.A. Bookstore here.

About the Author

Caroline Pfützner is a passionate hobby gardener with many years of terra preta experience. As a young entrepreneur at TerraTirol KG, which has been producing high-quality soil using the Terra Preta method since 2014, she was awarded the Tyrolean Regional Environment Prize in 2016. She passes on her practical knowledge in numerous workshops and lectures. 

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