Healthy soil full of earthworms

The fact that earthworms in the soil are desirable organisms is probably known to everyone. Their importance in shaping soil fertility and health is discussed in agricultural press, spoken about in podcasts, and presented in lectures on topics related to organic matter and soil biological life. However, it is worth remembering that, in addition to their positive impact on the soil environment, the abundance of earthworms is also an indicator of soil health. Let's take a moment to refresh this knowledge.

Basic biology

Earthworms are animals that, according to taxonomy, belong to the family Lumbricidae, which in turn belongs to the class Clitellata, a part of the annelid type, i.e., invertebrates whose bodies are composed of a series of rings. It is worth realizing that there are about 32 species in Poland alone, with 13 species found in agricultural soils. Someone might say that this is a small number, but because there can be up to 4 million of them on one hectare of meadow, it is a good moment to understand how common these animals are in the environment. Their presence is noticeable mainly after rainfall or when we need to dig something in the field. However, many species regularly come to the soil surface at night without exposure to direct sunlight. Their continuous presence in the soil and partially on its surface is associated with their often underestimated role in the cycling of matter in the environment and the shaping of soil fertility and health. Their importance was appreciated, among others, by minds such as Aristotle, who called them the "intestines of the earth," or Charles Darwin, who attributed to them a tremendous role in the natural history of the Earth.

Division of earthworms

Earthworms can be divided into three ecological groups, distinguished based on similar feeding behaviors and the character of burrow construction. This division is uncomplicated and practical, as understanding its foundations allows us to fairly accurately determine the group of earthworms we are dealing with and helps define their impact on the soil.

The first group is called epigeic earthworms. They live close to the soil surface, usually in the litter layer. Additionally, they can be found in compost heaps, manure, or decaying wood. They are small, dark-colored, and of modest size. Their main diet consists of decomposing plant residues. The next group is endogeic earthworms. They also live relatively shallow in the soil, but slightly deeper than epigeic ones. They create horizontally arranged burrows, often in the root zone of plants. They are usually of medium size, pale or greenish, and primarily feed on soil, which at this level of the soil profile contains more organic matter than the lower layers.

The third group includes the longest earthworms and is called anecic. They dig long, vertical burrows that reach 1.5 meters into the soil profile or sometimes deeper. They feed on soil, dead leaves, and decomposing plant residues. To obtain food, they come to the soil surface at night, searching for nourishment but also to eliminate digested food. They are characterized by a dark red or black color, mainly on the dorsal side, and this color is more intense in the head region.

There are many different types of earthworms living in the soil.

Additionally, in a more detailed approach to taxonomy based on the above criteria, groups with intermediate characteristics, combining features of epigeic and endogeic earthworms, can be distinguished. Nevertheless, the main division of earthworms is as described above, and each of these groups often fulfills distinct but significant functions.

Earthworms and carbon farming

One might wonder what connection earthworms have with carbon farming or if they contribute to carbon sequestration in the soil. It is essential to remember that regenerative agriculture is a holistic approach to the agroecosystem, taking into account interactions among various organisms and the relationships between different processes occurring in the soil. Although earthworms themselves do not capture carbon dioxide from the atmosphere, they play a significant role in the process of transforming organic matter into a form that is the most stable form of organic carbon in the soil, namely humus.

Earthworms play crucial role in carbon farming

Thanks to earthworms, organic residues (both fresh and decomposing plant remains) are moved down the soil profile, and during this process, they are enriched with microorganisms. These microorganisms subsequently secrete enzymes that act on organic matter. The humification process is a biochemical one, involving a series of complex processes of decomposition, restructuring, and synthesis of various organic compounds. However, these processes occur with the participation of microorganisms and soil fauna.

The greater the contribution of earthworms in this chain of transformations, the faster this process occurs. While plants indeed can capture carbon dioxide from the atmosphere and incorporate it into the form of organic matter, the animals discussed here play a role in facilitating the conversion of fresh plant residues into humus. This humus, in turn, can be considered a condensed form of organic carbon, stored in the soil and originating directly from the atmosphere in the form of carbon dioxide.

Impact of earthworms on soil

Earthworms have a multifaceted impact on the fundamental working tool of a farmer – the soil, and in the overwhelming majority of cases, this impact is positive and desirable. These effects can be categorized as physical, chemical, or biological.

First and foremost is their influence on soil structure. By creating horizontal and vertical caves, earthworms reduce soil compaction. Despite the argument that certain species may contribute to soil compaction, the interaction of such species with soil-loosening ones ensures the balance in soil structure. Furthermore, the network of channels in the soil, especially in no-till cultivation systems, allows for increased water absorption from rainfall. This is particularly crucial during seasons characterized by short, intense rains. Thanks to the work of earthworms, the soil can absorb more water in a short period, simultaneously reducing surface runoff and water erosion. Additionally, these channels are eagerly utilized by plant roots during their growth.

Earthworms benefit the soil from multiple different aspects.

The impact on the chemical properties of the soil is closely related to the fact that earthworms leave numerous coprolites in the soil environment, consisting of their excrement, deposited on the soil surface or within the soil itself. This material significantly contributes to soil fertility, with coprolites having much higher levels of nutrients available to plants than the surrounding soil. The concentration of nitrogen in coprolites is five times higher, phosphorus seven times higher, and potassium eleven times higher than in the soil. Moreover, coprolites have a higher pH than the soil due to the action of calciferous glands. Every day, earthworms pass through their digestive tract plant residues and soil weighing between 20 to 100% of their body weight. It is estimated that on one hectare, one ton of earthworms can produce 30 tons of coprolites in a year, thereby increasing soil productivity.

On the other hand, the impact on the biological properties of the soil is associated with the process of consuming organic matter and the microorganisms residing in it. In the digestive tract of earthworms, there is an enrichment of matter with other microorganisms, thereby increasing the biodiversity of soil microorganisms. Consequently, both in the coprolites themselves and in the space around the excavated channels, there is a greater proliferation of soil microflora.

Summary

After presenting this handful of information related to earthworms, it is worth reappreciating their impact on the soil and its mechanical, chemical, and biological properties. Let us not regard them merely as organisms "attached" to our soil but rather as the soil life that cultivates and fertilizes our crops in the best possible way.