Living topsoil

Plants at the center - rhizosphere and phyllosphere

All life in the soil needs energy to live. Apart from a few bacteria that get their energy from sulphur, nitrogen or iron compounds, everyone needs something containing carbon to provide energy to live. Most organisms eat more than one prey organism, which is why there is what you might call a network of food chains in the topsoil.

Plants largely control this. Much of the energy that plants obtain via photosynthesis in the leaves is distributed by the plants to produce chemicals that they secrete via the roots into the root zone or rhizosphere. These are mainly carbohydrates and proteins, the presence of which awakens, attracts and cultivates specific beneficial bacteria and fungi. These bacteria and fungi further attract and are eaten by larger microbes such as nematodes and protozoa (a group of single-celled organisms, such as amoebas) that need them to fuel their own metabolism. What they don't need is excreted as waste, which in turn acts as absorbable nutrients for the plant roots. In this way, the plants act as a center for life in the soil.

The plants can control the amount and timing of which types of fungi and bacteria are attracted to the rhizosphere via this root exudation. Soil bacteria and fungi act as small fertilizer bags that are opened and spread by nematodes and protozons. It is also a whole system where the parts depend on each other to survive.

Life in the soil creates structure and nourishment

Bacteria are so small that they have to attach themselves to things to avoid being washed away. To do so, they produce mucus that helps to bind the soil together.

Fungal hyphae travel through soil particles and also help to bind them together.

Earthworms, insect larvae and other burrowing animals move through the soil in search of food, creating air and water channels. Earthworms also drag organic parts down into the soil where they are broken up into smaller pieces by beetles, larvae and other insects, which in turn make the organic parts more susceptible to decomposition by fungi and bacteria.

Without this system, most of the important nutrients are drained away from the soil. For example, when using artificial fertilizers, most of the nutrients will disappear into the groundwater because they are not bound in the same way by the organisms and structure of life in the soil.

Soil life controls disease and pests

Not all organisms in the soil are beneficial. Damaging diseases and pests cause plant damage and plant death.Healthy soil contains an enormous number and variety of organisms that control the trouble makers by competing for the same root-derived carbohydrates and proteins, other nutrients, air and water. When competition is high, the pathogens are kept in check and can also be outcompeted and die. Each group has its place in the system, so when one group disappears, the whole dynamic changes.

Bacteria and fungi create a physical barrier around plant roots that prevents attacks by pathogens. If a factor contributes to reducing beneficial bacteria or fungi, the plant can easily be attacked. Mycorrhizal fungi form symbiotic, highly beneficial relationships with the plants via the roots and if the fungal community is weakened, the plant's immune system and nutrient supply changes drastically.

Sometimes fungi and bacteria form alliances to form protective layers for the plant, not only in the rhizosphere, but also in the phyllosphere (leaf surface). The leaves also secrete substances that attract microorganisms in the same way as the roots do. Some fungi produce inhibitory substances, such as vitamins and antibiotics, which help to increase the plant's resistance.

All nitrogen is not equal

The most important nutrient for plants is nitrogen. Nitrogen is the basic mineral in the construction of amino acids and therefore life. The total biomass of fungi and bacteria in the soil is the most decisive factor for how much plant-available nitrogen there is in the soil. As a rule of thumb, there are more fungi than bacteria in the least disturbed soil - where there is old forest that has been left undisturbed for a long time (10:1 - 10 times more biomass of fungi than bacteria or more). While plowed soil is on the opposite side of the scale with far more bacteria relative to fungi (1:1 - as much biomass fungi as bacteria or more biomass bacteria than biomass fungi).

Different plants have different preferences when it comes to growing in soil with a biomass ratio of fungi to bacteria. Most perennials, shrubs and trees prefer soils dominated by fungi, while annual plants, grasses and vegetables prefer soils dominated by bacteria.

What does it have to do with nitrogen?

When organisms are eaten, some of their nitrogen benefits the eater, while much is released as waste in the form of plant-available ammonium (NH4). Depending on the soil environment, this can either remain ammonium or be converted to nitrate (NO3) by special bacteria.

When there are a lot of bacteria in the soil, the soil tends to become alkaline because the mucus covering the bacteria is very alkaline. This is an environment in which nitrogen-fixing bacteria thrive, so when ammonium is released in soils dominated by bacteria, this conversion of ammonium to nitrate will take place. The acid that fungi excrete when they dominate an environment lowers the pH value and thus the occurrence of nitrogen-fixing bacteria. The nitrogen will therefore typically be found as ammonium in fungus-dominated soils.

Negative influence on the soil food chain

Artificial fertilizers and herbicides, fungicides and insecticides have a major impact on the soil food chain. They can have a poisonous or repellent effect on individual organisms and thus change the entire food chain. Moreover, the plant's important symbiosis with bacteria and fungi will not always take place when the plant has readily available nutrients from artificial fertilizers, which also means that the rest of the microbe chain adapts.

The problem then becomes that you constantly have to add fertilizers and herbicides to compensate for this imbalance. Earthworms move out of the soil if there is too little food and too much synthetic nitrates that act as irritants. As a result, the soil structure changes and irrigation and drainage become a bigger problem. Pathogens and insect pests become established.

Plowing also kills much of the life in the soil. Fungal hyphae are torn apart, earthworms and arthropods are damaged and mashed. The soil structure is destroyed and eventually depleted of air.

What exactly is soil?

Weather such as wind, snow, rain, sun and heat, ice and cold physically break down rocks into small mineral particles and thus initiate soil formation. Chemicals in the weather break down molecular bonds in different rocks and make them more susceptible to the ravages of the weather.

Fungi and bacteria also contribute to the chemical breakdown of rocks via the chemicals they secrete to break down matter they need for consumption. Microbes produce ammonia and nitric acid which act as solvents.

Soil contains over 80 elements, but only 8 make up the majority: oxygen, silicon, aluminum, iron, magnesium, calcium, sodium and potassium. All are electrically charged at the molecular level and can form combinations that become different minerals.

Organic matter, water and air

The soil needs more than decomposed minerals to form the basis for life. When plants and animals die on the surface, they are broken down by bacteria and fungi and eventually transformed into carbon-rich humus. Humus contains long chains of carbon molecules that are difficult to break down. They have a large surface area that contains electrical charges that attract mineral particles. The void between individual minerals and organic particles is filled with air and/or water.

Water moves between the soil pore spaces in two different ways - via gravity or via capillary action. Water drawn by gravity moves freely through the soil. Large pores encourage water to move in this way. As the water fills the pores, air is pushed away. When the water hits roots that act as sponges, it is absorbed. Smaller pores contain a membrane of capillary water that is not affected by gravity. This water therefore remains after water pulled by gravity has passed. The water is bound via the attraction of the molecules to each other and surrounding earth surfaces. This creates surface tension that causes the water to form a thick film on the particle surfaces. Capillary water can move upwards and make itself available to plant roots after all gravitational water has passed and is a significant source of water for the plants.

Hydroscopic water is a thinner film of water that is only a few molecules thick. These also bind to extremely small soil particles by virtue of their electrical properties. This bond is very hard and cannot be absorbed by plant roots. But this membrane of water is extremely important for many microbes' living conditions and ability to move. This membrane also exists in extreme drought. About half of the pore spaces in the soil are filled with water, while the other half is filled with air. Water movements push away old air and suck in fresh air from the surface. A supply of fresh water therefore also means a supply of fresh air, which is very important.

Microorganisms use oxygen in their metabolic activity to create carbon dioxide. The presence of carbon dioxide is a good sign that the soil is alive, but it must be constantly replaced to maintain life.

In some soils, the pore spaces are isolated so that air is not exchanged as the water moves through the soil. Such soils have little porosity in that they lack spaces between the soil particles. Organisms that thrive in such oxygen-free environments often produce alcohols that kill plant roots.

Soil profile and soil structure

Soil is constantly exposed to different types of weather. For example, rain will cause various minerals or organic substances to leach out of the soil by being transported out with water. These substances can encounter barriers when transported by water, which in turn causes them to concentrate in zones or soil layers. A soil profile is a map of these soil layers. The color of the soil can be a good indicator of what is in the soil layer since certain components have very distinctive colors. When iron is oxidized it becomes rust-colored, oxidized manganese gives a black-purple hue. When these colors are present, it often indicates good drainage and air flow. Gray soil can indicate a lack of organic matter and a lack of air flow.

The size of the soil particles can be described as texture - sand, silt and clay are such textural descriptions. Texture has nothing to do with soil composition, although sandy or silty soils often consist of a lot of quartz, all types of rock can be broken down into a sandy or silty texture. Clay, on the other hand, consists of a limited group of minerals. The point is, however, that texture has a big impact on how things work. Clay has far more surface areas than sand, and clay has smaller pore spaces between the particles but also many more pore spaces than sand - so the pore space surface area is larger on clay than on sand. Silt lies in between these.

The ideal soil structure will be a roughly equal distribution of sand, silt and clay - it retains nutrients well while ensuring good drainage and air flow. Equally important to soil structure is the shape these particles take when grouped together. Bacteria produce mucus that binds particles together to form colonies, and fungi bind networks of colonies together with their mycelial network of hyphae. For example, commonly occurring fungi within the order Glomales (the endomycorrhizal order that belongs to our oldest fungi) produce a highly adhesive protein called glomalin. When the hyphae grow through the soil pores, the soil particles are covered with glomalin, which then acts as a superglue that creates clumps of soil particles. The soil pore space changes and makes it easier for the soil to retain capillary water and plant-available nutrients that benefit the plants over a longer period of time.