The incredible soil aggregate
You might have noticed the way stepping stones seem to “disappear” into the lawn, and the way organic material builds up under the carpet of grass. Activity in the root zone results in small and large crumbs, called aggregates, which have significant consequences for soil health.
In field edges where plows and fertilizers are not used, we often find the kind of topsoil we would like in the field. The soil is darker, with crumb structure, channels and pore spaces without compaction. Plants here have deep roots, and we see more earthworms, worm castings, beetles and spiders on the surface.
What happens there, but not here
Soil loses its functions when the collaboration between plants and soil life is disrupted. Just think; this relationship has been developing since the first plants were established on land about 450 million years ago.
The difference between topsoil and subsoil is space, life, and the remains of life. When we dig into topsoil with living plants, we can see that the soil is darker. It clings together in large and small clumps, with plenty of pores and spaces. Where there are roots, there is life, and life builds structure. There must be good reasons for this partnership to have evolved, I think.
Life is the glue
Aggregates form when soil organisms reproduce, hunt, graze, and produce waste. The largest ones are called macroaggregates. These range from 0.25 to 2 millimeters in diameter, and they are made up of microaggregates, which are tiny clusters of particles cemented by chemical and physical bonds, and are less than 250 μm in diameter.
The size and composition of mineral particles in the soil is a significant factor which largely determines whether the soil will form clumps or aggregates. The composition of sand, silt, or clay also plays an important role in structure development. Primary particles are bound together with glues and slimes in the form of polysaccharides and bacterial or fungal ‘skeletons’, and fungal hyphae weave them together. Microaggregates develop and mature over time. Eventually, the activities of living organisms and the remains of dead ones bind clusters of microaggregates into macroaggregates. This process continues as long as plant roots remain active in the structure of soil aggregates.
Shelter, ventilation, and drainage
Over time, structures and spaces of various shapes and sizes will develop, creating habitat for plant roots, bacteria, and more diverse soil organisms. The underground metropolis needs ventilation to grow, and continuous pore systems enable the exchange of gases and fresh air, while providing additional habitat for bacteria.
Good structure and cooperation with soil life gives plants access to larger portions of the soil volume and more nutrients. Mycorrhizal fungi contribute to the structure with ‘superglue’, and they help distribute water and nutrients into the plant community. Fungal hyphae that reinforce aggregates are more resistant to degradation. Hyphae are also hydrophobic, and give the soil sponge-like characteristics, allowing it to absorb water and retain it for dry periods without dissolving, while allowing excess water to drain away.
With and without oxygen
From an ecological perspective, crumb structure can also solve chemical challenges. Manganese deficiency is often a problem in agriculture, but rarely in natural systems. The conditions inside aggregates are more reductive than outside, with higher and more consistent levels of carbon dioxide and water, and less oxygen. In the spaces between aggregates, there is increased oxygen availability and manganese is found as MnO4. Therefore, more of the manganese can be in a plant-available form inside aggregates.
There is surprisingly little nitrogen deficiency in nature (and among ecologists?), and this can also be connected to aggregates. It’s well known that nitrogen fixing bacteria live in the root nodules of legumes, without oxygen, but free-living forms exist too. Nitrogenase, the enzyme used in nitrogen fixation, has been found inside aggregates, and soil ecologist Christine Jones says that there are favourable conditions for nitrogen fixation there. The energy needed by bacteria to fix nitrogen is provided by mycorrhizal fungi, which then transport the nitrogen back to the plants.
Plants “create” topsoil
Many of the most fertile soils in the world were formed as the result of coevolution between grazing animals, plants, microorganisms, and predators. By spreading manure and trampling plants, grazing animals create microclimates which support decomposition. Many species in the grass family are especially suited to grazing. Grasses have large root systems, and they will form new roots and leaves in response to moderate grazing. Roots have a greater C/N ratio, and decompose more slowly than plant remains on the surface.
The area surrounding roots is rich with biological activity. There are “hotspots” of bacteria colonies, grazing protozoa, nematodes, fungi and worms. The collaboration is so tightly linked that it is almost impossible to separate root-associated microorganisms from plants.
Aggregate formation is important for cultivation, for carbon storage in soil, and to protect organic compounds from decomposition. We can influence these processes by the way we farm, and with active agronomic decisions.
Biological aggregate and soil structure formation is the “dark side” of ecological succession. A landslide in nature, or continuous intensive agricultural activity is a setback to the development of soil structure. In the early stages of succession, soil is disorganized and similar to subsoil. The structure begins to form with the establishment of the plant community, beginning with annual pioneers and eventually shifting toward perennial grasses and herbs. If there is no grazing, bushes and trees will take over. Species-rich, grazed meadows are one of the natural ecosystems which can sequester the most carbon and build topsoil.
We begin to see more stable aggregates and deeper structure with the establishment of diverse perennial meadows, which shows that plant diversity is essential.
Why take care of aggregates?
Biological aggregates or ‘crumbs’ are functional units in the soil. They prevent the soil from washing away with water, or blowing in the wind. Structure creates pores, allows air exchange, and enables water to distribute more evenly and flow through the soil profile. Aeration in the soil improves root development. Aggregates allow soil to retain organic material and plant-available nutrients, preventing them from leaching out into the water or air. They also increase the soil’s cation exchange capacity. All of these effects contribute to improved yields and better health and quality of plants, and are therefore important to the farmer’s economy in the long term.
Table: an overview of what is needed for the development of biological aggregates, and how they can be applied on the farm
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What |
How |
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Living plant cover |
Include meadows in rotation, or sow/plant new crops to replace harvests. Catch crops in grain and vegetables are a good option. Soil should be exposed as little as possible, both within and between crops. |
|
Reduce disturbance |
Minimize soil work such as plowing, harrowing, and tillage. |
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Protect soil and soil life |
Living plants or other soil cover. Prevent compaction; Avoid traffic and soil work in wet conditions. Maintain fixed driving paths, efficient routes, gentle loads and tire treads. Avoid rapid or strong chemical treatments, such as easily soluble fertilizers or large applications of lime. |
|
Diversity at all levels |
Diversity in plant communities, rotations, and varied organic material returned to the soil. Maintain natural meadows with appropriate grazing pressure and harvesting. |
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Complete the cycle |
Return animal manure, compost, and harvest residues, and other organic materials to the soil. Include grazing where possible, but avoid overgrazing. Return organic material to the soil. |
|
Livestock as soil builders |
Manage livestock to encourage large root systems and good growth in pastures. Avoid overgrazing. |
Photo caption for root with aggregates: Aggregates change the physical, chemical, and biological characteristics of the soil. They improve structural integrity, facilitate drainage and aeration, reduce erosion, and they protect microorganisms, life processes, and organic materials from degradation. Photo: Hege Sundet.
Author: Hege Sundet
Translator: Katelyn Solbakk
Title: Advisor for plant and soil culture and ecology
Organization: Norwegian Agricultural Advisory Service Østafjells
E-mail address: hege.sundet@nlr.no