3 Key Factors for Successful Nutrient Cycling in Soil
The key to creating productive, regenerative pastures is to understand the processes that move nutrients into, out of and within soil. Nutrients are usually in the form of water-soluble ions. They can be transported through the soil by mass flow, intercepted by roots that contact exchangeable ions on organo-mineral surfaces and immobilized in plants.
1. Soil Structure
The way in which sand, silt and clay particles are assembled into structural units, called soil aggregates, is what determines the structure of the soil. Aggregate size, shape and distinctness are the basis for classifying, describing and grading soils, as well as their behavior under varying conditions.
Most people are aware that a soil’s texture – the proportions of sand, silt and clay – has an impact on its performance and incredibly effective in promoting plant growth; however, few realize that soil structure is equally as important. Soil structure controls the movement of air, water and roots within the soil. A soil with excellent structure is easily permeable while a soil with poor structure is impermeable.
Soil structure is a result of the size and type of soil aggregates, the number of pores between them, and the manner in which they are cemented together. It is also controlled by the amount of organic material, water, fungi and calcium ions. Soil structure influences soil permeability, porosity and the rate at which water moves through the soil.
A good analogy is to think of a city; the layout and structure of a city influences how fast and efficiently traffic moves through it. The same is true of a soil; its structure influences how easily water can move through it, and therefore, the speed at which nutrients are cycled.
Soils are three dimensional, with layers of material extending horizontally from the land surface to the parent material. These layers are referred to as horizons, with the topsoil (the uppermost layer) being called the A horizon and the subsoil the B horizon. A soil profile is a two dimensional vertical section through the horizons showing the different soil types and their characteristics.
There are five general types of soil structure, defined by the shape and size of the structural aggregates: granular, blocky, prismatic, columnar, and platy. Each of these structures has a unique effect on the way in which water flows through the soil.
Soils characterized by a granular, or crumb, structure have high permeability and a large amount of pore space. These are soils that allow for easy movement of water, air and nutrients.
2. Soil Water Movement
The movement of water within soils is critical to their nutrient cycling. Soils act as a sponge to take in water and store it, as well as a conduit for moving water through the soil profile to reach plants’ roots. Water is moved into the soil by infiltration and downward flow through pores. The rate of this water movement is determined by the soil texture, pore space and hydraulic conductivity.
Soils are complex structures that comprise solid, liquid and gaseous materials. They are also home to billions of microorganisms that drive biogeochemical processes. They are the fulcrum upon which the wheel of life spins, providing essential ecosystem services such as water filtration, water storage, nutrient cycling and erosion control [1, 2].
Water moves through soil by capillary action from areas with high to low potential energy. Similar to how water at a higher elevation on a street will tend to run downhill, water in soil profiles move from areas of saturation to air dry regions through the forces of gravity and capillary attraction of the soil surfaces for water [3, 4].
Soil texture determines how much water is available to plants by controlling the number of large particles (clay, silt, sand) in the soil and the size of those particles. Soils can be categorized by their texture as sandy, loamy or clayey [1, 5].
The size of the smallest particle in a soil determines its granularity while the amount of organic matter affects its porosity. The size and texture of soil particles also influence the speed at which water moves through a soil, as does the tortuosity of a soil – how direct or indirect is the path that the water follows.
A sand-sized particle takes less time than a clay-sized one to travel through the soil, as it can more easily fit into the spaces between larger particles. The sand-sized particle will also have a smaller surface area than a clay-sized one, and as a result, it will have a lower hydraulic conductivity.
As the water in a soil moves through the soil, it picks up ions and other molecules along the way. These nutrients are adsorbed to the soil particles and are carried away by the water as it flows downstream, where they are released into the surrounding ecosystem.
Identifying the dominant mechanism for water movement in soils is important because it affects how much and how quickly water can be taken up by crops, which is vital to the crop’s health. Unraveling these processes can help us better predict plant responses to water and nutrient availability and guide management strategies for enhancing soil fertility, reducing erosion, halting biodiversity loss, and preserving our natural resources. This article is based on work supported by the Canadian Foundation for Innovation and the Natural Sciences and Engineering Research Council of Canada.
3. Soil Microorganisms
Soil microorganisms are crucial to the performance of soil functions including nutrient cycling, soil structure and sequestration of carbon. One teaspoon of soil contains around 1 billion microscopic organisms and 10,000 different species. The diversity of microorganisms in soil provides many essential benefits including boosting crop fertility, decomposing organic matter, releasing plant hormones and degrading complex molecules. Microorganisms also play an important role in the control of greenhouse gas emissions.
Soils contain many different kinds of organic material, both living and dead, including plants, animal manures, sludge from sewage treatment plants and food residues. These are transformed into humus or other stable organic matter (SOM) by soil microorganisms, primarily bacteria and fungi. This SOM provides nutrients, especially nitrogen and phosphorus, to crops in the form of rhizodeposits or mineralization. Moreover, some microorganisms provide specialized nutrient services such as nitrogen fixation or plant-arbuscular mycorrhizal fungi foraging soil phosphorus.
The number and biomass of microorganisms in a sample of soil varies widely depending on the quantity and type of organic material and the way it is handled. The most abundant groups of microorganisms in healthy soil are bacteria and fungi. Fungi grow in a branched network of filaments called hyphae and they release a distinctive earthy smell as they decompose organic residues. Bacteria are much more compact and have lower biomass than fungi, but they have the ability to break down structural carbohydrates such as lignin, chitin and cellulose, which are difficult for fungi to decompose.
Most research on soil microorganisms focuses on a subset of the population called the rhizospheric community, which comprises the microbial taxa that are associated with and attached to the roots of growing plants. These are able to interact directly with the plant and help it access the root zone and enhance its growth. Rhizospheric communities are extremely diverse and can be defined by a large number of different attributes, which makes studying them difficult.
A key challenge in this area is that only a small fraction of the resident microorganisms can be cultured in the laboratory, so most studies use cultivation-dependent approaches that involve inoculating soil or root-associated microorganisms with a medium and analyzing distinct strains in the laboratory. However, such assays may select for rapidly growing organisms and only give a snapshot of the active microbial population.
To function effectively, microorganisms require a constant supply of labile carbon (e.g., from root exudates) to meet their energy demands for cell maintenance and metabolization. If they are starved, they become dormant and no longer perform vital soil functions like breaking down recalcitrant nutrients or producing hormones that boost plant growth. Fortunately, providing a rich source of carbon (such as PhycoTerra) can awaken dormant soil microorganisms and put them to work enhancing soil health and yield potential.
