Soil looks simple. But a small clump contains an entire world teeming with life. Understanding microbial life in soil changes how people think about growing plants.
Microbial biomass carbon varies around a median of 206 micrograms per gram of soil.
The Invisible Workers Underground
Soil microorganisms, including bacteria, fungi, and archaea, drive essential soil functions such as nutrient cycling, organic matter decomposition, and disease suppression.
Bacteria often represent the most numerous group. They break down dead plant material and transform nutrients into forms plants can use. Some bacteria fix nitrogen from the air, turning it into fertilizer that plants need for growth.
Fungi contribute heavily to soil structure and the break down organic matter, significantly contributing to the conversion of carbon to stable organic matter. This makes fungi extremely efficient at building long-term soil health.
How Do Bacteria Help Plants Grow?
Bacteria do several important jobs in soil. As they decompose organic matter like leaf litter or dead roots, nutrients locked inside dead material are released and become available for plants to use.
Nitrogen-fixing bacteria work with plants in special partnerships. Bacteria like Rhizobium form symbiotic relationships that fix nitrogen, converting atmospheric nitrogen gas into usable ammonia that plants absorb through their roots. This free fertilizer helps plants grow strong without chemical additions.
Some bacteria dissolve minerals in soil. Bacteria such as Micrococcus, Enterobacter, and Pseudomonas play crucial roles in phosphorus solubilization, making phosphorus available for plant uptake. Plants need phosphorus for root development.
Understanding Fungi’s Critical Role
Fungi look different from bacteria; not only are they larger, but they have slightly different pigments. Fungal biomass is necessary for healthy soil—their size and structure give them special abilities.
Fungi break down tough plant materials like wood and tree bark. They produce special enzymes that dissolve lignin, the substance that makes wood hard. This decomposition creates rich, dark soil called humus that holds moisture and nutrients.
How Farming Practices Affect Soil Microbes
Fungi and bacteria keep each other in check through symbiotic relationships. Different plants prefer different ratios of fungi to bacteria. Annual crops may prefer lower fungal-to-bacteria ratios, while perennials prefer higher ratios. Forests have the highest ratios because trees depend heavily on fungal networks for nutrients.
According to a study by Lori et. al. in 2017, organic farming systems show 32 to 84 percent greater microbial biomass compared to conventional systems. Adding compost, manure, and cover crops feeds soil microbes and helps grow their populations.
Chemical fertilizers and pesticides harm soil microbial communities. Fungicides kill both harmful and helpful fungi. Without beneficial fungi, plants struggle to access nutrients and water. This forces farmers to add more chemicals, creating a cycle that damages soil health.
Understanding Soil as a Living System
Soil microbial biomass represents the foundation of productive agriculture and healthy gardens. When people protect and feed these microscopic workers, they foster plant-soil interactions and receive a stronger and healthier soil community.
Learning about soil microbes transforms how people garden and farm. Every decision—from whether to till, what to plant, and how to fertilize—affects billions of organisms working underground. Making choices that support microbial communities creates healthier soil, stronger plants, and better harvests that last for generations. Use the microBIOMETER® soil test to estimate your soil microbial biomass and ensure you have the healthiest soil possible.
Seasonal dynamics are a major driver of soil microbial communities. Much like you and I, microbes are more active during some seasons, and more dormant during others. This can be attributed to the different responses microbes have to nutrient inputs, climatic conditions, and other soil properties. As there are a lot of factors that affect microbial activity, it can be difficult for farmers or researchers to make definitive statements regarding the relationship between their soil microbial communities and seasonal changes. Specifically, temperature, moisture content, and the existence of plant life are considered the most important factors affecting microbial growth and activity within a season.
The presence of plants on the soil has a large impact on microbial life. As plants form, they begin to cultivate microbes surrounding their roots by producing nutrients for the microbes to essentially feed on. As the microbial community grows, they undergo a series of processes allowing them to obtain nitrogen and mineral nutrients from the soil and then provide the nutrients back to the plant to stimulate growth. This is part of the symbiotic relationship between plants and microbes– they support each other through the mining of nutrients from the soil and sun.
Just like plant presence, temperature greatly influences soil microbial properties. During cold seasons, temperature is considered a major limiting factor of microbial activity, whereas water availability could be a limiting factor during the summer season. Soil temperature can affect organic matter decomposition and mineralization rates, thereby impacting microbial biomass and activity levels. Bare soil, or soil without any plants growing, will have lower microbial activity occurring, regardless of season. This is why researchers and land stewards have emphasized the planting of cover crops between growing seasons in regenerative agriculture– as cover crops can alter soil properties and increase the biomass and diversity of microbial communities. In the warmer or hotter seasons, the addition of cover crops can also help to mitigate how much heat the soil is absorbing.
Studies show that microbial activity in agricultural soils increases in the fall when compared to other growing seasons–likely due to an increased level of nutrients and soil organic matter from crop and plant residue post harvest. Throughout the wintertime, or non growing season, microbial activity and composition is thought to be stagnant, but stable. An increase in microbial activity is said to occur after the thawing of frozen soils and can be linked to the freeze-thaw cycle (FTC) that colder climates experience. As snow freezes over soil, it inhibits air diffusion from occurring, creating anaerobic conditions for the microbial communities and therefore altering the soil community structure. In turn, this causes an increase in denitrification, respiration, and production of greenhouse gases, which are being trapped under the frozen layer. Once temperatures begin to rise, the soil begins to thaw, allowing oxygen into the soil. This provides labile carbon and other nutrients to the soil, which increases microbial activity and biomass. However, once thawing occurs, those greenhouse gases that were once trapped, are released into the air. This exact dynamic between microbial activity and the FTC is still being debated due to different soil properties greatly affecting freeze/thaw rates and as researchers use different methodologies, making it difficult to compare results between studies.
But despite the controversy surrounding the exact relationship between microbes and seasonal temperature changes, researchers do agree that microbial biomass and activity are related to seasonal temperature fluctuations. They’ve found that generally, microbial biomass decreases once the temperature increases past a certain point. As temperature increases, there is also an increase in CO2 being released from the soil, which we refer to as respiration. So when more respiration occurs, more carbon is being put into the air. This respiration process is sensitive to temperature change, which is why it’s imperative to have a better understanding of the seasonal dynamics of microbial communities.
As soil microbial life varies naturally by season, it might be hard to differentiate the natural seasonal changes from the changes related to your regenerative growing practices. Understanding the short term seasonal dynamics of microbial communities in various soil conditions is key in furthering our understanding of soil biology. Documenting and analyzing periodic readings with microBIOMETER® can assist you in differentiating between natural and seasonal changes in your soil.
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