Biological Soil Health

It is widely known that healthy soil is the foundation of agricultural productivity and long-term profitability. Less known is the importance of living organisms within the soil. Bacteria, fungi, and other soil life drives decomposition, releases nutrients, suppresses certain diseases, and builds soil structure that improves water movement and storage. Understanding soil biology not only helps explain why management practices—such as cover crops, manure additions, or reduced tillage—improve the biological health of soil, but also demonstrates how life found in soil can improve input efficiency, reduce risks, and strengthen a farm’s bottom line. The goal of this fact sheet is to introduce the basics of biological soil health, helping farmers connect the science of soil biology with practical tools that support stronger and more profitable soil.

What Are Soil Biological Properties?

Soil biological properties refer to the abundance, diversity, and activity of living organisms in soil. These living organisms perform critical soil functions such as decomposition, nutrient cycling, disease suppression, and building of soil structure. The living organisms in soil make up the soil food web, connecting soil to plant health and ecosystem functionality. Therefore, biological soil health is related to specific functions that can be performed and the rate at which those functions can occur. For example, is the soil biology mineralizing (i.e., releasing) organic nitrogen to a plant-available form, and if so, at what rate? In practical terms, soil biological properties can be indicative of the ability for a given soil to provide specific functions that may be financially beneficial to a producer.

Key Soil Biological Functions

Decomposition

• breakdown of complex carbon (C) (e.g., leaves and stems) to simple forms (e.g., sugars)
• contributes to the long-term stable soil organic matter pool
• food for microbial communities

Nitrogen and Phosphorus Mineralization

• microbially driven transformation of organic nitrogen (N) and phosphorus (P) (e.g., P and N in plant detritus or manure) to inorganic (i.e., plant-available) N or P
• key contributer to the plant-available N and P pools

Aggregation

• fungi build hyphal networks that also help glue soil particles together
• particles held together (Figure 2) lead to the development of soil structure that improves water dynamics and nutrient availability
• some microorganisms, such as fungi and earthworms, bore holes into soil minerals and rocks, creating more macro and micro pores that further increase the water-holding capacity and air exchange of soil.

Disease Supression

• healthy soil biota promotes beneficial microbes that outcompete pathogens
• certain microbes can trigger plants to strengthen their natural defenses against disease
• soil organisms break down plant residues that may contains pathogenic microbes

Key Soil Biological Attributes

1. Abundance. Amount of microbes in the soil, commonly measured by microbial biomass carbon. Sometimes measured through the carbon dioxide (CO₂) burst test which may serve as a proxy test for abundance.
2. Diversity. Type of organisms in the soil. DNA sequencing can provide a comprehensive profile of biological diversity.
3. Activity. Measures how active soil biology is in the system. Enzyme-activity analysis can determine how effective microbes are for decomposition of organic substrates.

As they maintain the chemical and physical health of soil, biological organisms can effectively carry out key functions such as C and N cycling, disease suppression, and improving aggregate stability. However, when the chemical and physical health of soil is neglected, biological organisms will struggle to perform their functions, inhibiting the soil from providing services critical to agricultural and ecological success.

Testing for the Biological Health of Soil

The biological health of soil is typically more sensitive to changes in management than the physical or chemical health of soil. Soil testing for biological health indicators has increased dramatically with the advent of DNA sequencing. Testing for the biological health of soil often includes a suite or subset of a variety of tests:

1. Soil organic matter (SOM). Total amount of organic material including living and dead components. This test reflects long-term carbon inputs.
2. Soil respiration. CO₂ released by microbes during decomposition. This test is used as a proxy for the rate of microbial activity in the soil.
3. Microbial biomass carbon. This test is also used as a proxy for the amount active microbes in the soil.
4. Active carbon (POX-C). Readily available carbon pool accessible to microbes. This test indicates the amount of food present for soil microorganisms.
5. Autoclaved citrate-extractable (ACE) protein. A proxy measurement for the organic nitrogen pool. This test is related to the storage and cycling of nitrogen over time.
6. Potentially mineralizable nitrogen (PMN). Measure of the amount of organic nitrogen converted to inorganic nitrogen (i.e., plant-available nitrogen) by soil microorganisms. This test could potentially offset the amount of inorganic nitrogen fertilizer needed by plants.
7. Beta-glucosidase enzyme activity. Measures the degradation rate of carbohydrates/ cellulose into simpler sugars that can be utilized by microorganisms. This test is used to indicate soil carbon cycling and potential nutrient availability.

Practices to Improve the Biological Health of Soil

When targeted management practices are aimed at improving soil health, biological indicators in the soil will typically improve before other physical or chemical indicators. Best Management Practices (BMPs) to improve the biological health of soil are rooted in carbon inputs and reduced disturbance, both chemical and physical. Adding more carbon or “food”—and a diversity of food with a proper nutrient balance —for microbes while also maintaining the structure of their “home” is key to improving all aspects of soil health, but particularly the biological health of soil. Over winter, cover crops that continue to exude or provide a year-round microbial food source help promote the biological functions of soil. Reducing tillage aids in maintaining the structure of the microbes’ “home,” therefore enhancing their abilities to perform key functions. Using a combination of BMPs to target improvements in the biological health of soil can typically be observed within the first 3–5 years following their implementation. As a result of implementing BMPs, soil biology will aid in the management of N and P, maintaining more in the field rather than allowing it to leach or run off.

Conclusions

The biological health of soil is a vital component of overall soil functionality, directly tied to processes that support plant growth, nutrient availability, and soil stability. Soil microorganisms drive key functions such as decomposition, nitrogen cycling, aggregation, and disease suppression, each essential to resilient and productive agricultural systems. Because soil biology is highly responsive to changes in management, testing for biological soil health indicators may show a response over shorter time periods (i.e., within several years). By adopting BMPs, such as using cover crops and reducing tillage, producers can foster healthier soil biology to improve key soil biological functions. Strengthening soil biology can help achieve higher yields or maintain yields during times of environmental stress, improved input efficiency, and long-term farm productivity.

Additional Resources

• Baseline Assessment of Soil Health in Ohio
  (soilhealth.osu.edu/sites/soilhealth/files/imce/WhitePapers/Baseline%20Ohio%20Soil%20Health.pdf)
• Chemical Soil Health
  (ohioline.osu.edu/factsheet/anr-0204)
• Physical Soil Health
  (ohioline.osu.edu/factsheet/anr-0203)
• Soil Health
  (ohioline.osu.edu/factsheet/anr-0202)
• Soil Health Testing
  (ohioline.osu.edu/factsheet/anr-0206)
• Soils and Soil Health
  (ohioline.osu.edu/factsheet/anr-0136)
• NRCS Soil Health Fact Sheets
  (nrcs.usda.gov/resources/guides-and-instructions/soil-health-fact-sheets)
• NRCS Biological Indicators and Soil Functions
  (nrcs.usda.gov/sites/default/files/2022-10/biological_indicators_overview.pdf)