By Kristy Borrelli and Terrence Bell et.al
A basic overview about soil microbes, their communities, and functions including opportunities and challenges associated with their management.
Soil biology is important for soil health and the soil biological community encompasses all living things including earthworms, insects, nematodes, plant roots, animals, and microbes. Beneficial soil microbes perform fundamental functions such as nutrient cycling, breaking down crop residues, and stimulating plant growth. While the role of microbes to maintain soil health and contribute to crop performance is clear, the soil biological component is extremely difficult to observe and manage. Soil microbes, biological communities, and the functions they perform are dynamic, complex, and not easily interpreted for field practices. Yet, they are getting more attention as farmers are encouraged to practice more holistic management approaches on their farms.
Two articles published in eOrganic by Penn State scientists, Soil Microbes in Organic Cropping Systems 101 and Management of Soil Microbes on Organic Farms, help farmers and people without a background in microbiology simplify information about soil microbial community functions and specific aspects of their management. Although the articles target organic agriculture, their information will be useful to anyone who is interested in learning more about the fundamentals of soil microorganisms. Both articles are briefly summarized here.
Most soil microbes can be classified as fungi, bacteria, archaea, protozoa, or viruses. It has been estimated that a single gram of soil can contain up to several billion bacteria alone. In agricultural systems, soil microbes are directly associated with soil carbon because of the essential role they play in residue decomposition and nutrient cycling. In soils, carbon-based materials include plant litter, compounds released from plant roots, living or dead soil organisms (including microbes), and larger soil fauna like worms and insects, as well as their waste. Collectively, all carbon-based materials, including living things, are known as soil organic matter.
Like all living things, soil microbes need access to nutrition and water and therefore tend to concentrate near sources of soil organic matter. Microbial biomass and diversity tend to be much higher in soil immediately surrounding plant roots, an area known as the rhizosphere, and other organic matter deposits. Due to their close proximity to plant roots, soil microbes significantly affect soil and crop health. Some of the activities they perform include, nitrogen-fixation, phosphorus solubilization, suppression of pests and pathogens, improvement of plant stress, and decomposition that leads to soil aggregation. However, soil microbes can also be harmful to crops if they cause disease or compete for nutrients.
Although microbial diversity is necessary for multiple microbial functions to occur in the soil, including disease suppression, clear relationships between microbial diversity and soil or crop health continues to be difficult to decipher. In general, soils with higher microbial abundance and diversity have a greater chance of containing individuals that perform a particular function under a range of conditions, but there is often a saturation point where more diversity does not result in noticeably improved soil function.
Despite understanding their importance on farms, the diversity and complexity of soil microbial communities make them incredibly difficult to manage. Many strategies do exist for managing soil microbes, however, and range from adding beneficial microbes (inoculants or commercial products ) to suppressing harmful microbes (soil fumigation, soil steaming, anaerobic disinfestation, and solarization). These approaches range in cost, labor and equipment needs, scale of application, and measurable efficacy. Additionally, common crop management practices often target other agronomic needs and simultaneously influence soil microbial communities. Examples include tillage, crop rotation, cover cropping, and adding compost, manure, or mulch.
Benefits, challenges, and considerations associated with managing soil microbes using different methods are thoroughly outlined in the aforementioned articles. Predicting the overall response of the soil microbial community to any of these practices is challenging. Microbial management practices can help growers achieve specific goals however, any intervention can yield complex or unintended consequences. Farmers have very few tools available to monitor the impacts of microbial soil interventions and are encouraged to focus on the following considerations.
- Microbes play an important role in crop and soil health but can be beneficial or harmful.
- Current research has not yet optimized microbial contributions to soil systems.
- Soil microbial impact on crop and soil health will depend on composition and activities of existing soil microorganisms as well as other soil characteristics.
- Farmers are urged to be selective when using time-intensive or financially-intensive management practices.
- Performance of some products or practices can be monitored on individual farms but should be considered in small trials.
- It is possible to introduce pathogens or other microbes with negative impacts when attempting to introduce beneficials.
- Soil building practices can have overall beneficial impacts on soil biological communities and are encouraged to build soil health on farms.
Source : psu.edu