Microbial Inoculants

A program of the National Center for Appropriate Technology • 800-346-9140 • www.attra.ncat.org Microbial Inoculants The symbiotic relationship By Barbara Bellows, A growing number of companies between plants and Texas Institute for Applied are selling biofertilizers or bios- soil microorganisms. Environmental Research; timulants: microbial inoculants Source: Meena et al., 2019 Mike Morris, NCAT; and intended to enhance soil health. Colin Mitchell, NCAT In laboratory and greenhouse Published Sept. 2020 studies, the microbes con- ©NCAT tained in these products have IP604 proven eff ective in enhancing nutrient availability, decreasing Contents pest infestations, stimulating Introduction ......................1 plant growth, protecting plants against stress conditions, and Overview of enhancing plant competition Plant-Benefi cial Microorganisms ...............2 against invasive species. These benefi ts have not always been Reasons Why seen by farmers in “real world” Microbial Inoculants conditions, however—making May Not Work ...................5 the value and cost-eff ective- Overcoming Microbial ness of these products a topic Inoculant Limitations .....7 of considerable uncertainty. Summary, Future To help you reach your own Directions, and conclusions, this publication Recommendations .........8 explains how microbial inoculants work, reviews current (in 2020) scientifi c literature and studies, and References ...................... 10 off ers practical recommendations: factors to consider before buying and using these products. Further Resources ........ 12 Introduction vast number of soil microorganisms infl u- contain benefi cial microorganisms that are ence nutrient uptake, water access, disease intended to help plants access their own nutrients A resistance, and many other factors related from the soil. Some of these products contain a to plant growth and health. For example, some single organism, others include a mixture of organ- bacteria, fungi, and viruses have been shown to isms, and some include proteins, enzymes, or other This material is based upon work increase competition against weeds, while others supported by the Southern Sustain- compounds intended to serve as food sources for able Agriculture Research and Edu- can remediate or immobilize soil contaminants, the microorganisms. cation program through Research and Education grant award number including heavy metals (Mishra and Sarma, 2017) LS14-264: Indicators and Soil Conser- Interest in these products is growing rapidly. vation Practices for Soil Health and and hydrocarbons (Janczak et al., 2018). Carbon Sequestration. According to Fortune Business Insights, the global Intense land use, tillage, and synthetic agricultural microbial inoculant market was valued at $1.34 inputs can greatly diminish the number, diversity, ATTRA (www.attra.ncat.org) billion in 2018 and is projected to reach $3.15 is a program of the National and benefi ts provided by soil microorganisms. Center for Appropriate Technology billion by the end of 2026. Seed treatments of (NCAT). The program is funded Many farmers and ranchers who want to restore through a cooperative agreement rhizobia and other nitrogen-fi xing microorganisms with the United States Department plant-microbial interactions are exploring microbial of Agriculture’s Rural Business- accounted for 82.7% of this market. Cooperative Service. Visit the inoculants: powders or solutions containing live NCAT website (www.ncat.org) Th e use of rhizobia bacteria to enhance nitrogen for more information on or dormant microorganisms. Unlike chemical fer- our other sustainable tilizers that contain plant nutrients (like nitrogen, fi xation by legumes (the Fabacae family, including agriculture and energy projects. phosphorus, and potassium), microbial inoculants peas, beans, alfalfa, clovers, and vetch) has been www.attra.ncat.org Page 1 thoroughly studied by researchers since 1896, their food from plants, and directly involved in and rhizobia inoculum has been widely used to plant growth processes. Still other microorganisms, enhance nitrogen fi xation since the 1950s. In con- like mycorrhizae, live partially within plant cells trast, there has been far less research on other and partially within the rhizosphere. microbial inoculants, especially under fi eld conditions. Controlled studies in greenhouses have Rhizobia shown benefi ts such as enhanced nutrient uptake, Rhizobia are a genus of rhizobacteria: bacteria resistance to drought and saline conditions, and that live in the rhizosphere. Th ey live in large colo- protection against pest and disease infestations. nies in nodules on the roots of legumes, forming a Unfortunately, these benefi ts are not always seen Related ATTRA symbiotic (mutually benefi cial) relationship with in the fi eld, under real-world conditions. Th is Publications their plant hosts. Rhizobia take nitrogen from the publication explains how microbial inoculants air (N2) and transform it into ammonia (NH4), www.attra.ncat.org work, reviews current (in 2020) scientifi c litera- which plants can use to form proteins and other ture and studies, explains several possible factors Alternative Soil biomolecules needed for growth. In return, the that can limit their eff ectiveness, and off ers practi- Amendments plant provides a place to live on its roots, as well cal advice about buying and using these products. Nutrient Cycling as carbohydrates that the rhizobia use as energy. in Pastures Overview of Plant-Benefi cial To promote the formation of nodules and also Sustainable Soil nitrogen fi xation, rhizobia inoculum in a powder Management Microorganisms form is mixed with peat or biochar, wetted, and Plant roots provide an excellent habitat for soil ATTRA Soil Resources applied to legume seeds immediately before plant- organisms. Osmosis and root pressure pull soil ing. Th ere are eight diff erent rhizobia-legume nutrients towards plant roots, making the root cross inoculation groups, and it’s important to zone a nutrient-rich area. At the same time, plant use the correct type of rhizobia for the legume roots exude many organic chemicals that serve as being planted. (See the Further Resources section food sources for microorganisms. for guides to successful inoculation.) Plant-benefi cial microorganisms living in the rhizosphere (the area close to plant roots) include Mycorrhizae decomposers and bacteria involved in nitrogen Mycorrhizae are fungi that form symbiotic rela- transformations. Other microorganisms, called tionships with almost 80% of all land plants. endophytes, live inside or between plant cells: Th e mycorrhizae most important for agriculture protected from predators, getting most or all of are the arbuscular mycorrhizae (sometimes seen abbreviated as “AM”). Mycor- rhizae begin their life within the cells of plant roots, where they extract carbohydrate energy and produce hyphae, thread-like structures that grow through the soil. Since these hyphae are considerably thinner than plant roots, they can grow more easily through soil, obtaining hard-to- reach phosphorus and water that are transported to plant roots. As hyphae grow through and around soil particles, they bind soil particles into aggregates: clumps of soil mineral and organic matter that are key to soil nutrient and water-holding capacity and also enhance soil tilth or friabil- Plant-benefi cial activities of endophytes. Source: Brader et al., 2014 ity, decreasing soil compaction. Page 2 Microbial Inoculants Rhizobium bacteria. Source: McMahon, 2016 Common mycorrhizal networks. Source: Jansa et al., 2013 Plant Growth-Promoting Rhizobacteria (PGPR) Plant growth-promoting rhizobacteria (PGPR) are rhizosphere bacteria that (as the name implies) stimulate plant growth. Some PGPR protect against pathogens, stimulate production of growth regulators, or enhance a plant’s ability to detoxify poisons. Others enhance the availability of nutrients by fi xing nitrogen, solubilizing phosphorus and potassium from soil minerals (Khan et al., 2007), or facilitating uptake of iron (Dob- belaere and Okon, 2007). PGPR can also work with mycorrhizae during droughts to enhance nodule formation on legume roots and nitrogen Mycorrhizae. Source: Karas, no date fi xation by rhizobia (Pandey et al., 2016). Among their other benefi ts, mycorrhizae can produce chemicals that make iron and other micro- nutrients more available (Saha et al., 2015), make plants more resistant to saline conditions (Vuru- konda et al., 2016), or produce phytohormones that signal the plant to create chemicals hindering the growth of pests and diseases (Hohmann and Messmer, 2017). Mycorrhizae can also connect with the roots of other nearby plants, including plants of diff erent species. Th ese common mycorrhizal networks can transfer nutrients, protective substances, and signals among plants—helping neighboring plants resist infestations from pests, diseases, or invasive plants. Some studies have shown that these mycorrhizal networks may be partially responsible for enhanced yields of multi- cropping systems (Walder et al., 2012). Plant growth-promoting rhizobacteria. Source: Kumar and Dubey, 2012 www.attra.ncat.org Page 3 Like mycorrhizae, PGPR can produce phyto- growth of pathogens or their ability to enter hormones, improving tolerance for salinity, and damage plant cells. PGPR can also stimulate drought, and extreme temperatures (Vurukonda protective responses to pest or disease infestation, et al., 2016). One group of PGPR produces such as exuding exopolysaccharides (Vurukonda

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