Inoculant Importance for Legumes
Legume species are unique because with the help of rhizobia, they transform atmospheric nitrogen (N) gas into the plant useable NH3 form.1 Nearly 15% of the plant’s photosynthetic energy goes into N fixation. Rhizobia bacteria are living organisms that occur naturally in the soil; however, agronomic practices and environmental stress may limit their availability. If the specific rhizobia for the legume planted is missing or limited in the soil this could negatively impact N fixation.
Rhizobia colonization is tightly controlled by legumes and doesn’t occur until soil available N is used. After soil available N is removed, the legume’s roots secrete flavonoids into the soil which signal the rhizobia to begin colonization. Since each legume species has specific rhizobia strains that maximize fixation, signals sent by the plant and rhizobia must be in synch or the N fixation process is disrupted. When specific rhizobia are not present or present in limited quantities, commercially seed applied inoculants can fill the void, allow for rhizobia to reproduce, and enhance the nodulation process.
Commercial Seed Applied Inoculants
Seed treatments including N fixation inoculants may be applied upstream by the seed company or downstream by retail-applied systems. Regardless of the applicator, the application process is important for rhizobia livelihood and integrity before and after planting. The time between the application of the inoculant, planting, and the subsequent time needed for the inoculant to start working in the soil is referred to as the “planting window”. There are several factors that can impact an inoculant’s planting window including desiccation, chemical resistance, environmental factors, equipment, manufacturing, and packaging.
Following best management practices and applying a quality, high rhizobia colonization product are the first steps for a successful inoculant application. Achieving a high inoculant rate on the seed can be difficult because space on the seed is limited and inoculants compete for this space with other products such as polymers, pesticides, flowability agents, and seed-applied nutritional products. Holding capacity is a function of the properties of the seed treatments including viscosity, suspended solids, and drying time. Consequently, there is a pressing need to concentrate inoculants to preserve space on the seed for other necessary treatments.
Although holding capacity can be a concern, there doesn’t seem to be any limiting factors, like reduced germination, from “over” applying too much inoculant. However, nodulation studies and field performance research in large-seeded legumes seem to indicate that counts somewhere between 50,000 to 100,000 colony forming units/milliliter (CFU)/seed are a common target at planting to reliably achieve product performance.
Inoculant performance is strongly influenced by the seed-applied rhizobia count (CFU/seed) which can be influenced by application rate, concentration applied, and preservation of the applied rhizobia; therefore, the manufacturing and application process is important for rhizobia survival. As living organisms, rhizobia bacteria require oxygen for survival and must be handled carefully. Allowing freshly treated seed to dry (desiccate) too quickly can contribute to a high desiccation rate. The larger the load of rhizobia on the seed, the higher the probability that enough rhizobia survive desiccation. During the treatment process, rhizobia survival can be influenced by the total seed load of seed treatments, their composition, the relative dryness of the seed, the seed coat, and the environmental conditions at the time of application. Treatment drying times are affected by the ambient air temperature and humidity.
Equipment type, and operating procedures can have an impact on rhizobia preservation. Balancing the need for dry seed and not damaging the rhizobia by drying too fast is an art. Rather than adding fans, heaters, ladders, or other drying aids, it is best to slow and level the barrels on continuous treaters. Additionally, bypassing the atomizer on the treater and reducing dwell times in batch treaters can help preserve rhizobia. Certain sugars (osmoprotectants), like trehalose, can help bacteria resist desiccation. The fermentation and other processes should be optimized to help ensure that high concentrations of these substances are present.
Because of the need for oxygen, most liquid inoculants are packaged in plastic films that allow for the transfer of oxygen and carbon dioxide. If the product is packaged in a non-gas permeable plastic container, sufficient air needs to be included in the package to support the rate of metabolism. Knowing the air amount needed can be difficult to estimate because of variable environmental and metabolic rates.
Most liquid inoculants are packaged in clean rooms into sterile packaging because contaminants compete with the target bacteria for air and food. Contaminants can also overrun the bacteria and change the pH of the broth or produce metabolites that are harmful to the target bacteria. Aseptic packaging and those with breathable films can offer increased viability and potentially higher product quality. Package convenience may come at the expense of product quality.
The commercial label for inoculants MUST be read when the product contains an EPA-registered ingredient. Since inoculants are not the same, it is important to understand for which legume species the inoculant provides benefits.
The plant itself ultimately controls nodulation and regardless of how much inoculant is applied, the plant only allows for as much nodulation development as it needs to meet N requirements. Rhizobia strains and the fermentation process heavily influence the durability of the rhizobia and are very important to nodulation success (Figure 1). Strain selection determines the infectivity and effectiveness of the rhizobia, the two most critical criteria in an inoculant product.
Quality commercial inoculants can help achieve nodulation potential and capture the benefits of N fixation. Some inoculant products may also contain bacteria to help enhance mycorrhizal colonization. Mycorrhizae can help to increase functional root volume which ultimately helps the plant uptake water and nutrients. Other products may contain bacteria that can help enhance the uptake of nutrients such as phosphorus.
1Evers, G.W. Need for legumes. Texas A&M AgriLife Research & Extension Center at Overton. Texas A&M University. https://overton.tamu.edu/faculty-staff/gerald-wayne-evers/cool-season-legumes/need-for-legumes/.
Flynn, R., and Idowu, J. (original authors Lindemann, W.C. and Glover, C.R.) 2015. Nitrogen fixation by legumes. Guide A-129. New Mexico State University. https://pubs.nmsu.edu/_a/A129/.
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