The bacterial strain of Rhizobium used by soybean is Bradyrhizobium japonicum, and the process by which it gains access to the plant is very complex, but fairly well understood. Soybean plants release chemical signals (flavanoids) into the soil as they germinate and the plants emerge. These signals are picked up by the rhizobia in the soil and in inoculation material, and produce a return signal “Nod factor signal” to the plant. This return signal results in the plants’ roots preparing for infection by the bacteria. The Nod factor signal causes the root hairs to curl, trapping rhizobia that are present on the surface of the root. An infection thread develops, and the rhizobia multiply until they reach the inside of the root. The cells of the root also begin to divide and form a specialized structure called a nodule. The rhizobia continue multiplying inside the nodules increasing their size. The enzyme (nitrogenase) that fixes nitrogen from the air to form ammonium cannot function in the presence of oxygen. In order for nitrogen fixation to occur, the plant must produce Leghaemoglobin inside the nodule to absorb oxygen in the root to prevent it from interfering with the nitrogen fixation process. Since Leghaemoglobin is red, the inside of an actively fixing nodule is pink. Sugars produced in the leaves travel to the root system and move into the nodules to provide energy that the rhizobium uses to extract nitrogen from the air to make the nitrogen compounds which the plant then uses to produce protein. The better this process works, the greater the grain yield and amount of protein produced.
Both plant and bacteria genes govern the entire nitrogen fixation cycle. Stresses on the bacteria or the plant such as cold, flooding, drought, and low soil pH can all interfere with this process. Cold temperatures delay the recognition of both the plant and bacteria signals. The plants and bacteria have to produce more signals in order to begin nodulation, which delays the onset of nitrogen fixation. Once enough signal is received and
nodules develop, ammonium is produced and provided to the crop.
Drought reduces the moisture in the soil that protects the bacteria while it lives on the seed surface after planting. A dry seedbed and dry seed will quickly draw moisture from the inoculation material causing the bacteria on the surface of the seed to dry and die. If enough bacteria die, there will be little nodule formation and insufficient ammonium production for a good yield. Other stresses, including low soil pH, seed treatment chemicals, and starter fertilizers, can also kill the bacteria or inhibit nodulation. In fact, soil nitrogen levels in excess of 30-50 lbs per acre can stop both nodulation and nitrogen fixation altogether.
Caring for inoculation products
Rhizobia cells survive best at temperatures of 40 - 80 degrees F. Prior to application, inoculants should be stored in a cool place and out of direct sunlight. Packets exposed to sunlight during the planting season will overheat rapidly due to the greenhouse effect and all the bacteria can be killed in less than an hour of exposure. When transporting the inoculant to the field, try to keep it cool. The back of the cab and out of the sunlight is best, and under a parked truck in the cool shade when in the field. Sunlight is a problem of heat, not radiation, so keeping the inoculation material in its box will stop some heat absorption. Under normal conditions the shelf life of inoculation materials varies from a few months to two years depending on the formulation and additives in the product. Dry materials generally have shorter shelf lives than liquids, but some sterile products have a shelf life approaching that of liquids.
Applying inoculation materials
Before applying an inoculant, it is important to check for compatibility with chemical seed treatments, fungicides and insecticides. Chemical compatibility can vary with seed treatment brand. Anchor and Vitaflo 280 both contain the same active ingredients; Thiram and Vitavax; however, they react to inoculants very differently. Compatibility is measured as inoculant survival on untreated seed vs treated seed. Seed treatment compatibility is influenced by specific formulations of both the fungicide and the inoculation material and by application method, e.g. slurry application vs sequential and/or simultaneous applications. Inoculants with an “extender type” additive have significantly improved on-seed planting intervals.
Seed inoculated with a product not having an “extender material” should be planted as soon as possible after treatment (12 hours or less) so the bacterial cells will remain moist and survive long enough to infect soybean roots following germination. Seed inoculated with a product having an “extender” can be held up to 30 days before planting depending on the product and which fungicides may be present. Always check the inoculat
ion label for fungicide compatibility and the recommended planting interval. Most inoculation companies are working to increase compatibility with fungicides to give producers more management flexibility in the use of inoculation materials. It is notable that some fungicides, such as Apron-Maxx RFC (Rhizobia Friendly Concentrate) are designed for lower Rhizobia Toxicity. When applying a fungicide or using fungicide treated seed, be sure the fungicide has dried before applying inoculation material to the seed. Some inoculationproducts may be mixed with some fungicides and applied to the seed together, and many of the good liquid inoculants allow a 4-hour tank mix with fungicides so they can be simultaneously applied from the same tank. For specific product information refer to the product label.
When loading a drill or planter using an auger; liquid or dry inoculation materials should be added to the seed as it enters the auger for thorough application. While over-application of a dry inoculant should not be toxic to seed, as it can with chemical seed treatments, it increases the cost and may interfere with seed flow through the planter or drill. Seed inoculated at the recommended rate with peat inoculation materials will appear untreated from a distance. Twenty to forty small specks of inoculation material on each seed are adequate. Over-application of liquid products is also costly and may cause bridging in the seed box. Calibration of the application system is a must if the proper rate of inoculation material is to be applied. First determine the delivery rate of seed through the auger by weighing the amount of seed passing through the auger in 30 seconds. Then calculate the amount of inoculation material needed for that amount of seed and set the inoculation applicator to apply that amount of inoculation material in 30 seconds.