- Soybean inoculation is critical to optimum yields
- Seeds should be supplied with the correct species of inoculant (do not use alfalfa or any other legume inoculation!)
- Seed treatments commercially applied should be handled as live product, controlling temperatures in storage and out of direct sunlight, as well as planted as quickly as possible after treatment.
- BMP for growers is to use liquid carrier of water and introduce inoculant onto plants as a pop-up placement.
- Purchased inoculant should be protected from temperature and direct sunlight until inoculant time.
- pH of soil should be maintained and 6.5 preferably 7.0 and if questionable sampling or testing or below 6.5 the addition of Molybdenum at .25lb of sodium molybdate per acre will aid in infection.
- Scout soybeans from V2-R1 to assess nodules with ideal infection of 10 on the main stem. If much less, consider application of N in season up to 50lb per acre.
The seed sprouts lateral and hair roots develop and send a signal that matches needs of the B Japonicum inoculant and the roots open a pathway for the bacteria to invade and nodules form. Properly functioning nodules will be pink in color because Leghemoglobin is red, the inside of an active nodule is pink. Sugars produced in the leaves move down the stem and into the nodules as an energy source for the bacteria so they can fix nitrogen. Stresses such as cold, drought, flooding, and low soil pH interfere with the establishment of the nodules and the with the nitrogen fixation process.
One bushel of soybeans requires 3.5 lbs of N for a 100 bu crop that would be 350 lbs of N per acre. The nodules fix and supply this demand from the soils if handled properly. Soils do not contain the rhizobium bacteria specific to soybeans called Bradyrhizobia japonicum. The soybean root will send signals out to nearby B Japonicum to trigger and genetic link between the two and begin to form a relationship where the plant feeds the bacteria and the bacteria in turn turns atmospheric N into usable N for the soybean plant to use for growth.
This picture (G. Roth 2014) illustrates the dramatic effect it has on soybeans if this process of inoculation does not occur. Soybeans on the right with N deficiency.
The nodules are typically formed on the main stem and the fine root hairs from emergence to V2 but the process continues but the main infection occurs early and growers can scout at V2- R1 to count for nodules that should be present. If not perhaps additional N might be responsive. A properly nodulated soybean plant should have five to seven nodules on the tap root two weeks after emergence or twelve total root nodules per inch of tap root at flowering (R1) (R. Elmore 2007). Typically on high yielding fields I have focused on the main stem and get about 10 large nodules on the main stem at R1. To evaluate nodule performance, cut nodules in half. Nodules that are actively fixing nitrogen will be colored pink to bright red, while nodules that are white or green are not producing or have not begun to fix N.
To get the rhizobium B japonicum in the soil growers need to provide this. Manufacturers of the B japonicum have refined the process and improved delivery and even the sub species with better more efficient B japonicum. The modern sterile products are four times more productive than the older non-sterile materials. (Also more profitable) (Ohio State Extension 2009) There are advantages to inoculants and there are numerous configurations of ingredients that can be applied to seeds.
Newer Products May Contain Improvements:
- Plant growth promoting hormones.
- Disease control materials and other bacterial Bacillus etc.
- Signals to induce/speed nodulation in cold soil.
- Extenders to protect the bacteria, allowing inoculation up to 30 days before planting.
- Binders to improve adherence to the seed
- Nourishment for the bacteria.
Ideal Environment for Inoculations
It is important to understand the environment for which will best suite this relationship of the root and the B japonicum. Elevated aluminum in the soil can be alleviated with maintaining the soil factors as below.
- pH- 6.5-6.8 pH of soil should be maintained and 6.5 preferably 7.0 and if questionable sampling or testing or below 6.5 the addition of Molybdenum at .25lb of sodium molybdate per acre will aid in infection. The Pa On Farm Network over three years tested moly on fields with pH lower than 6.5 and found a significant response to moly applied either on the seed and or in the V 1 stage. Here is one year’s results. A 2.5 bu/acre response over 3 years and 38 replications on both farm and research station work.
- Optimum P and K
- Moist but not wet
- Pa On Farm Network research with dairy, swine, and poultry manure confirm applications prior to planting may impact successful nodulation but it is not always a detriment. It is wise to avoid manure prior to planting or place in the fall and or at reduced rates to avoid potential complications. University of Delaware found applications of 45lbs of N applied with commercial fertilizer can lead to a 40% or greater reduction in N fixation over the maximum achievable N fixation when no supplemental N is applied.(Taylor, 2014 Nitrogen and Soybeans)
- Ideal 77°F for most species of B Japonicum but advances may be coming to aid with cooler soils.
- Rapid germination and colonization by rhizobium will take place if the soil and root systems are conducive to infection. This may not line up with extremely early planting as many times soils are much colder than normal.
First Year Syndrome
Virgin Soils (soils without soybeans in rotation for 5 or more contiguous years) require special treatment and a willingness to live with lower yields the first season.
- Some have used small grains in the fall treated with inoculant to kick start system.
- Set yield goals in half of normal production in the area.
- A planned 50lb applied per acre of N (R Elmore Nebraska Extension, 1984) N pre flower has proven effective to provide some avoidance of drastic loss in yield where plants fail to respond to double inoculated seed.
Caring For Inoculant is Important!
It is true that millions of bacteria are supplied within the bag of purchase inoculant however those bacteria can die quickly with major swings in temperature and direct sunlight so here are some tips. The majority of the B Japonicum cells survive best at 40 to 80 F. and should be stored in a cool place out of direct sunlight. The shelf life of inoculation materials is from a few months to two years depending on formulation and additives in the product. Follow these storage parameters to maintain viability.
- Storage temperatures (40-80 F) (do not allow to heat above 80) bottom of bank barn out of sunlight is a great spot if in plastic packaging.
- Keep out of direct sunlight.
- In field temperatures
- Poor inoculation occurs at 50-59 with some species that will not grow at all at those temperatures. So if issues occur delayed planting may prove useful in ensuring ability of the inoculant to survive.
Causes of failures
- inoculant heating in storage
- Flooding(prior to planting most severe)
- Drought shortly after planting or at planting
- Low pH
- Below 6.5 pH annual inoculation highly recommended
- Fungicide interactions
What About Annual Inoculation?
Answer these questions for your specific field: This list provides some insight into where annual inoculation is most beneficial and is a work in progress. Any points means better to inoculate.
- Is it in a consistent corn–soybean rotation? ... Y___ (0) N___ (2)
- Will the field be planted early (before May 1)? ...Y___ (1) N___ (0)
- Do parts of this field experience long periods of flooding (more than four days)? ... Y___ (3) N___ (0)
- Is the pH of the field below 6.0 or above 8.0? ... Y___ (3) N___ (0)
- Does the field coarse soil? ... Y___ (3) N___(0)
- Did the last crop of soybean appear yellow, especially during the second half of the season? ... Y___ (4) N___ (0)
- Have seed-applied fungicides been used with each previous soybean crop? ... Y___ (2) N___ (0)
- Do the crops often experience severe water stress? ... Y___ (3) N___ (0)
- Is the field in a no-till system? ... Y___ (3) N___ (0)
GIP (Good Inoculant Practices) for Applying Inoculants
Before applying an inoculation material it is important to check its compatibility with any other seed delivered technologies.
When non-compatible materials are on the seed, plant within four hours of inoculant application if possible. Compatible fungicides are: Apron, Allegiance, Maxium, and biological materials. All new materials use a sterile media and have up to 108 bacterial cells per gram of inoculation material. Use application rates provide over 800,000 cells per seed. Most products have an adherent and anti-desiccant to protect bacteria cells. All have nutrients to maintain cell viability and activity. Over application is not toxic to the seed, but is expensive and may cause metering problems for the drill or planter. Seed can be inoculated when loading a drill with an auger or when loading by hand or from bags.
What is the best method to get inoculant applied?
Liquid and dry materials perform equally well when used properly. However, liquid materials have a longer shelf life and are easier to use. Some newer materials can maintain viability on untreated seed for sixty days or more, and for seven or more days when applied over some fungicides. This is important as even if commercial applied if fungicides are utilized at the same time that may affected inoculation then direct planting is advised.
Many growers are using a 15-inch row planter with pop-up liquid applications and using water 3 gal/acre as a carrier to direct-apply liquid inoculants on the seed in the field at the same time, which is the most preferred method of inoculation.