By Travis Faske, Extension Plant Pathologist and Peanut Specialist
Peanut planting in the South-Central region and Arkansas typically begins in April when the soil temperatures at 4-in. deep are consistently above 65°F. So, planting season is just around the corner for much of the South. Each year there are a few fields with inoculation issues, which can have a significant impact on yield. Some stand out like those yellow strips through the field (Fig. 1), while others are more subtle with fewer nodules per plant than anticipated. With the recent formation of the Delta Peanut Cooperative and subsequent discussion of new buying points, peanut acreage is expected to increase in Arkansas allowing new farmers to incorporate peanut into their production system. Thus, now is a good time to point out a few important facts about peanut inoculants.
Like other legume crops, peanuts have a symbiotic relationship with nitrogen-fixing bacteria, Bradyrhizobium. These bacteria fix atmospheric nitrogen into a form that is utilized by the peanut. Peanut inoculants (not to be confused with soybean inoculants) contain a specific strain of these bacteria that is essential for nodule formation and nitrogen fixation in peanut. Inoculants are most commonly applied as a liquid formulation below the seed in the seed furrow. Nodules can be observed on the root system about 35-45 days after planting. This is a critical time to check peanut for adequate nodulation (12-25 nodules/root system; Fig. 1) as peanut will not show symptoms of nitrogen deficiency until it is too late (Fig. 1), at begin pegging and pod development stage of growth. Moreover, applying fertilizer at this stage of development will not allow for maximum yield potential of a peanut crop. For proper use of inoculants, here are a few reminders:
- DO NOT MIX WITH FERTILIZER. Fertilizers will kill the bacteria. This includes all fertilizers, those without nitrogen and those with only micronutrients.
- Proper storage and selection. Inoculants contain living bacteria that will die when exposed to extreme temperatures (heat and cold) for extended periods of time. Store between 40 and 77°F. Use inoculants before expiration date and those that are specific for peanut. Applying expired inoculant is prone to failure and using soybean inoculant will not work for peanut.
- Water. Use non-chlorinated water or neutralize chlorinated water before use. Chlorinated water will kill the bacteria.
- Tank mix inoculant for a single day of use. Do not use inoculant that has been stored overnight in the spray tank. For twin-row equipment twice as much inoculant will be used per acre as these bacteria have limited soil mobility.
- Place inoculant into moist soil. Inoculants placed on dry soil will cause the living organism to dry out and die. Though some may survive there and produce nodules the number of nodules may be less than adequate.
Some pesticides (imidacloprid (Admire Pro) or prothioconazole (Proline)) can be tank mixed with inoculants to provide early season insect and disease suppression, but refrain from experimenting with products that have not been proven by a university to be tank mixed with inoculants. Moreover, there are products that are marketed to enhance peanut yield and though some may have merit most have yet to be tested. Currently, Arkansas has the greatest yield/acre in the U.S. because the yield potential is not limited by disease or water. Thus, these products that are market to “boost” yield may not provide the same performance as it does in other areas with a long history of peanut production and limitation in their production system. Overall, peanut inoculants are the most important fertility decision a new farmer will make when planting peanut, so take the time to make sure it is done right the first time.