Soybean Inoculants: The Other Seed Treatment

UT’s standard recommendation is to include an inoculant where soybeans will be planted into fields that have never been planted to soybean or where soybean has not been grown in the field in the past 3 to 5 years.

There are several products on the market that contain a single or even multiple strains of Rhizobium bacteria designed to improve root nodulation and help soybean roots fix nitrogen from the atmosphere into a usable form for the plant.

Since the soybean must make 50 to 75% of its required nitrogen through this process this is pretty important stuff. With the hectic weather this spring creating flooding and instances where lime was not applied timely folks should include an inoculant in the following situations:

  • Flooded fields going to beans. Some data indicates bacteria may live for a few days under flooded conditions but require oxygen to survive long-term. Fields flooded more than a week would definitely need an inoculant. It might also be cheap insurance to inoculate fields that have been under water for 3 or 4 days as well.
  • Fields with low pH (<5.8) or very high pH (>7.0). Bacteria do not reproduce and thrive where pH is out of kilter. Also, in a low pH situation where lime has recently been applied and has not had time to react with soil, consider adding molybdenum to the seed along with the inoculant. We have had mixed results with foliar moly products and I prefer to put it on the seed if possible.
  • Fields with high sand content. We don’t have many of these in TN but some data suggests that a field with low organic matter and high sand content should be inoculated each time it is planted to soybeans.

Which product is right for me? Newer retailer-applied inoculants (Optimize, Vault, Launcher Pro) have replaced the older powder or granular materials for a lot of folks. They cost a little more (around $3.00- $3.50/bag) but are convenient since we don’t have to put them on ourselves the day of planting. Where commercial treatment isn’t an option, peat based products have replace granular materials and can be metered through a hopper box into the furrow with seed (line yours up early- some retailers don’t carry these in supply).

Powder type inoculants are applied to the seed just prior to planting. I think any product will work fine as long as it is applied correctly. We don’t always see yield responses to retailer-applied material over the do-it-yourself products but they are very convenient to use.

Does an inoculant give us Higher Yields?  Not always. In the stress type situations bulleted above you are more likely to see a benefit (i.e. higher yields). However, fields with balanced pH and good fertility that have not been planted to soybeans in recent years also have ‘native’ populations of bacteria that can do what seed inoculants do—create root nodules and fix nitrogen for the soybean. And, in these fields I have seen inconsistent improvement in nodule formation and seed yield compared to untreated checks relying on native bacteria. We have run inoculant studies in long term cotton ground where we would see up to a 4 bushel increase in soybean yield with a product one year and less than 1 bushel increase the following year using the same inoculant in a nearby spot in the same field.

So, returns can be inconsistent on good productive fields. Bottom line: use an inoculant in fields that need one since they are cheap insurance- increased yield is a bonus.

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