Tuesday, August 26, 2014

Poor or No Soybean Nodules in Many Fields

Chad Lee, Extension Agronomist, University of Kentucky

A field with soybeans with nodules
and soybeans without nodules. 
Several soybeans have turned yellow in the past few weeks and further examination revealed little to no nodules. The problem has occurred with seed inoculated in the bag and seed inoculated at the farm. It has even occurred in cases where a 2x rate was applied at the farm. Several brands of inoculant have been indicated. It has occurred in fields all across the state. The problem has occurred in full season soybeans and in double-crop soybeans.

In most cases, these are new fields for soybeans, coming out of pasture, hay or continuous corn. Some fields were in pasture until last year, then in corn followed by wheat and now double-crop soybeans. These are all fields where inoculant was needed. A few fields had very low pH values and that may be to blame. Low soil pH will bind up molybdenum (Mo or "Moly"). Moly is needed for inoculation to work. But, most fields had soil pH values that were 6.2 to 6.8, so Moly should not have been a problem.

Some are suggesting dry weather and that could be the case in some fields. But, we have had too many fields at too many planting dates for it to be only dry weather. Compaction was suspected and could have been a problem in a few fields. But, again, most fields had adequate pH, adequate water and no signs of compaction.

Three soybean plants and only one nodule. Not good. 
Once we identify a lack of nodules, good soil pH, and no compaction we need to address a few other questions. The first question is, what do we do now? The second is, how to we keep this problem from happening next year?

What do we do now?
Soybeans will remove about 3 pounds of nitrogen (N) per bushel. So, 50 bushels of soybeans will remove about 150 pounds of N. Soybeans yielding 50 bushels per acre likely use a little more than 150 pounds of N per acre in a season. But, depending on the field, we may have some N there to help. According to our Lime and Nutrient Recommendations, fields in sod/pasture/hay for 4 or more years will have about 25 to 50 pounds of N per acre to contribute. Soils that have been in other annual crops (corn or tobacco, for example) will provide little to no N to the soybean crop. So, if there is a good stand of soybeans, and we think we have a good season, we may need to use as much as 100 to 150 pounds of N per acre if we want excellent yields.

One study from Kansas State investigated N rates on soybeans with no nodules and observed that 120 lbs N per acre gained 21 bushels per acre in a good year and only 11 bushels per acre in a dry year. We have had a few farmers in Kentucky who applied about 100 to 140 pounds N per acre and had yields over 60 bushels per acre. One of those producers had a check strip where the non-nodulated soybeans yielded less than 20 bushels per acre. These are the best stories. They had good stands, good weather and they had plenty of time to get good yields.

If someone is finding a field just now (late August) with no nodules, then yield losses can be expected and it may even be too late on full season soybeans. For double crop soybeans with adequate moisture, some fertilizer N may help. We have been suggesting 50 pounds of N per acre in these situations. If possible, please leave a check strip with no N applied and let us know the yield differences.

What do we do next year?
No nodules on most of these soybeans.
The darker green area has nodules and is
the lower area in the field. Lack of water
may have been a problem in this field. 
Fields with no nodules this year are going to be fields that need inoculant the next time soybeans are planted. Pull soil samples from the no nodule areas and soil samples from the nodulated areas of the fields. If soil pH is low, get the soil pH corrected this fall. Next spring, treat the inoculant as the living organism it is, Bradyrhizobium japonicum. Don't allow the inoculant and seed treated with inoculant to get very hot. Keep both out of direct sunlight. Keep both cool and dry before planting. Check with your seed representatives about any other seed treatments being applied and if they will harm the inoculant. Moly (which is needed on low pH soils) will harm the inoculant if both are on the seed too long before planting. Adjusting soil pH this fall will remove the need for for a Moly seed treatment in the spring.

Plants without nodules in the front
and with nodules in the back
and then without nodule in the far back.
Photo: Gary Hamilton, ANR Extension Agent. 
For now, that is about all we can suggest for next year. We are trying to determine any commonalities across the problem fields this year. So far, we do not have a "smoking gun".  Unless we find out more from this season, keep track of both the inoculants used and all seed lots planted next season. Keep the seed bags (or records of the seed bags, including seed treatments) until we know if nodulation occurred. Check for nodulation at about V4 growth stage. If you don't see nodules at V4, then we have plenty of time to manage the field.
Left: Green plant with healthy nodules.
Right: Yellow plant with no nodules.

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