What’s the Soybean Plant Telling Us?

Dan Davidson, Agronomist PhD, CPAg, CCA

Soybeans are up, so what’s next? Let the crop direct you.

Most likely your full season soybeans were planted in good time this year with little or no risk of replant. While our warm spring arrived on about May 1, April was a cool month, setting low temperature records for the month, and some experienced unusual April snow showers. One of the lessons we are learning is that soybeans can be planted as early as corn. And in seasons like 2018 when April soil temperatures are too cold to plant cold and the soil is in good condition for planting, it is okay to plant treated soybean seed and get a head start on building bigger soybean plants and getting all your acres planted.

Whether you planted early or late this year, here are four assessments to help you make sure you are learning the language of your soybeans.

Stand Assessment: Once soybeans emerge, check emergence and do some stand counts. Your goal is an evenly distributed stand with no gaps. Soybeans can easily branch and compensate for a few missing plants in row, but long empty gaps or spots can hurt yield if the area is significant enough. Replanting is always an option but rarely economically justified unless large swaths of fields are lost.

You can count number of seedlings in a length of row, in a square of known dimensions, or circular hula hoop of known diameter. If you drop 140,000 seeds per acre, your goal should be an average count of 120,000 seedlings per acre. If less, something was wrong with the seed (low germination), weak seed treatment, poor soil conditions at planting, or perhaps your planter.

Walking an 80- or 160-acre field is a time-consuming task, so get a drone and scan the field for spots with poor stands and walk there. If stand losses exist and follow a replicated pattern, it is probably man-made and related to planter, compaction or tile line plugging issues. If the pattern is random, then nature is the cause and it is time to do some sleuthing. While you probably can’t make any amends for stand losses this season, you can learn from these losses and be proactive next season.

Color Assessment: Soybeans can often appear discolored and symptoms could be a nutrient deficiency or something else. Most nutrients, when deficient, reveal specific telltale signs, and by the time the symptoms are seen, some damage has been done. Instead relay on tissue testing to identify hidden hungers and make necessary correction. There are other visible symptoms such as nitrogen malaise, iron chlorosis, SCN infection or herbicide damage.

  • Nitrogen malaise: When soils and wet and cool during June, nodulation is slow and nitrogen fixation begins later. The crop will assume a yellow hue showing a deficiency of nitrogen. As soil warm and dry and roots expands and nitrogen fixation kicks in, the crop recovers and grows out of it.
  • Iron chlorosis: In high pH and calcareous soils, the micronutrient iron (Fe) can be tied up and become unavailable creating symptoms of iron deficiency. Iron chlorosis appears in spots that are stunted and yellow. The solution is to increase Fe solubility in the soil, around the roots.
  • Visible SCN (Soybean Cyst Nematode) symptoms are very similar to iron chlorosis with general yellowing within spots in a field. High population densities of SCN results in large portions of soybean fields with plants that are severely stunted and yellow. The solution is to rotate crop, plant resistant varieties, and consider some of the new seed treatments on the market.

 

Nutrient Assessment: Plant analysis is an excellent diagnostic tool to help understand some of the variation seen in the field of even if your plant is well fed and there’s no hidden hunger.

The general rule is to sample whole plants before R1, and after R1 pick fully developed trifoliates leaves from the top of the plant. Early in the season, when plants are 8-10 inches tall, collect whole plants from 15-20 different places in the sampling areas. Later in the season, collect 20-30 sets of top, fully developed leaves.

Another approach is to pick the best and worst areas and mark those two spots and collect top trifoliates weekly or biweekly, package and label separately and send to a testing lab.

Lastly, some nutrients like calcium and boron are consider immobile in the plant. Even though a tissue test may show levels adequate levels, the plant can’t reuse those reserves to support new growth at various stages of growth.

Node Assessment: A good practice is to monitor node count on the plant’s main stem. While final yield is mainly determined by pod and seed count per acre at harvest, it is the number of nodes that largely determines potential number of pods per plant. That is why planting soybeans in April makes sense because the plant has more time to add nodes before summer solstice, the kickoff of flowering (R1). In the image below, you can see that the greater the node number accumulated over the season the higher yield with the difference in node number and yield being related to high management.

University of Illinois Crop Physiologist Dr. Fred Below said, “My guess on the optimal number by June 21 is 6 or more nodes, and that 18-20 at the end of flowering would be optimal. Total node number is affected by the relative maturity with the longer RM varieties typically having 1-3 more nodes. We don’t have data on these planting date effect (on node number).” Below has shown that yield is determined by pod number per plant. The difference between a 50- and 62-bushel yield is largely the number of pods, and particularly in the middle portion of the plant.

About 60% of soybean yield comes from the middle nodes (nodes 7 to 13) of a plant. That is why Dr. Below advocates protecting leaves at those nodes. The closest leaves provide most of the energy for pods at those nodes.

Learning the language of the plant is an important process. It is about monitoring ongoing growth and development processes, while observing and seeing if the development of this year’s crop is meeting targets. Using these four assessments can help you learn the grammar and syntax of your soybeans and help increase yield.

Fortalis® is an essential crop input

Your soybean yields can also benefit from Fortalis®, an advanced crop enhancement foliar spray that works by mobilizing calcium already existing within plant tissues, resulting in higher pod retention. When applied as a tank mix, Fortalis® with a fungicide or insecticide contributes to better plant health and higher yield potential. It’s an essential crop input that you need to get the most out of your crop.

Dan Davidson is a PhD agronomist, part-time farmer and soybean expert from Nebraska who consults with soybean checkoff and industry on crop production, marketing, and product development projects.



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