Early in June we were discussing the possibility of drought intensifying causing issues with stand establishment and early rooting and nodule development. However, we caught enough spotty showers to get through that brief scare.
Over the past several weeks we have gone to the other end of the spectrum in many places across Indiana with rain events of 2+ inches and total accumulations of 6 to 8 inches (Figure 1), which was 50 to 75% more than we typically get.
Again, much of this precipitation came within short periods over the last few weeks. Then, the sky let loose of additional buckets of rain on July 8.
Needless to say, many fields of soybean are struggling with wet feet (Figure 2). Chances are you will see the off-green to highlighter green soybeans throughout the fields where the field has been saturated to ponded. Saturated soils will limit oxygen supply to soybean roots and bradyrhizobia. Soybeans grow and develop by burning plant energy (photosynthates) with the assistance of oxygen.
Thus, soybean (and rhizobia) growth is limited under wet conditions. In fact, nodules can die under prolonged saturation (see video at the end) and limit N supply.
I suggest digging up the plants in these fields to assess if the nodules are white, red, or dead. White nodules are immature, but developing. Red or pinkish interior of the nodules indicates that N fixation has started. Brown to mushy nodules are dead and will not supply N. Young soybeans, V2 to V3, may only have three to five actively-fixing nodules (pink to red interior).
Soybeans from V4 and onward should have eight or more actively-fixing nodules with more nodules developing. Nutritional analyses of the most recent mature leaves of soybean will help to pinpoint if N or another nutrient is deficient.
Under normal growing conditions, soybeans accumulate ~10 lb N/acre by V4 then accumulate another ~3 lb N/acre daily until R2 (full bloom). Thus, soybeans normally accumulate another 60 lb N/acre by R2 (full bloom). Nitrogen stress during this period will impact yield.
We have tried rescue treatments under similar situations in 2015 when we received nearly 20 inches of rain in June up near LaCrosse. We had a uniformly, poor-looking soybeans (Figure 3) that we evaluated 18 treatments to rescue the soybeans that were off-green with compromised root system compared to those soybeans that were normal (Figure 4).
We evaluated N-based treatments; individual foliar sprays of nutrients, fungicides, and growth regulators; and tank mixes of some of those foliar sprays (Figure 5). The N-based treatments were to provide the shot in the arm since we were compromised on nodule and N supply.
Urea was applied at 40 and 20 lb of N/ac with no yield effects. Granular AMS (21-0-0-24S) was applied at two different rates 20 and 10 lb N/ac, which were the only treatments to increase yield above the untreated control (UTC) (Figure 5). These are also the only treatments that supplied sulfur (23 and 11.5 lb S/ac, respectively).
Please note this was our first glimpse into soybean response to sulfur (S). In fact, this site has become our S-deficient site to evaluate S management of soybean. Thus, it is difficult to tease out if the rescue treatment for soybeans under saturated conditions was beneficial because of the saturated conditions that compromised root development, nodulation, and N fixation OR if it was beneficial because the field is responsive to sulfur treatments year in and year out.
If you are considering any rescue treatments of soybean, please be sure to assess the root system and nodule activity (or the lack thereof). If you deem a field worthy of rescuing or trying to rescue some of it, I would start with a S and/or N+S based approach to stimulate nodulation (S is needed as a co-factor) and to provide a shot in the arm for the limited N supply until N fixation takes over.
It is reasonable to use 10 to 20 lb S/ac from a soluble source like granular AMS or pelletized gypsum. If you use granular AMS, 8.8 to 17.5 lb N/ac would also be supplied. If you would like to increase the N portion applied, I suggest using urea to reach a total of 40 lb N/ac. We do not want to overload the field with N to the point of delaying nodulation and N fixation.
Soybean Assessment After Wet Conditions
In this video, three different areas of a soybean field that have been subjected to extended periods of saturated soils to flooding are assessed for damage. Primarily, once soils allow foot traffic, one should evaluate the health of the root system and nodules. Root and nodule recovery is possible once soils drain and aerate (oxygen supply for respiration and growth).
White roots are indicative of being healthy, whereas dark roots are compromised. Nitrogen-fixing nodules will either be white (new and developing), red (pink and producing N), or dead (hollow and mushy).