YELLOW SOYBEANS were a big subject of conversation this past year.Though many acres experienced this phenomena there were many different reasons why it occurred.Perhaps you were one of those growers who had yellow beans and have been wondering why.My objective in the following expose of shots will be to help you better understand why this may have happened on your farm.Your challenge will be to look through the shots as well as notes I've posted to see if you can match up what your fields looked like and the reasons why it may have happened. In the above field shot taken near Dresden Ontario the soybeans on the left were No-tilled while the soybeans to the right were conventionally tilled.As you can see the conventionally tilled soybeans are greener.The reason for this is the greater soil warmth in the conventionally tilled ground.It allowed soil bacteria to nodulate the roots sooner as well as allow faster nitrification of soil organic matter .Soil temperature differences between No-til and conventional tilled fields this past spring ran in the 3-4 degree F range.
No til soybean fields planted into corn stubble remained much cooler than usual in the spring of 2009.First the amount of corn residues remaining from the 2008 bumper corn crop was much greater than normal and secondly the well below normal temperatures for May , June and July lead to much slower growing cooler seedbeds.Yellow soybeans were the end result.Slower nodule formation as well as slower nitrication of soil organic matter kept beans yellow for an extended period of time.
Very early yellowing of many first time soybean fields occurred because soil temperatures were too cool to allow proper nodulation of the roots.Research has confirmed that soil or seed applied rhizobium will not be effective at forming N fixing nodules on soybean roots until soil temperatures reach 21-23 Celsius.In fact little to no nodulation will occur at soil temperatures below 19 Celsius. The shot taken above was of a first time soybean field planted on a tobacco sand south of Courtland Ontario.The grower involved had done a thorough job of inoculating his beans prior to planting but root digs confirmed that very few nodules had formed.To remedy the problem the grower was advised to apply 50 pounds of actual N per acre as NH4NO3.(soybean leaves need to be dry when doing this so as to not cause sticking and burning of the leaves )
Much of the yellowing noted in other fields,like the one shown above here near Ingersoll Ontario, can be traced back to differences in top soil structure and subsoil textures.These differences were more pronounced in 2009 because of our much cooler and wetter May, June and July weather.The field above expressed symtolmology of potash deficiency in areas of the field where water had laid and plant stands had been severely thinned.Early plant stand reductions is indicative of a cooler and wetter seedbed giving rise to early damping off organisms like pithium,fusarium and rhizoctonia.Upon doing root digs the reddish brown lesions of rhizoctonia were very evident as well as poor root nodulation.In instances like this additional potash applications are recommended if soil tests depict low readings. Continued use of seed treatments including Cruiser is also recommended to help stand establishment.It is hoped that a warmer,drier spring will give a much more favourable result in another cropping season.
The plant on the left of this photo was dug from an area of shorter growing yellow soybeans.As you can see there are very few nodules.The plant on the right was dug from an area of healthy growing green plants.As you can see there is a significant amount of root nodulation on this plant.My observations lead me to believe that when soybean plants are not able to fix sufficient N for normal plant growth they appear to be more predisposed to expressing other soil nutrient deficiencies, particularly potash.
Overcast cool growing conditions gave way to above average rainfall for many localities. This often compacted the soil leading to standing water.Nothing is more compactive or detrimental to soil health than excess soil water.
When evaluating yellow areas of fields one can gain a much greater appreciation of the soil tilth and health by using a soil pentrometer.A soil pentrometer measures soil resistance to root growth. Areas of fields that expressed more yellowing this past summer often registered much higher soil pentrometer readings indicating a much more consolidated top soil layer.Tighter soils do not drain as well making for cooler slower growing roots. Restricted roots are much less efficient at searching through the soil profile to pick up valuable soil nutrients and producing good nodulation.Less than healthy actively growing roots can give way to disease infection.Another good way to get a handle on soil health is to dig a 15 inch deep hole with a shovel and then insert a soil thermometer in the side walls at different depths to get an indication of soil temperatures.Wetter pastier soils will be cooler.( Photo coutesy of Pioneer Hi-Bred International Agronomic Image Library )
The above photo provides a clear illustration of a tighter compacted soil.Rather than roots running vertically downward through the soil profile these roots are running laterally to the soil indicating that there is something obstructing their downward movement.Soil compaction is still growers number #1 yield robber.We need to be conscious of avoiding fall or spring tillage on less than completely fit ground and we need to be installing tile drains where water problems persist.
When roots are restricted from properly growing down through the soil profile it often paves the way for root rot organisms to invade them.In this case these roots are expressing a significant infection of Rhizoctonia root rot. The red brown lesions formed on the root surface can lead to other soil pathogens infecting the root as well.Improving soil drainage and using proper crop rotations will help reduce rhizoctonia root rot infections.
Manganese deficiency can cause a significant yellowing of the soybean canopy.Fortunately it can be easily remedied with foliar spray applications of MnSO4.Usually one spray application is sufficient to green beans back up but in the odd instance it may take up to 2 spray applications 10-14 days apart.
A more detailed shot showing the interveinal yellowing that Mn deficient soybeans will express.
A close up shot showing the early symptomology of Manganese deficiency.Note the prominent interveinal yellowing.
Manganese deficiency will often show up in soybeans under drought prone environments.In the shot above you can see how more Manganese was made available from the soil tightness created by the Tarragator wheel tracks from spring.
Parts of this field near Strathroy Ontario are expressing Sudden Death Syndrome ( SDS).One needs to be very conscious to assess the roots of such a field as often soybean cyst nematode infection can act as a precursor to SDS infection. The wounds created by cysts will often serve as portal of entry site for fusarium solani infection that can lead to SDS development.
This is a close up shot of what Sudden Death Syndrome ( SDS ) appears like.Note the start of the interveinal chlorosis of leaf tissue between the midveins.
A more progressed shot of Sudden Death Syndrome .
When Sudden Death Syndrome ( SDS )strikes hard you may be able to obseve differences in varietal tolerance much like the field shown above here growing near Chatham, Ontario.When this happens you may have to evaluate the SDS scores of the varieties you'd prefer to grow.
The only true way to diagnose SDS is to cut open both healthy and non healthy plants and examine the non healthy plants for any brown discoluration in the plants xylem and phloem system in the outer rind or the presence of an oily silvery grey apperance to the lower stem and root crown much like that shown in the above photo.
Though soybean cyst nematodes will generally make a field appear yellow and variable in plant height it doesn't always have to express itself in this manner.Sometimes you can find cysts on the roots of a reasonably uniform crop much like the one grown here near Tilbury,Ontario. The only true way to get a handle on cyst pressure is to take a soil sample and have it analyzed for cyst pressure.If cysts are hanging around they will generally make a crop mature earlier and yield less.This is because cysts will reduce nodule formation.Cysts actually make soybean roots a less conducive symbiotic host for N fixing bacteria that form the roots nodules.With reduced root nodules less N is fixed and a less healthy plant will mature sooner.
The only unique sign of Soybean Cyst Nematode ( SCN ) is the presence of cysts on the roots like those shown in the above photograph.This is why a close examination of the roots is crucial in identifying cyst damage.When in doubt a soil sample should be sent to a soils lab for cyst screening.( Photo courtesy of South Dakota State University )
Cysts are extremely small in size and have a tan yellow to creamy white colour depending on their age.As you can see in the above photo they are extremely small in comparison to soybean nodules.It often takes a good pair of keen eyes to spot them on the soybean plants fine root hairs. ( Photo courtesy of McGawley )
This is a microsopic shot showing how cyst juveniles can penetrate the root hairs of a soybean plant.The key difference between a cyst resistant variety and a non cyst line is that once the cysts penetrate the root they are unable to set up their feeding sites and reproduce.Rotation with non host crops and cyst resistant lines is the key to keeping SCN populations low and yields high.( Photo courtesy of J.D.Eisenback )