Phosphorus is responsible for transporting energy created during photosynthesis throughout soybean plants. This essential nutrient is critical in promoting growth, increasing water use efficiency and supporting high yields.  

It can be a challenge to identify symptoms of phosphorus deficiency. Agronomists at Iowa State University confirm that phosphorus deficiency is less common in soybeans, but it can still present negative results at the end of the growing season without revealing foliar symptoms. 

Scouting for Deficiency

Signs of discoloration are the best way to identify phosphorus deficiency. A dark green, blue-green, or purple-green color can be exhibited in phosphorus-deficient soybeans. Discoloration will begin on the oldest leaves and move to younger leaves.  

Leaf cupping and lesions on soybean leaves are additional signs of phosphorus deficiency. The phosphorus level in the soil can also a key indicator of future phosphorus deficiency in plants.  

When soils have adequate moisture, signs of deficiency may be more apparent as nutrient uptake is stimulated by moisture. Stunted growth is the most common result of phosphorus deficiency in soybeans.   

Treatment Options

As phosphorus deficiency can appear to be asymptomatic, it is important to collect soil samples to capture soil nutrient levels. Once captured, you can decide if additional nutrient application is necessary.   

If a deficiency appears in season, the best practice is to pull tissue samples to determine the exact cause. Take twenty leaf samples from the newest mature leaf. The leaf is usually 2nd or 3rd from the top. If plants are in early vegetative stages, send in the whole above-ground portion of the plant. Remember, always pull two samples: one from the problem area and one from a healthy area. Correcting phosphorus deficiency in–season is not typically cost effective. Once you determine phosphorus deficiency is the problem, it is best to apply phosphorus in the fall or spring before planting in dry granular form. Contact your Latham or Data Forward™ Advisor to fine tune your fertility in each field.  

Even as nitrogen-fixing plants, nitrogen deficiency in soybeans can be problematic for farmers. Soybeans can fix up to 50% of their own nitrogen while the other 50% is sourced from soil. Root development and soil health are key drivers of nitrogen deficiency.  

A primary cause of nitrogen deficiency is poor inoculation. Rhizobia – the nitrogen-fixing bacterium responsible for prompting nodulation – cannot survive well in coarse, sandy or saturated soils. Cold and wet soils can also suppress rhizobia activity and reduce nodulation.  

Soils with low pH can prevent root nodulation and reduce nitrogen absorption Flooding or highly saturated soils can also prevent nitrogen uptake, even if nodulation occurred successfully.  

Scouting for Nitrogen Deficiency  

Nitrogen is a mobile nutrient, moving throughout the entire plant. It moves to the newest leaves first, leaving the oldest leaves more susceptible to deficiency. In nitrogen-deficient soybeans, the lowest leaves will appear a pale shade of green or show signs of yellowing. Some leaves will also have dark green veins.  

Be sure to check nitrogen levels in the soil before R1. As nodules can appear as soon as V1. Check the health of nodules by squeezing them open. If A pink color on the inside indicates that nodules are active and healthy.   

Treatment and Implications  

A single bushel of soybeans uses 4.2 pounds of nitrogen. Purdue University observed a 38% decrease in yield from nitrogen deficiency – a decline from 65 bu/acre to 40 bu/acre. Rhizobia population can carry over from season to season and continue to support soybean growth. We advise applying inoculant in fields where soybeans have not recently grown. The inoculant can help to stimulate rhizobia activity and promote nodulation. 

It is advised to apply 20 to 40 pounds of nitrogen per acre during the R3 growth stage. However, applying nitrogen at planting – especially too much nitrogen – does not increase yield and can actually reduce nodulation. It’s important to determine the best nitrogen application and treatment plan to fit your unique needs. Contact your Data Forard™ Advisor or Latham Representative to discuss your options.

Iron deficiency chlorosis (IDC) is a disorder that appears in soybeans during the early stages of the growing season. IDC can reduce yield and lead to stunting if it goes unaddressed year to year. Every year, farmers lose over $100 million to IDC and yield loss estimates range from 20-30% in severe cases.  

What to Look For  

Iron is an essential nutrient that supports key plant functions including nodulation, energy transfer and plant metabolism. Iron deficiency can reduce the absorption of other essential nutrients including nitrogen. Deficiency in iron prevents chlorophyll from forming which leads to the yellowing of leaves.  

In soybeans with IDC, the youngest leaves will begin to turn yellow or white while the veins remain green. Unlike mobile nutrients such as nitrogen or potassium, iron is immobile within the plant. As a result, yellowing and signs of IDC will only be seen on newer growth. Beyond yellowing, severe cases of IDC can cause leaves to turn brown.  

Causes and Implications  

Upon testing, soils may have abundant levels of iron, but must be in a usable form. Wet soils, salty soils and soils with high pH (greater than 7.5) will cause IDC to occur as these factors prevent iron absorption. Calcareous or chalky soils are also favorable conditions for IDC.  

IDC occurs most often in shallow areas that are wet where salts and carbonates have built-up overtime. Under these conditions, calcium particles bind to the soil, making the iron inaccessible to soybeans. Calcium carbonate neutralize acid in roots that is needed to make usable iron. For this reason, soils containing limestone with carbonate levels higher than 5% can make iron insoluble.  

High nitrates can worsen IDC since absorbing nitrogen decreases the amount of usable iron. Fields with over 100 pounds of nitrogen per acre can be problematic for soybeans and lead to IDC. 

Treatment  

IDC cannot be managed in-season. We recommend selected a portfolio of defensive varieties to fight this in your fields. Check our line of Ironclad™ soybeans. Our Ironclad brand has been designed to fight this disorder. 

Reducing stress levels for soybeans is critical for preventing IDC or worsening its impacts. Other factors including compaction, over-tillage and herbicide injuries can make soybeans more susceptible to chlorosis or lower their chances of overcoming the deficiency.   

Soybean varieties that are resilient to IDC should be planted, especially if a field has produced soybeans with chlorosis in the past. In addition to selecting the right varieties, planting iron-coated seeds can be a way to combat the onset of IDC. In-furrow application of iron chelate fertilizer can also address IDC.  

Rotational planting or companion cropping with oats are also solutions for preventing IDC. Intercropping soybeans with oats and wheat can reduce levels of soil nitrates that would otherwise lead to chlorosis. Contact your local Latham Representative to determine a treatment plan that meets your specific needs.