Dennis Lange has had lots of calls recently from all parts of agro-Manitoba about yellow soybeans, which in many cases was likely iron deficiency chlorosis.
“With the cool weather last week, and some areas have had a bit more moisture, many fields have been yellowing and we’re seeing those IDC symptoms,” Lange, Manitoba Agriculture’s pulse crop specialist, said in an interview June 21.
The bad news is farmers can’t do much now, but the good news is IDC can be mitigated with variety selection in future years.
“The first thing we can do is look at matching our fields (based on the IDC risk) to our (soybean) varieties,” Terry Buss, Manitoba Agriculture’s Beausejour-based farm production specialist for pulse crops, said during CropTalk Westman’s June 21 webinar.
IDC occurs most years in Manitoba, but its severity can be affected by the weather, residual nitrates in the soil and the stress soybeans are under. Stresses include cool, wet weather, herbicide applications and crusted soil.
“In years when we have a lot of IDC it comes in early, it is intense and it lasts a long time and catches us really unprepared,” Buss said. “Yes, there are things we can do to help ourselves prepare for IDC years. But every year is different in terms of how intense IDC is.”
Some years soybean crops will quickly grow out of the problem, but when IDC is severe and present at the fifth to sixth trifoliate, American research has found it can significantly reduce yields, he said.
There are two main contributors to IDC — carbonate and soluble salts levels in the soil, Buss said. But another critical factor is variety. Soybeans are rated as IDC tolerant, semi-tolerant and susceptible, he said.
High-risk fields for IDC have carbonate and salt levels of more than five per cent and one millimho (a unit of electrical conductance) per centimetre, respectively. (Low-risk fields have carbonate and salt levels of less than one per cent and 0.3 millimho per centimetre, respectively.) (See IDC risk table.)
In high-risk fields farmers should select tolerant or semi-tolerant soybean varieties, Buss said. Seed Manitoba rates soybean varieties for IDC tolerance.
IDC can also show up where either just carbonate or salt levels are high, especially if the crop is under stress, he said. And some years, under ideal conditions, IDC symptoms might not show up, but farmers shouldn’t be lulled into complacency.
It’s also important to diagnose IDC. Yellow soybeans can also result from a lack of nitrogen. IDC symptoms begin with yellow leaves but then leaf veins turn dark green, Buss said. That’s the classic symptom.
“We just can’t diagnose this one from the road,” he said.
“Patience and close observation are the key.”
Once IDC is identified Buss suggests farmers don’t look at the field every day.
“The plants do grow and as the plant gets bigger it overcomes this challenge of iron absorption and taking up iron from the soil and we start to see better plant health,” Buss said. “That is generally the scenario.”
IDC symptoms often show up during the first trifoliate, when the plant is trying to get iron from the soil instead of the seed, Buss said. The plant does this by acidifying the rhizosphere (root area). The plant lowers the soil pH significantly, using an iron-reducing protein the plant secretes, along with acidifying agents, which let the root oxidize iron into soluble ferrous iron that the plant can take up.
“When we have high carbonate levels in the soil, and the soil is getting wetter, that carbonate gets changed into bicarbonate and the bicarbonate reduces the acidity, raising the pH of the rhizosphere. Then that iron-reducing protein can’t do its job,” Buss said.
High soil salinity makes IDC worse, but the reasons aren’t well understood, he said. Salinity is associated with wet soils, which aren’t good for crop production. Soybeans aren’t saline tolerant either.
Experts used to link IDC with high soil pH, which is often associated with high levels of carbonate and salts, but IDC is also present in fields with low pH.
American researchers have found foliar applications don’t cure IDC, although in some cases they alleviate the symptoms, Buss said.
“I would encourage producers, if they are approached about a foliar product, that they really ask questions around replication and statistically analyzed research — in a climatic zone that is similar to theirs — that actually shows them that there is a real, statistically significant yield increase,” he added.
However, American researchers have demonstrated applying orth-ortho-EDDHA Fe chelate with water, in furrow at seeding time, can reduce IDC, but only when with semi-tolerant varieties, Buss said.
“This is a proactive thing,” he said. “It is not done after you see symptoms. It is done on problem fields ahead of time.”
Buss repeated the best approach is assessing the field’s risk for IDC and selecting the appropriate variety.
Farmers can ask for carbonate and salt level tests when they submit soil samples for nutrient analysis.