A three-year University of Guelph project is working to give soybean growers the first clearly defined critical leaf tissue-test levels for seven micronutrients: boron, cobalt, copper, iron, manganese, molybdenum and zinc.

Hugh Earl, a crop physiologist in the university's department of plant agriculture, launched the study because diagnostic tools for these conditions barely exist. Soybeans are thought to be prone to boron toxicity and deficiencies in several other micronutrients, but the rarity of these problems means neither diagnostic criteria nor management recommendations are fully developed.

"The challenge is that these types of studies usually use field locations that have random issues," Earl says. "Often micronutrient deficiencies aren't as simple as just one thing missing from the soil. It's actually not very common to get a pure situation where only one thing is wrong in a field environment. More often, whatever is causing a deficiency of one element might be causing deficiencies in others as well. So when you try to look diagnostically, it's complicated by all the other things that might be going on."

Total control below ground

Funded in part by Grain Farmers of Ontario, the Ontario Agri-Food Innovation Alliance and the Ontario Soil and Crop Improvement Association, the study is now in its second season outdoors on the University of Guelph campus. Soybeans grow in sand-filled pots placed on landscape fabric over a gravel surface outside greenhouses. A fertigation system designed by technician Erik Glemser supplies reverse osmosis water that is nearly free of all the elements under study, giving the team complete control over the below-ground environment.

"We're trying to create a growth environment where we know for certain that the only thing different is that one micronutrient is missing or in overabundance," Earl says. "So sunlight, temperature, humidity, wind – all the things that affect plant morphology – are identical across all treatments except for whatever deficiencies we introduce."

Getting there took persistence. Early attempts in a controlled indoor environment produced manipulable boron levels and little else. The first outdoor season in white silica sand induced only boron and zinc deficiencies. The breakthrough came from undergraduate research student Peyton Nailor, whom Earl mentored through a series of greenhouse experiments with different grades of so-called "glass sand". She identified a specific glass sand that, used as the growing medium, consistently produced boron, iron, manganese and zinc deficiencies.

"We've now got that experiment set up outdoors using the glass sand, and we're getting responses for all four of those major micronutrients," Earl says, adding that the team has now also introduced molybdenum and copper deficiencies.

The molybdenum result carries agronomic weight. Nailor's work showed plant performance plummeted under molybdenum deficiency, but only where soil nitrogen was also limiting. That aligns with expectations, since molybdenum plays a critical role in fixing atmospheric nitrogen in soybean nodules. The team is trying to repeat the result in this season's outdoor pot study.

Copper is behaving differently. Eliminating it from the fertilizer solution has greatly reduced tissue test levels, but so far that has not produced visual deficiency symptoms or reduced plant growth. Earl is confident the team will hit its targets for every element except copper and cobalt.

"For everything else, we've been able to induce clear deficiencies," he says. "And we know that if you've got a problem that you suspect may be caused by a micronutrient deficiency, tissue tests can work very well to confirm that."

What it means in the field

For farmers and agronomists, the practical output will be clearly defined sufficiency ranges for interpreting leaf tissue-test values for boron, zinc, manganese and iron, plus toxicity levels for boron — numbers that currently do not exist in usable form. That, in turn, should let growers decide whether an intervention is worth making to protect yield.

Earl cautions on one front: it remains unclear how effective foliar applications of any of these micronutrients actually are on soybean. Every one has a specific foliar fertilizer product available off the shelf, but the efficacy data is thin.

"Going forward, what I'd like to do is set up deliberate deficiency scenarios then test whether foliar application of these micronutrients can correct them and if you then get the expected effects on yields," he says.

His headline message to growers is direct: micronutrient deficiencies are real, and soybeans respond to them. In the near future, tissue sampling for micronutrients should be a viable basis for an application decision. But precautionary spraying without an identified deficiency is a different matter.

"You shouldn't necessarily apply micronutrients as a precautionary measure if you don't have an identified deficiency, because micronutrient toxicity is also very real for some of them – certainly for boron and iron," Earl says.

Photo library and AI ambitions

The project extends beyond numbers. As part of establishing the sufficiency ranges, Earl is building a comprehensive library of high-resolution photographs showing the range of visual symptoms each deficiency produces, so farmers and agronomists can diagnose problems — and boron toxicity — more easily in the field.

He also hopes to collaborate with colleagues on a smartphone application using an artificial intelligence image-analysis model trained on that photo collection.

"The idea being that you could develop AI tools where you take a picture of a crop or a leaf and AI then tries to deduce, based on the visual symptoms, what's going on there," he says. But he is candid about the obstacles, noting the idea is still in the thinking stages: "I don't know if that's going to work or not because there are lots of things that cause symptoms that look pretty similar and I don't know whether AI could really tell the difference between, for instance, iron chlorosis and a mild boron deficiency."

Growers should watch for the finished sufficiency ranges and the diagnostic photo library as the study completes its remaining seasons, with foliar rescue trials the likely next stage once the deficiency scenarios are reliably established.