The ‘ghostly nature’ of phantom nutrients

Micronutrients are the acid test of crop fertility. Some believe in them. Others just don’t

For the most part, our Prairie soils aren’t short of micronutrients. Deficiencies are rare, and they are also difficult to pin down, in part because such shortages are usually associated with highly localized soil conditions and because some of these conditions change with varying moisture or pH levels. As well, since these micronutrients are only needed in tiny quantities, most soil tests don’t even look for them.

“I always like to say they’re a little bit ghostly in nature,” says University of Saskatchewan soil scientist Jeff Schoenau. “They’ll appear and manifest themselves, and then environmental conditions change and they disappear.”

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The ghostly nature of micronutrient shortages breeds a typical “out of sight, out of mind” mentality, which is part of the reason why soil tests rarely look for them. Soil tests are based on macronutrients like nitrogen, the cornerstone of protein, or phosphorus, part of the framework of DNA. The contributions these elements make are well understood, and we know they’re needed in large quantities to make tissue and to set seed.

So our soil tests revolve around the macronutrients. But the micronutrients are quite different.

“They’re the nutrients that plants require in very small quantities, and this has nothing to do with the amount in the soil or even the amount that the plant will take up. It’s just the amount that’s required — and that’s very small,” says Don Flaten of the University of Manitoba. “For example, iron is a micronutrient required in very small amounts but in real life it’s the most abundant nutrient in the earth’s crust.”

Even so, that small amount of iron is crucial. It’s an important component of cytochromes, the proteins in the cell that act as electron carriers in the transport chain. It’s also a tiny part of many of the enzymes that drive the chemical reactions within the cell, so a shortage of iron means a weakened plant that may not survive.

There is a whole host of other micronutrients that plants can’t do without, but since they’re present in sufficient quantities there’s really no concern about shortages. Nickel is a classic example.

The really important micronutrients include molybdenum, boron, chlorine, copper, zinc, manganese and iron. “There’s a category that we can classify as the micronutrient metals, things like copper, zinc, manganese and iron,” Schoenau says. “They play an important role in electron transport and enzyme activation in the plant.”

Enzymes are proteins that work as catalysts, and they’re absolutely essential to the chemical workings of living things. A catalyst initiates a chemical reaction and then disconnects itself to look for another set of molecules to begin the reaction all over again. It’s sort of like a railway locomotive.

After a fashion, a train is like a long-chain molecule, assembled from a collection of smaller particles called freight cars. Once the train is assembled, the locomotives pull it to another location where it’s broken into smaller units. Some of it is reassembled into another train while other cars are sent to a final destination where their cargo is delivered.

Even though the locomotive is only a small part of the train, none of the assembly, hauling or switching happens without it. At the end of it all, the locomotive will disconnect from the train and then couple to another to start the process over again. In other words, the locomotive may be used to pull and process several trains, much the same as an enzyme which will pop out of the reaction after completion, find another molecule and start the process over again.

If the locomotive is the enzyme, then the various micronutrients are like the couplers that fasten the engine to the train. Even though it’s a tiny, almost insignificant part of the whole system, the locomotives still can’t pull a train without them. In the micro world of biochemistry, the iron, copper or boron in that enzyme is just that crucial, and the whole system would break down without it. And just like a coupler failure on a train, a shortage of micronutrients is relatively rare.

“Micronutrient deficiencies can occur but they tend to be fairly isolated and patchy within one field. It’s rare to find an entire field deficient in a micronutrient,” Schoenau says. “Instead they tend to occur in localized areas within a field, maybe in a gravel lens or a highly eroded knoll with low organic matter where the C horizon has been exposed.”

Often these shortages are geographic. These deficiencies are rare in the southern Prairies but they sometimes do occur in the northern fringe where the land was once under boreal forest. If the farm is located on some of those sandy grey soils, the coarse texture and low organic matter (i.e. below two per cent) can be a factor. Oddly enough, high organic matter above 30 per cent may also contribute, so those high-peat soils may cause some trouble. Soil temperature, pH and moisture content may also influence availability.

“And that would be the same for all the micronutrients. Not only are they very sensitive to geographic variability, but also to time or temporal variability,” Flaten says. “Environmental conditions play a huge role. Iron and manganese in particular are very sensitive to soil temperature, flooding stress and that sort of thing.”

It also depends on the crop type. Copper deficiency is probably one of the most frequent micronutrient problems in crops especially on sandy soils or on highly organic peat soils. Copper deficiency in wheat does happen and farmers should watch for that distinctive pig tailing along the leaves where the tips die off and twist leaving a brown frond instead of a healthy green leaf.

Manganese deficiencies often show up in the same kinds of soils, and may be seen with a pale green or yellowing in legume crops. It may also be seen as a grey speckling in oats. Zinc deficiency is most likely to show up in corn and may be seen as light yellow bands on the youngest leaves.

“Boron is a nutrient that’s sometimes deficient in canola,” Flaten says. “But the deficiency is extremely rare and I am aware of only one case in Manitoba and one case in Saskatchewan where a boron deficiency has been diagnosed with authority.”

Flaten also adds that chlorine deficiencies may show up in cereal crops, but this too is dependent on the variety. Some strains will respond to chloride fertilizers while others grown side by side will have no response at all.

With so many complicating factors, it’s no wonder that micronutrient problems can fly below the radar. It takes a lot of experience, observation and field notes from both farmers and agronomists.

Still, such things are site specific, so these spots can probably be teased out over time. There is also a number of ways to narrow down a diagnosis.

“The best approach to diagnosis is to use a multiple evidence approach,” Schoenau says. “You use a soil test, a tissue test, a visual inspection and, if you suspect a micronutrient deficiency, you may try a test strip with a micronutrient across a field area just to see if there is any response.”

There are some places that might recommend a more proactive approach with a micronutrient seed treatment. This is relatively new, however, and may require some verification before science can really say that it works.

“I know that in some parts of the world they have worked reasonably well, and if you have an acute deficiency maybe they have some potential,” Flaten says. “In a number of cases, I think that micronutrient-based seed treatments are being recommended for soils where micronutrient supply is sufficient.”

“The other time micronutrient deficiencies may show up is when you’re shooting for the top end of the yield curve, and you’re trying to squeeze every last bushel out so you’ve got your macro nutrients and other inputs applied to overcome any limitations,” Schoenau says. “Some growers will put on a micro-nutrient as a bit of insurance and there may not be a response, but they’ve got that there just in case. Some growers have that kind of philosophy.”

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