Features Fertility and Nutrients Soil
Chasing ghosts

Micronutrients are required in very small amounts but are still as essential as the macronutrients nitrogen (N), phosphorus (P), potassium (K) and sulfur (S). However, they can be notoriously hard to diagnose, and responses can be small and fleeting.

“Micronutrients can be rather ghostly. Deficiencies show up and disappear, and they are masters of disguises,” says Jeff Schoenau, soil science professor at the University of Saskatchewan in Saskatoon. “Symptoms are easily confused with other forms of stress, and responses to fertilization are often small, fleeting and variable.”

Micronutrients of concern in Western Canada include copper (Cu), zinc (Zn), boron (B), manganese (Mn), iron (Fe) and chlorine (Cl). Schoenau says diagnosing micronutrient deficiencies can be difficult. For most micronutrients, using visual inspection alone is risky and inconclusive.

“Soil and tissue testing are useful tools, and although they receive a fair amount of criticism and debate over their usefulness as a diagnostic tool, they are no better or worse than a lot of macronutrient tests,” says Schoenau. “A combination of soil and tissue testing, plus using test strips when fertilizing is the most conclusive way to identify deficiencies.”

Availability can vary greatly across fields, and deficiencies tend to occur in patches and localized areas within a field such as eroded knolls and sandy areas.

Soil conditions contribute to micronutrient availability and incidence of deficiency, with sandy, gray, peaty soils in northern agricultural regions having the greatest frequency of deficiencies. Sandy soils have a low content of minerals capable of releasing micronutrients through weathering. Calcareous (high lime), high pH soils will fix micronutrients like Cu and Zn into insoluble forms. Low organic matter can contribute to low B availability while high organic matter peaty soils can have Cu, Zn and Mn deficiencies.

Nutrient imbalances in the soil can also contribute to deficiencies. For example, a soil with high soil P content from fertilization can interfere with Zn and Cu uptake. On P-deficient soils, adding Cu and Zn can reduce plant growth unless P fertilizer is also applied.

Copper
Of the micronutrients, Schoenau says that Cu is the element most likely to show up as a limitation in Saskatchewan and Alberta. Cereals, especially wheat, are the crops most susceptible to low Cu fertility. Research done near Porcupine Plain, Sask., on a Gray Luvisol soil found that a foliar application at the flag leaf stage was most effective in correcting a deficiency in hard red spring wheat. A 1.8 lb./ac. (two kg Cu/ha) application incorporated into the soil yielded the same as a no-fertilizer control, but a 0.22 lb./ac. (0.25 kg/ha) foliar application at the flag leaf increased yield by 63 per cent. Some other studies have found similar results with foliar application more effective than soil applied when soil fixation potential is high.

A 2015 and 2016 greenhouse study by Ryan Hangs investigated the response of 47 soils from Saskatchewan, Manitoba and Alberta to Cu, Zn and B fertilization in wheat-pea-canola rotations. Foliar copper sulfate (CuS04) and soil-banded copper sulfate were statistically higher yielding than the other treatments. A foliar application of chelated-Cu was also statistically higher than the control and a banded application of chelated-Cu but still lower than the copper sulfate applications.

“What we saw in our studies with Cu and B was that there was a fine line between the crop suffering from a deficiency or toxicity from adding too much fertilizer, especially when the fertilizer and crop roots were in close proximity as in a pot,” says Schoenau.

In the following pea crop in Hangs’ study, pea yield was significantly higher when Cu fertilizer had been applied the year before to wheat. Schoenau says the response could be from a fungicidal or nutritional effect, and studies are underway to further investigate the yield response. There was also a trend to higher pea yield to soil and foliar Zn application, but it was not statistically significant.

Zinc
An analysis of 23 field trials in the 1980s on dryland crops found no significant yield response to zinc sulfate fertilizer applications. However, two studies, one in 1993 and the other in 2002, found significant cereal crop responses to zinc application selectively made on eroded knoll soils in greenhouse studies.

Corn has a high Zn requirement, and research has found a yield response on low testing soils. Schoenau says that high rates of P application to soils that are marginal or deficient in Zn can induce severe Zn deficiency.

A greenhouse research study in the mid-2010s looked at lentil response to Zn on 10 soils from Saskatchewan. It found a few positive yield responses in Brown soils from southwest and south-central Saskatchewan, but on higher organic matter soils in the Dark Brown and Black soils further north, there was no response to Zn fertilization.

Boron
Canola and alfalfa are the crops that are most susceptible to B deficiencies. Large responses to B in research studies are quite rare. Some studies found that even on soils with very low extractable B, no significant canola responses were found with B fertilization. However, in one recent study with an orthic Dark Gray Chernozem, a large yield response to B fertilization was observed that was predicted by soil analysis.

Manganese
When cereals, especially oats, are grown on peaty soils in the northern agricultural fringe, these crops can respond to Mn. However, deficiencies on mineral soils are rare.

Schoenau cautions that Mn toxicity has shown up especially on very sandy, acidic Gray soils in east-central Saskatchewan. A U of S research study in 2018 found that on an acidic soil with a pH of 5.5, canola yield doubled when lime was applied to the soil to raise the pH to 6.1, and tissue Mn dropped below toxic levels.

Iron
Soybeans are inefficient users of Fe and are susceptible to iron deficiency chlorosis (IDC), with symptoms of interveinal chlorosis. Some varieties are more susceptible to IDC, and IDC tolerance ratings are available for registered varieties. High pH, poor drainage, nitrates, carbonates and salts aggravate Fe deficiency.

A U of S field trial in 2015 and 2016 compared IDC sensitive and IDC tolerant soybean variety responses to Fe fertilization. The soil was slightly saline with high nitrates and a high pH on the lower slope of the field. In 2015, the weather was very dry, and there was no response to Fe fertilization. The weather was wetter in 2016, and the IDC sensitive variety responded significantly to foliar Fe, but the IDC tolerant variety did not.

“Genetics are the best defense when IDC is a concern,” says Schoenau. “Foliar Fe may be suitable as a rescue treatment.”

Chloride
Cereals can respond to Cl fertilization on highly leached soils with low extractable Cl. The response may be related to disease pressure, although the responses are variable. A U of S study in the late 1990s found an application of 40 lb./ac. (45 kg/ha) of KCl (potassium chloride) increased wheat yield in a highly leached depression in a wet year, but no response in the following dry year.

“The more widespread use of potash in recent years has likely reduced the incidence of Cl as a crop limitation,” says Schoenau. “And nearly all Cl remains in the straw and is recycled back into the soil.”

Schoenau says that micronutrients, in general, become more important when farmers are aiming for the top of the yield curve. Their use as a fertilizer should be considered when backed up by soil and plant evidence of deficiency in the short term, and a single application to the soil may provide benefit over a number of years in the rotational cycle.

“The patchy and variable nature of micronutrient deficiencies makes them an obvious target for precision fertilization,” says Schoenau.