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Micronutrients and prairie agriculture

As we increase expected yields, we need to consider micronutrient availability

In my many years of involvement in agricultural and horticultural pursuits, I have repeatedly come across cynicism when I talk about the need for micronutrients. North Americans, as well as Europeans, are slow to realize the absolute role that micronutrients play in plant and animal health and well-being.

Unlike horticulturalists, particularly the Dutch horticulturalist specialists, human and animal nutritionists were slow to realize the absolute role played by specific micronutrients in plant and animal health. Because they were needed in such small quantities, their role was disregarded. Agriculturalists in particular seemed to ignore the picture of the needed “nutrient” determining the final crop yield.

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For example, if you buy a proprietary houseplant fertilizer such as 25-10-10 and check the nutrient label, you will see sulphur finally listed and in alphabetical order boron and molybdenum salts and as chelated nutrients, copper, iron, manganese and zinc. Horticulturalists cannot afford to make any plant production mistakes, particularly in the greenhouse industry.

Not many years ago on the Canadian and U.S. Prairies, it was not uncommon to see ailments such as goitre — thick swollen necks on farmers and their families. This was caused by a lack of iodine in the diet. Death and disability and lowered intelligence was the outcome. To counter this, governments world-wide added iodine to table salt, as iodized salt, potassium iodide. Vegans and vegetarians who avoid iodized salt and instead espouse sea salt or pink salt or some such money-making enterprise are now endangering their own health, especially in areas of the world where iodine is naturally deficient in the soil.

Crop plants have no need for iodine but if it’s present in the soil it is taken up into all the plant parts and consumed by man and animals.

Way back in the 1970s and earlier, the average wheat yield on the Prairies was around 20 to 25 bushels per acre and corn was around 50 to 75 bushels an acre. In Nebraska and Oklahoma, wheat yields were 19 and 17 bushels, respectively. When standards for micronutrient levels were set up at this time and earlier it was no problem to say that 0.5 ppm (parts per million) of copper, two ppm of zinc and 1 ppm of boron were adequate soil levels for crops in general.

Now we are into 60 to 150 bushels an acre for wheat and 75 to 200 bushels an acre for corn. Therefore, the amount of micronutrients available must double, triple or even quadruple to meet their achievable yield goals. Copper should move up to at least two ppm, zinc up to eight ppm for corn and boron at two to three ppm, especially for canola.

Corn, for example, is poor at extracting adequate zinc, wheat is very poor with copper uptake and flax is very poor at taking up both copper and zinc. In the case of flax, I have seen data in Alberta where the flax yield was doubled from 20 to 40 bushels an acre when adequate levels of copper and zinc were added to the cropland along with additional macronutrients.

The essential nutrients

You have all heard the mantra that there are 16 or so plant essential nutrients including hydrogen, oxygen and carbon. The soil-borne nutrients are nitrogen (N), phosphate (P), potash (P), sulphur (S), calcium (Ca), magnesium (Mg), boron (B), chlorine, (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn). You can also add at least three more to the plant micronutrient requirements: nickel (Ni), cobalt (Co) and silica (Si).

When it comes to humans and animals, we can add arsenic (As), chromium (Cr), fluorine (F), iodine (I), selenium (Se), silicon (Si), vanadium (V) and of course salt, sodium (Na).

All of these elements must be present in plant parts in order to be acquired by humans and animals. On the Canadian Prairies, as mentioned before, there can be a shortage of iodine but we can also have shortages and excesses of some of the above animal and plant micronutrients in various areas.

We have areas of selenium shortage leading to white muscle disease in animals or an excess leading to all kinds of animal health problems in several areas of the Prairies. Most soils on average have only 0.1 ppm of selenium. Sulphur can interfere with the normal plant uptake of selenium so high S-fertilized soils will reduce the availability and uptake of this nutrient. Some plants on Prairie pastures like astragalus can accumulate selenium. Livestock feeding on these toxic plants or on pastures naturally high in selenium can die, a condition known as alkali disease.

Adding nutrients

Continuous crop and animal removal gradually reduces the availability of essential micronutrients in the soil for crops, animals and man.

Prairie soils do not have inexhaustible levels of any nutrient. The bigger the crop yields, the more plant- and animal-essential nutrient you extract from the soil. Soil depletion of the macronutrients such as N, P, K and S is more or less fully understood. Soil depreciation of plant essential micronutrients such as copper, zinc and boron are less well understood as we remove crop after crop often including nutrient-rich straw.

Naturally, the most depleted soil zones of the Prairie cropland are the zero- to six-inch and six- to 12-inch zones. Reserves of these micronutrients may be present deeper into the soil zones at 12 to 36 inches, but in wet seasons crop roots of cereals in particular seldom go deeper than 12 inches. In dry seasons, these self-same cereal roots, often in association with mycorrhiza, will penetrate up to 36 inches or more, in the process picking up the necessary micronutrients such as copper or zinc left deeper down in the soil.

Nonetheless, present-day farming worldwide is slowly depleting crop-productive soil profiles of these essential micronutrients while we ply them with essential macronutrients, expecting higher and higher crop yields.

Here is an explanation of why many Prairie crops in good wet seasons result in unexpected poor yields yet in dry seasons yields may exceed expectations: In a wet season, the crop roots (wheat, canola, flax, barley) are concentrated in the top six to 12 inches of soil where the micronutrients have, over the last 100 to 200 years, been depleted by crop removal. In drier years, most crops root much deeper into the subsoil, picking up the deeper reserves of these micronutrients.

Copper deficiency is a primary and proven cause of severe lodging in wheat, particularly in wet seasons. A perfect example of missing the boat on an actual role of a micronutrient in crop vigour and health.

About the author

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Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]

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