“If you look at all the micronutrients in soil, there’s enough to last your lifetime. The problem is that only a fraction of that is available at any one time.”
– JOHN HEARD
As if farming on the Prairies wasn’t already complicated enough.
Micronutrients are one aspect of plant nutrition that is fiendishly difficult to measure and understand, but can cause potential headaches when overlooked.
Especially in sandy, grey wooded, peat or low-organic matter soils, deficiencies may exist in the more obscure list of the difficult-to-pronounce, yet still essential nutrients needed by plants for fulfilling their mission of converting sunlight and water into food.
“Without them, in plants, the wheels would just fall off. Although they are just required in very small amounts, they are critical to many of the major plant growth processes,” said MAFRI soil fertility specialist at a recent soil fertility workshop in Brandon.
Micronutrients are generally present in abundance in the soil parent material. But problems may arise when they aren’t broken down fast enough by natural processes to satisfy crop needs.
“If you look at all the micronutrients in soil, there’s enough to last your lifetime. The problem is that only a fraction of that is available at any one time,” he said.
For alfalfa, it’s often a shortage of copper and boron that results in stunted, slow regrowth, and purplish leaves. In barley, copper is usually the unexplained culprit.
Clover sometimes runs short of molybdenum. Corn needs its zinc, and oats and wheat can’t thrive without enough manganese and copper.
The good news is that canola, as a very effective nutrient scrounger, seldom needs supplementation. Also, many micronutrients are present in adequate amounts in the seed itself.
Boron, which is present in soil as a negatively charged anion and therefore prone to leaching, has been the subject of much research recently.
The Carrot River area of northeastern Saskatchewan has been identified as a boron-deficient region, and patches have been spotted in sandy lands west of the Red River and north of Brandon.
Unfortunately, boron is also a “darned good” herbicide if it is applied at too high rates, Heard said.
“Generally it shows up in the second cut (of alfalfa) if it’s dry,” he said, adding that it is usually diagnosed by a tissue or soil test and fixed with a foliar boron spray.
Sometimes, the problem goes away with the application of 22,000 gallons of water per acre – one inch of rain. That’s because alfalfa roots under dry conditions that have been reaching down for water have gotten out of range of the mineralized boron present in the organic matter near the surface.
Alfalfa is the most boron-tolerant crop, but if sensitive crops such as peas and dry beans are to be included in a rotation, farmers should be wary of overapplication, he added.
Copper deficiency is another common problem in Manitoba’s sandy or peat soils, usually showing up as twisted, dying leaf tips, poorly formed kernels, and shorter, darker brown straw. When cereals are short on copper, a greater proportion of the pollen is sterile, and the delayed fertilization that results can leave the crop susceptible to ergot spore infestation.
Studies in Western Canada have shown that in fields with a soil test result of less than 0.4 parts per million, there was a 94 per cent chance that adding copper would result in a yield response, and 62 per cent of the time it would have made economic sense.
“Almasippi soils are almost a textbook case. With low organic matter to begin with, what copper that’s there is reduced in solubility because of the high pH,” said Heard.
The problem is that copper deficiencies are often scattered around in isolated spots in any given field, he said, and soil testing can be hit or miss.
In one case, soil test samples were taken at a distance of one metre apart across a field, for a total of 15 samples. The average came back at 0.7 to 0.8 ppm, but two were below critical levels.
“That just shows that over a short distance, there can be a fivefold difference in the amount of available copper in the soil,” he said. “So, sometimes it’s best to use a combination of soil testing and field scouting to observe whether it warrants going in there to make corrections.”
High pH, again, is linked to manganese deficiency, particularly on peat soils. The elevated acidity binds the manganese so tightly that crops can’t pry it out of the organic matter. On mineral soils, one ppm is generally sufficient.
To add to the complexity, “antagonism” between nutrients can occur.
“If there’s less manganese than copper, then you get a manganese deficiency. But if you have 15 times more manganese than copper, then you start to see a copper deficiency.”
Soil-applied zinc is best for severe deficiencies, which generally show up in corn growing on exposed subsoil that is low in organic matter. Applied zinc on land with soil tests below one ppm has resulted in a five-bushel-per-acre yield increase, he said.
Iron and molybdenum deficiencies tend to be related to short-term, environmental conditions, such as cool, wet seasons. When soils warm up, root uptake improves, and the problem often goes away.
Some fertilizer manufacturers have lately been offering “cocktails” of micronutrients that are touted as offering farmers the ability to dose their fields with a kind of multivitamin fix-all.
Studies in wheat and barley have shown a yield increase in one-third of all cases, but only a few cases would have had an economic benefit for the grower.
If the soil is only deficient in one micronutrient, then it would be better to concentrate on fixing that one instead of using a shotgun approach, said Heard, adding that any cash invested in micronutrient fixes should be considered “fun money.”
“Spend your dollars first on nitrogen, phosphorus, potassium and sulphur. I don’t like to see it used as a substitute for the products that will really build the yield of the crop,” he said. [email protected]