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Soil micronutrients get a closer look

There’s a growing realization that soil fertility means more than just N, P and K

Some days, it’s hard to keep track of the latest word on soil health and fertility. One minute the word on sulphur is that it’s no longer a secondary nutrient and that’s it right up there with nitrogen, potassium and phosphorus. The next, we’re hearing that boron or molybdenum levels need more attention.

According to a document published by WinField United in August 2018, one of the key nutrients in boosting corn yields turns out to be zinc. The article, authored by WinField technical seed manager Corey Evans, refers to zinc as “an unsung nutrient hero that helps maintain some of a plant’s most important physiological processes.” Among its attributes, zinc is the catalyst for enzyme creation and it kick-starts auxin hormones to initiate seed germination and helps with healthy root development.

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Zinc can also be a barometer for other nutrients, providing balance and aiding in a plant’s chemical processes. Soils that have good zinc levels tend to have good manganese levels. If zinc has been built up with organic materials, the soil may have correspondingly higher levels of phosphorus (P), potassium (K), residual nitrogen (N) and sulphur (S). The one proviso is that zinc’s impact can be limited with P levels of greater than 45 parts per million (ppm).

But the list doesn’t stop there. Zinc is also involved in protein synthesis, the formation of chlorophyll and the metabolism of carbohydrates, and it can detoxify superoxide radicals produced under stress conditions. Part of its role as a growth regulator is the production of indoleacetic acid (IAA) as part of the auxin response, and IAA is synthesized as a response in zinc-deficient soils. Zinc is mostly available in soil organic matter, so any enhancement to those levels is a positive step. It’s always present in the soil but its availability is limited in those soils that are compacted, cool and wet, or with a pH above 7.

Corn production is where zinc proves itself an essential nutrient. Crop removal in corn is about a quarter to half a pound per acre and its relative immobility in soil means that roots need to come in contact with zinc in its available form (Zn2+) for uptake or absorption. Yet a little goes a long way.

Its importance can’t be underscored enough, says Tim Welbanks, market development manager with WinField United/Croplan Genetics. The company’s field tests in the U.S. showed a 3.2 bu./ac. yield advantage, and Welbanks believes similar results apply to Canadian conditions. He adds that Croplan provides a zinc seed treatment called Nutriseed Zn to its corn seed to provide the best value to their customers.

“We make sure in our lab testing that it’s compatible with the current seed treatments, that it doesn’t affect planting ability and seed flow in planters, and that it doesn’t have an antagonistic effect with other seed treatments,” says Welbanks. “And we can treat small batches of seed with various treatments to make sure they work.”

Welbanks adds that the goal with Nutriseed Zn is to have zinc at the point where the radicle breaks through the seed coat, receiving that added boost of the micronutrient.

Asked if the knowledge base on micronutrients is getting better and whether agriculture is learning more about why soils behave the way they do, Welbanks concedes that funding can often affect fertility and agronomic research. But there’s definitely a need for generating more information and more resources.

“The industry is offering tools that can help growers decide if they want to take these next steps in crop production,” he says. “We’ve done alright at monitoring the macronutrients — nitrogen, phosphorus, potassium and now sulphur. But these micronutrients, although needed in relatively small amounts, play key roles, and if they’re short (in supply in the soil), they can hinder the utilization of the macronutrients.”

Different formulations

There are different options for adding zinc to the cropping regimen: Croplan has its seed treatment while Mosaic offers MicroEssentials SZ (12-40-0-S10-Zn1). In research conducted by Dr. Ismail Cakmak, various fertilizer blends were tested and compared to MicroEssentials SZ, including diammonium phosphate (18-46-0) and DAP + Ammonium sulphate (AS) + zinc sulphate (ZnSO4). In terms of phosphate, using DAP + AS + ZnSO4 versus DAP + AS, there was a 50 per cent increase in P uptake. With MicroEssentials SZ, the P uptake improved to 100 per cent.

Additional research conducted by Mosaic from 2010 to 2012 indicated that the addition of two lbs. of Zn per acre increased yield by 4.4 per cent, an average of 7.4 bu./ac. When using the MicroEssentials SZ product and balancing the rate with the other nutrients, the resulting yield boost was 11.3 bu./ac.

“The biggest thing is availability for plant uptake and source, and the combination or distribution of nutrients,” says Aaron Stevanus, Mosaic’s technical sales manager for Eastern Canada. He makes further reference to research by Dr. Fred Below of University of Illinois and Ross Bender, senior agronomist for Mosaic. “Only 50 per cent of the zinc needed in corn production is taken by the plant between V18 to VT, so a season-long source of zinc is needed to fulfil the crop’s demands.”

Nutrient science has improved considerably in the past decade as a path to higher yields. Growers have also benefited from elite germplasm and superior hybrids and varieties. The “next level” of agronomic advancements is coming from a better understanding of activity and interactions below-ground, and Stevanus believes the agri-food research community is diligent in learning more about micronutrients.

“We know fertilizer is responsible for up to 60 per cent of the crop yield, so we want to provide every resource we can to aid producers to achieve this,” he says. “As we continue to push yields, we’re looking for the next steps and the building blocks to get there. We cannot push yields any higher if we do not provide balanced crop nutrition.”

Helpful strides

In terms of finding that balance, one of the pillars in determining those nutrient levels in the ground is based on soil testing. According to Jonathan Zettler, a zinc soil test can determine if organic amendments have been applied in the past. Yet as much as zinc may be a key nutrient that’s overlooked, it’s a part of the total picture, which can be helped by taking a fundamental approach: namely the right source, at the right rate, at the right time and in the right place.

“Typically, you would not sample for zinc on its own,” says Zettler, managing agronomist with Fieldwalker Agronomy in Clifford, Ont. “The fertilizer industry continues to work on new ways of improving consistency of response, and using 4R Stewardship as a model will help.”

As an example, Zettler cites the relationship between phosphorus and zinc. Excessive levels of P can induce zinc deficiency in soils. Other cation metals such as copper (Cu) and iron (Fe) have competition with zinc as well. In response to that, using zinc in combination with nitrogen and sulphur can improve uptake and availability. In some situations, foliar zinc applications can be more effective than soil-applied.

“One area where I see fertilizer manufacturers making improvements is bringing products to market that improve zinc or other micronutrient availability,” says Zettler. “Growers want to use high analysis products to reduce the amount of material they want to handle, but then they suffer the issue of having root interception for those that do not really move through the soil profile, zinc being one of them.”

Liebig’s law of the minimum

In 1840, Carl Sprengel, a German botanist, first formulated the “theory of minimum” as it pertains to agricultural chemistry. It stated that growth in plants is not dictated by the total resources available but by the scarcest resource — also known as the limiting factor. Increasing concentrations of more abundant nutrients does not automatically increase plant growth, yet increasing the amount of the limiting factor does.

Years later, the concept was popularized by fellow German scientist Justus von Liebig and was dubbed “Liebig’s law of the minimum.” Illustrated using an image of a barrel of water with uneven staves, it shows how the shortest stave has the greatest impact on the water level inside the barrel. In spite of attempts to lengthen other staves, thereby increasing volume in the barrel, the shortest stave will dictate the actual capacity. In much the same way, the minimum amount of a specific nutrient can have the greatest impact on plant growth and ultimately, yield.

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CG Production Editor

Ralph Pearce

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