Different nutrient, different strategy

The nutrient in question, its source, and a host of other variables determine the right path

All nutrients are not created equal, and their management strategies should reflect that.

That’s been a core message from soil and fertility specialists this winter during a round of soil fertility updates held in late January and early February.

In recent years fertility and extension specialists have concentrated on the 4R message: right source at the right rate in the right place and at the right time.

They stuck with that message at a soil fertility workshop in Brandon Jan. 30 and 31, but noted that in the field that needs to take into account the nutrients being managed. What works best for nitrogen doesn’t directly transfer to phosphorus and while potassium management is close to phosphorus, it’s not identical. To further complicate things, the source of the nutrient in question can vary greatly and affect management practices.

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Phosphorus

Farmers have little choice when it comes to the phosphorus source compared to nitrogen, Dr. Tom Jensen of the International Plant Nutrition Institute said.

“You don’t worry so much about the form, but you worry about rate and placement,” Jensen said. “Even timing isn’t as critical for phosphorus.”

Less mobile in the soil, Jensen noted that an RTK-equipped farm could band phosphorus in the fall and plant near the band the following spring with little difference from spring banding.

Jensen pointed to a maintenance zone, in which available phosphorus is above the critical level needed for crops and application should balance what plants remove. Below 12 parts per million on the Olsen scale, phosphorus can be applied high enough to build up in the soil. Levels from 12-18 parts per million hit the maintenance stage and rates drop up to half of expected crop removal at very high phosphorus levels, a line set above 18 parts per million, according to Manitoba response data from the ’60s.

Other data has shown a sudden drop in fields responding to phosphorus in that maintenance range. In a western Canadian study of 33 site years between 1988 and 1995, researchers found that half of fields between 11 and 20 parts per million phosphorus on the Olsen scale responded to phosphorus, compared to all fields under five parts per million and 71 per cent between six and 10 parts per million. Only a quarter of fields responded when levels topped 21 parts per million.

That guideline will be of little help to a producer that is not soil testing.

For those looking to build phosphorus in their soils, pH may become a factor when it comes to rate, since higher pH may mean phosphorus gets tied up with calcium, the room heard. A University of Manitoba study estimated it would take 15-25 pounds an acre of phosphate fertilizer in the neutral soils near Carman to add one part per million of available phosphorus. In the alkaline, high-calcium soils near Brandon, the same change would take 30-40 pounds.

“On some of the soils that are extremely high pH, let’s say 8.3, a lot of carbonates in the soil, they may have to go with a little bit of higher rates of phosphorus compared to a soil that’s maybe more neutral,” Jensen said.

Phosphorus is often banded to place the nutrient closer to roots and to concentrate the nutrient, limiting tie-up with other elements in the soil. The right placement will depend on soil fertility, conditions like temperature, that soil’s capacity to store phosphorus, which crops are to be grown and how they are managed, the room heard.

Tillage may disturb mycorrhizal fungi, something some plants are reliant on to access phosphorus.

Farmers may want to keep phosphorus close to the seed in cold soils, or in fields that are already phosphorus deficient, compacted, or any time roots may be restricted, Jensen said.

Jensen warned farmers, however, about seed row toxicity in phosphorus, especially for canola and peas.

Farmers should take the long view on both phosphorus and potassium management, Jensen said. Both nutrients can be stockpiled in the soil, he pointed out.

“They’re better managed in the long term,” Jensen said.

Nitrogen

Market forces, such as market preference for high protein in wheat, may have some impact on nitrogen rate decisions, Manitoba Agriculture crop nutrition specialist John Heard said.

“Generally, Manitoba farmers would be looking to apply rates of nitrogen that are going to be close to optimizing yield,” he said, pointing out that crops with 13.5 per cent protein are generally considered to have been given enough nitrogen for full yields.

Manitoba Agriculture’s John Heard presents in Brandon during one of several soil fertility updates in the last month.
photo: Alexis Stockford

“The challenge right now is that we have some very high-yielding varieties and I think they’re producing yields that some are unaccustomed to and we’re unaccustomed to supplying enough nitrogen for both the yield capacity and the protein,” Heard said.

Producers may be unwilling to take the financial or economic risk to jump-start nitrogen application or risk lodging with too much nitrogen, he added.

“I think some of them are looking at other strategies,” he said. “They’re putting on a good base rate of nitrogen and then they’re maybe assessing the crop and saying, ‘No, this crop has extra yield potential,’ and they will do some in-season application.”

Farmers have more choice when it comes to sourcing their nitrogen, both Jensen and Heard said. Outside urea, products have emerged with nitrification inhibitors, designed to slow release to spread further through the season.

“There’s a lot of misunderstanding of some of these additives, inhibitors and coated material,” Jensen said. “They’re built and targeted at different loss mechanisms. So one that works under one condition of loss isn’t necessarily the best one on another.”

Ideal timing might depend on crop, Heard said. An application before or at seeding might lend itself to short-season crops, while a post-plant application boosts longer-season crops like corn.

Fall application is an option, although one that risks loss if wet conditions leach it away.

A 2010 study from Jensen found that both Super Urea and ESN returned high yields when applied in fall with winter wheat and, in fact, ESN plots applied in fall outpaced those broadcast the following spring. Ammonium nitrate, urea and a urea-Agrotain application all performed better if applied in spring.

At seeding, nitrogen should not be right on the seed and two to three inches should separate the band from the seed horizontally, Heard stressed.

Like phosphorus, Heard suggested banding for nitrogen, citing a 20 per cent jump in efficiency, largely by reducing contact between the fertilizer and straw and microbes and centralizing the nutrient to avoid nitrifying bacteria.

Heard warned, however, that high rates of nitrogen might delay phosphorus uptake when banded together because of toxicity in the band.

Acidity may also impact nitrogen application, he added, although Manitoba’s largely alkaline soils may see more movement toward neutrality.

Potassium

Decisions on potassium should also start with a soil test, Jensen told the room.

Soil test recommendations put critical potassium levels at 125 parts per million, Jensen said. At that level, up to 15 pounds of the nutrient may still be applied to avoid deficient soils. The nutrient does not store in the soil as well as phosphorus, he said, although it beats out the more mobile nitrogen.

“If we can get it to where the roots are going to grow, then we get better utilization and better results,” Jensen said.

Environmentally, potassium carries less of a red flag than either nitrogen or phosphorus, both of which have been tagged as contributing to water quality issues, such as those in Lake Winnipeg.

Light-textured, peat soils, high rainfall areas, high forage production and intensely cropped areas may all be at more risk for potassium shortfalls, the room heard.

Outside soil tests, Jensen suggested that farmers with intensive forage production, a history of straw removal, cold compacted soils, a history of disease or low chloride levels may want to explore potassium application.

Malting barley growers may also want to take a second look at the nutrient. Westco surveys in the ’80s found that adding 27 pounds of potassium to a nitrogen and phosphorus regime boosted malting barley yield, although only twice in 50 studies did it affect plumpness enough to boost grade.

“We try to match additions with removals and then we look at it more in the long term,” Jensen said. “What are our crops removing? If we want to maintain our soil test levels at a healthy, good level, then we have to replace those removals.”

Potassium recycles from crop residues easily, he added, something that ties into advice that farmers consistently removing straw should be on the lookout for potassium shortage.

Organic matter, salinity, pH, secondary nutrients and micronutrients also appeared on the agenda in Brandon.

About the author

Reporter

Alexis Stockford

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.

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