Spreading the word on nutrients

Even in droughty conditions, urea on the surface of the field is subject to losses that aren’t an issue when it is banded below the crop. |  Mike Raine photo

Putting it all on top isn’t right in all cases because not all fertilizer is created equal; some of it also needs to go underground

Broadcasting fertilizer and then harrowing it into the soil before seeding was once a standard practice on parts of the Prairies.

This system worked relatively well, but after the rise of conservation tillage practices subsequent nutrient management studies found deep banding to be more efficient for most plant nutrients when it comes to crop uptake and losses to the environment.

Advances in application equipment, including anhydrous rigs and one-pass seeding systems, also enabled producers to move away from broadcast applications for most plant nutrients.

Today, with increased fertilizer rates there can be a daunting pile of inputs to apply, especially on large farms. This logistics pressure has ushered in an era where broadcasting plant nutrients has again become common.

But deep banding remains the gold standard when it comes to placing fertilizer where crops can access it.

“I think a good general rule of thumb for fertilizer management, folks, is get it in the ground,” said Jeff Schoenau, a professor of soil fertility who works in the Department of Soil Science at the University of Saskatchewan.

Schoenau, who also farms in southern Saskatchewan, provided a presentation on the surface application of common fertilizers, during the University of Saskatchewan’s annual Soils and Crops workshop.

He said there are many reasons why it’s a bad idea to have fertilizer hanging around the soil surface, including the gaseous ammonia losses of nitrogen especially with 46-0-0 urea.

There is also increased microbial immobilization, or temporary tie-up, of nutrients that happens when fertilizer is used by micro-organisms while decomposing plant residue on the soil surface.

“It’s a temporary tie-up, but nonetheless one that can influence the availability of nutrients to the crop, particularly nitrogen, but also other nutrients like phosphorus and sulfur when the crop needs it,” Schoenau said.

There the right times, products and conditions for top dressing fertilizers, but there are also the wrong times. | Mike Raine photo

When fertilizer is placed and left on the soil’s surface there is also the potential for nutrients to be transported away in run-off water.

“That’s especially true that in spring snowmelt runoff here in the Prairies where the majority of nutrient transport off-site takes place; especially for phosphate and also nitrogen that is contained and dissolved in that spring snowmelt runoff from the field,” Schoenau said.

Growers should consider using farm equipment capable of banding fertilizer at an appropriate placement during seeding, where it’s harder for shallow-rooted weeds like kochia and green foxtail to reach.

“Having that fertilizer down in the moisture close to the (crop) roots is going to improve the root access. That moisture enables that nutrient to move more effectively by diffusion and mass flow to that root surface,” Schoenau said.

“When we’re thinking about application methods placement, it’s always that we need to be considering the trade-off between operational efficiency and the efficiencies of recovery.”

Surface application of nitrogen without incorporation can lead to sizable losses of a fertilizer investment to environment losses.

When a granule of urea is applied on the surface it undergoes dissolution followed by enzymatic hydrolysis and is broken down into ammonia gas.

Ideally, the ammonia gas reacts with the water and soil particles and is converted into ammonium ions and retained in the soil.

“When that happens on the soil surface that ability of that ammonia to be retained by reaction with water and soil of course is reduced, and as a result we have a lot greater potential for that ammonia to simply gas off and be lost to the atmosphere,” Schoenau said.

“When we broadcast urea on warm moist soils with lots of surface trash that translates into high potential volatile ammonia losses and that’s because that urease enzyme that’s responsible for that hydrolysis is very active under those conditions.”

Broadcasting phosphorus fertilizer, especially in conservation tillage systems where it’s just left on the surface, is agronomically inefficient and leaves that water soluble phosphorus on the surface and therefore prone to runoff, especially if applied in the fall, says Don Flaten, soil scientist at the University of Manitoba. | File photo

He said such losses due volatilization can increase on soils with high pH, because the high pH tends to favor free ammonia gas rather than the ammonium ion form.

“Some work out of North Dakota State University by Franzen et al, putting some numbers to this, showed up to 20 percent of surface-applied urea was lost in North Dakota’s soil over 10 days at 30 C. And when the pH was higher, when soil pH was greater than 7.5, those losses were increased upwards of 33 percent, or one third of that surface applied urea lost,” Schoenau said.

A strategy that’s been around for a long time to reduce losses of surface-applied urea is to shut down the activity of that urease enzyme for a period of time by adding an enzyme inhibitor or blocker.

Urease inhibitors including products like Agrotain, Anvol, and Limus that may contain inhibitors like NBVT. In some cases, other closely related molecules provide a larger period of time for rain to move that urea into the soil, Schoenau said.

There are also products like ESN that have a polymer coating to help slow the release of urea, while other products use nitrification inhibitors to block the conversion of ammonium to nitrate.

However, “those inhibitors tend to be less effective for reducing gaseous losses associated with putting nitrogen fertilizer on the surface. They are more effective instead for reducing leaching denitrification losses that are associated with having lots of nitrogen in the soil around in the nitrate form,” Schoenau said.

He said dribble banding urea ammonium nitrate solution 28-0-0 on the soil surface can be efficient.

“Some of the work that we did a few years back showed that it worked really quite well for forage crops, and part of that is because you’ve got some of that nitrogen in there in the nitrate form, which is not susceptible to ammonia volatilization losses,” Schoenau said.

For instance, he was involved in a study that added nitrogen in the 28-0-0 form, soil placed with a coulter versus dribble banded.

“At the 50 pound of nitrogen per acre application rate there was very little difference in the yield and the nitrogen recovery, whether that 28-0-0 was coultered in with a disc coulter or simply dribble banded on the surface. We found that in some other sites as well, and certainly a rain after application as we had at those sites really helps as well,” Schoenau said.

His message to producers was clear when it comes to surface application of urea to frozen soils.

“Don’t do it,” he said.

“Urea will not move downward through the snow if the soil stays frozen and it is susceptible to losses in the rapid runoff that may take place during a melt, or be blown away. And folks, we can get urea hydrolysis even in winter, under low temperatures.”

For surface applications of phosphorus, Schoenau led with a quote by soils researcher Don Flaten from the University of Manitoba.

“ ‘Broadcasting P fertilizer, especially in conservation tillage systems where it’s just left on the surface, is agronomically inefficient and leaves that water soluble P on the surface and therefore prone to runoff, especially if applied in the fall.’ ”

Work done in Saskatchewan by Saskatchewan agrologist Blake Weiseth, now of Glacier Media’s Discovery Farm, found significantly higher yield and greater recovery when the phosphorus was placed in the soil versus simply broadcast and left on the surface.

“In fact, the broadcast left on the surface didn’t give much better yields, really very similar to our unfertilized control. So, agronomically, that in-soil placement of P was superior,” Schoenau said.

Separate studies in Saskatchewan found seed-placed or deep-banded phosphorus had similar runoff compared to an unfertilized control, while when it was broadcast on the soil surface without incorporation there was significantly higher loss of soluble reactive phosphorus.

In Western Canada, the most common surface placement of potassium is with potassium chloride, KCl 0-0-60, and like phosphorus, potassium can only move a few centimetres in the soil.

“If we put it on the surface it will tend to stick there and can result in that potassium being hung up in dry surface soil, just like with phosphorus and it’s not good for root access,” Schoenau said.

“An old rule of thumb, and it’s been out there for a long time, is that to account for reduced efficiency of surface placed potassium, increase that rate by 50 percent to achieve the same response from potassium that’s placed in that soil in the year of application.”

Banding of nitrogen remains the gold standard for applying that nutrient to prevent losses. | Mike Raine photo

However, losses of potassium to the environment from surface placement are generally small and not a big concern when it comes to the environment.

“The chloride that’s in that potash in fact will move readily into the soil with precipitation and cereals may respond to chloride application in soils that have very low chloride levels. So that chloride that’s in that potash can move down with the water into the soil and be accessible to the crop roots,” Schoenau said.

The most familiar product when it comes to surface application of sulfur, is ammonium sulfate 21-0-0-24.

“This can work in broadcast application to non-calcareous soils, that gets relatively reduced losses of nitrogen through volatilization compared to surface urea. And for the sulfate there’s really no gaseous losses. That sulfate is mobile and so precipitation will move that sulfate like chloride into the soil for root access, provided that you get a rain,” Schoenau said.

Other forms of sulfur that are commonly applied are the elemental sulfur fertilizers, such as 0-0-0-90.

“Elemental S forms are insoluble. We require them to be dispersed into small particles followed by microbial oxidation to be turned into plant-available sulfate. That takes some time. You need some lead time in order for that microbial process to take place,” Schoenau said.

“What’s rather unique about the elemental S forms as a fertilizer is they often work best when they’re simply broadcast on the surface and left there to weather. That helps in the dispersion to break down into small particle sizes and the release of those micron-size particles from the granule, and small particle size means high surface area and high oxidation rate.”

The micronutrients copper and zinc are often applied to the soil as sulfate salts, copper sulfate and zinc sulfate.

“Our research work and others showing that particularly in soils with a high carbonate content that copper and zinc can get readily bound to those soil particles, react with carbonates, become precipitated and therefore get stranded at the surface,” Schoenau said.

“That’s why with those sulfate salts like copper and zinc, it’s best if you’re broadcasting to follow that by incorporation or band it to increase root access in many soils.”

Boron is typically applied as borate, chlorine as chloride and both of those are mobile and will wash into the soil if they’re surface applied.

“The thing about micronutrients is there really are not any significant gaseous losses to concern ourselves about,” Schoenau said.

He said it’s important to consider interactions among form, placement, rate and timing when deciding what is required to maximize nutrient applications.

“The particular form or type of nutrient can affect what particular placement will work the best and as I’ve shown in the case of some of those nutrient forms, a surface application can be an appropriate type of placement,” Schoenau said.

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