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Just say ‘no’ to variable rate fertilizer

Consultant says only about a quarter of prairie fields are hilly enough to benefit from the practice.
wp canola feilds
Ken Greer of Western Ag says that to get a response from variable rate fertilization, you need significant low, medium and high topography, something few prairie fields have.

WESTERN PRODUCER — The variable rate, or VR, component of precision agriculture can help deal with record-high fertilizer prices while maintaining or increasing yield. Therefore, farmers should be clamoring for VR equipment and knowledge. Right?


Ken Greer, founder of Western Ag in Saskatoon, says farmers are facing their highest-ever fertilizer prices yet they are not flocking to VR for savings. One reason he hears from his clients is that farmers don’t trust VR and aren’t convinced the return on investment is there.

“Western Ag has purposefully stayed out of VR for the very good reasons I give to 4R agronomists,” he says. “VR tied to big data is destined to only ever be marginally successful, despite the 4R push from lobby groups. That’s because only 25 percent of the land in Western Canada has enough topographic diversity to justify the investment in VR technology.”

For VR to be valid, he said, a field must have distinct low areas, side-slope and knolls. It has nothing to do with potential of the soil, but rather with variations in elevation between the lowest points and highest points. Western Ag is the company that developed the alternative soil test based on the Plant Root Simulator (PRS) probes and the CropCaster decision support model.

“To get a response from VR fertilizer, you need three well-defined topographic zones,” Greer says. “That means genuine hills. The closer your field is to being a homogenous flat surface, the less response you’ll get from VR. I’d say only a quarter of fields in Western Canada have enough low, medium and high topography zones to justify VR fertilization. In three quarters of the Prairies, our soil does not have enough variance from up slope to low slope. VR needs predictable variance to work.”

VR can realize some reduction in fertilizer rates in peaty hollows and saline spots. Greer’s clients often reduce N in those areas, which can be a significant cost saving.

Western Ag conducts all soil analysis using its PRS system. Greer adds that some producers have made VR pay by simplifying their system and cutting the fertilizer rate in saline zones.

“One of our franchises in northwestern Saskatchewan did a lot of detailed comparison work in 2017 using SWAT (soil, water and topography) map zones. They used strips and then compared the strip results of our dominant soil zone approach to the SWAT map results. Even with a very detailed SWAT map guiding your zones, the team concluded that after all the complexity, you are not money ahead doing variable rate. These kinds of experiences are what guide us.

“If you ask around, you’ll find a huge number of farmers who tried conventional VR companies. They couldn’t prove it really paid and they finally dropped it. You’re chasing a smaller number of acres where you can actually do something different, as opposed to a better average straight across the field using a standard rate.

“You’re financially better off focusing on improving your soil test program. Field by field there’s always variance, typically more difference field-to-field than within a field. That’s a result of different landowners over the years treating the soil differently. One quarter might have been pasture or hay land for a long time, while the adjacent quarter might have been disced to death until 1980.”

Greer says the water variance factor totally disrupts variable rate theory. He says the simple algorithms previously used by some companies recommended farmers take the low-yielding variants, which are the eroded knolls and hilltops, and add more fertilizer.

That works in wet regions, but it doesn’t work well in dry regions, he says.

“An algorithm has so much to do with where the water is. The SWAT maps give you information in hummocky terrain, but you are still limited by this less-than-useful fertilizer recommendation, in my opinion, based on conventional soil tests.

“I’ve become very vocal about the sacred cow of conventional soil test. People say it’s the best we have, so we shouldn’t criticize it, but Western Ag has been around for 25 years proving the PRS CropCaster is a better alternative.”

Greer quotes from a paper Terry Roberts presented to the Info Ag Conference in 1996, where he questioned the newborn practice of VR fertilizer. Roberts spent his career with Potash and Phosphate Institute, then lead PPI to become the International Plant Nutrition Institute.

“There are 87 million acres of cultivated land in the prairie provinces,” Roberts wrote. “About 14 percent of the landscape has slope exceeding nine percent. About 30 percent has slope greater than five percent. Moderate to severe wind erosion affects nearly 20 percent of the improved farmland. The soils are naturally fertile but variable.

“The other big factor is the seldom-spoken reality that conventional soil test calibrations are just not good enough to predict the response in multi-zone fields. The research that calibrated the response curves from conventional soil tests had faults. The return on investment just wasn’t there. The rate you apply in a particular zone was never validated or calibrated for that zone.

“I’d say only 25 percent of the Prairie provinces have those super rolling hills where VR fertilization gives a return on investment. We also have areas with peaty bottoms or we’re farming beach sand hills. That’s where VR can work. In the other 75 percent of prairie land, we just do not have the variance big enough to get responses to VR fertilizer.”

Roberts wrote that VR fertilization may not be the best application of precision farming in the Prairies. Moisture is always the most limiting factor and it cannot be controlled. VR technology might be most appropriate for spraying operations when the target is disease, insects and weeds.

“There are cases where VR fertilization can work,” Greer says. “Just not 100 percent of fields on 100 percent of years with 100 percent of crops. The success rate will be more like 25 percent of the acres, years, crops.

“Terry Roberts’ report got zero notice by farmers or farm media. Instead, the whole world of agriculture keeps running headlong into layers of maps and variable rate and prescriptions.”