Vexing Variability

Variable-rate technology can boost efficiency, but success hinges on individual field traits.

A horse’s head resting in a gangster’s bed isn’t just part of the 1972 movie The Godfather. Years later, this scene was slightly imitated – minus the blood-spattered gore – in one of Anthony Thilmony’s fields. 

“When we started looking at varying fertilizer rates, we had no equipment available to do it,” says the Valley City, North Dakota, farmer. “It was an experiment to answer questions.”

So, Thilmony worked with Dave Franzen, a North Dakota State University Extension soil fertility specialist, to pull soil samples in 100×100-foot grids on which to base variable-rate fertilization. Franzen grid-sampled 40 acres in one of Thilmony’s 160-acre fields. The mapped field revealed a peculiar horse head-shaped area where yield and nitrogen (N ) levels were concentrated. Since N is a mobile nutrient, Franzen predicted this pattern would never again surface.

Not so. The same horse head-shaped pattern surfaced the next year. Ditto for the next year, and the year after that. 

“Finally, we figured out what was happening,” Thilmony says. Moisture moving down from higher areas moved mobile nutrients like nitrogen and sulfur into the lower horse head-shaped area. 

“Where there’s moisture and nutrients like nitrogen, there’s yield,” Thilmony says. 

Causes of Variability 

Sometimes, yield variability occurs due to soil type differences. In Thilmony’s case, his farm is a hodgepodge of heavier soils laced with a share of low-producing sandy and gravelly soils. 

Still, it was field elevation differences – topography – that separated his high-yielding field areas from others. 

“The easiest way to create a script (prescription) is based on soil type,” says Lance Tarochione, an Asgrow/DeKalb technical agronomist. “The most worthless way to create a script is also based on soil type.”

That’s because soil type generally doesn’t explain yield variability, he says. 

“Even in a uniform field, it’s not uncommon to observe a 100-bushel spread in yield,” Tarochione says. “It might be due to drainage, it might be due to elevation, but not necessarily soil type.”

Variability can even exist within a soil class, says Bryce Baker, Precision Planting integrated marketing manager. 

“Just because an entire field contains the same category of soils doesn’t mean they’re all created equal,” he says. “Research we’ve done shows that not every silt loam, not every clay loam, not every sand is the same. Even within a soil that has similar organic matter levels, there may be some level of variability when it comes to water-holding capacity.” 

A soil type map often will not discern yield variability, Tarochione says. He advises farmers to use yield maps or imagery maps to identify high- and low-yielding field areas in order to plan a variable-rate strategy. 

That’s what Thilmony did when he shifted from grid sampling to a zone-based plan offered by Farmer’s Edge. Each of his fields now has three to five zones based on topography that are gleaned from yield maps and satellite imagery. 

“We now pull (several) samples out of each zone,” he says. “That gives us a good representative sample without pulling a sample every 100 feet.”

Variable-Rate Fertilizer

Thilmony’s journey in variable-rate fertilization hasn’t been an easy road, though. 

“The first time I varied rates was with a Raven 440 control system. I was literally manually flipping the switch between a high and low rate. Once in a while, I’d forget to switch from the lower rate to the higher rate (of nitrogen). The next year, you could go over the hilltop (into a highly productive field area) and see the nitrogen deficiency that resulted. Where I flipped the switch in time to the higher rate, the crops would look good. I thought, ‘Hey, we’re onto something!’” he says.

Seven years ago, though, he was finally able to tie satellite imagery and automatic application together. 

“I saw enough results from the variable-rate nitrogen that led me to make my dry spreader variable rate for phosphate application and also to apply dry urea,” he says. 

Initially, Thilmony planned to boost the productivity of low-yielding field areas. “We were going to put more fertilizer on to make them yield like the rest of the field,” says Thilmony.

Unless it’s possible to drain those areas, though, Thilmony found low-yielding areas will always be low yielding, no matter how much fertilizer is being added, he says. 

Not so on higher-producing areas.

“Before we began variable-rate fertilizing, high-producing areas were being underfed because we were just applying a blanket rate across the whole field,” he says. “Now, we are using the same amount of fertilizer but just putting it in the right places. Yields have dramatically increased on those fields, because now we’re finally feeding the good soils.

“Some of this is common sense,” he says. “I look at my father, who farmed the same land for 40 years. He can say, ‘This is where it would yield well and this is where it doesn’t.’ There was no hard data to look at, but he had an idea of what the field was like.”

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