Checking on the 2020 groceries

When it comes to assessing soil nutrient levels and determining appropriate fertilizer rates, today’s modern farmers have a seemingly endless number of tools and technologies at their disposal, from aerial and satellite field mapping systems to model-based rate recommendations and variable rate application technologies.

But regardless of what systems you’re using, there’s still no substitute for getting out there and sampling the soil, says fertility expert Jeff Schoenau.

A well-designed soil sampling program is the foundation upon which all other crop nutrient decisions should be based.

“There’s a lot of technologies that are out there but I think going right at the soil itself, and saying, ‘OK, I’m going to get … out in the field and find out exactly what’s out there,’ — I think that’s a pretty hard approach to beat,” says Schoenau, a soil scientist at the University of Saskatchewan in Saskatoon.

“All the … remote sensing technologies that are available can certainly be very valuable in terms of extrapolating (soil nutrient levels) across a larger land area, but at some point in time, you’ve got to get out there and test the soil.”

What’s more, routine sampling is a big part of responsible nutrient stewardship.

“When you’re talking about the 4Rs of nutrient stewardship — right source, right form, right place and right rate — identifying the right rate really relies on a solid foundation for making that rate recommendation, and certainly a soil test is a very good basis for that.”

Soil sampling is nothing new to prairie agriculture but the way samples are collected and analyzed has changed significantly.

Gone are days of traditional composite sampling, when growers collected samples randomly from a few areas of a field and bulked them together to arrive at a rate recommendation.

Today, most growers use a more scientific approach that relies on the collection of field data or the use of field mapping technologies that use drone imagery, satellite imagery or ground-based sensing technologies.

“I think what’s happening today is that a lot of agronomists that are out there working with growers are realizing that some efficiencies can be gained by using smart sampling or directed sampling systems … that typically involve the identification of different zones within a field,” Schoenau says.

Agronomist Alexis Adams with Westgreen Agronomy at Warman, Sask., checks waypoints while collecting post-harvest soil samples from a wheat field north of Dalmeny, Sask. | Brian Cross photo

Different zones within a field might be grouped together based on some similarity in characteristics, perhaps topography, production history or soil organic matter.

The groupings act as a basis for variable rate application models that can be used to determine area-specific rate recommendations for an entire field.

“The ability to identify these zones in a field is getting easier and easier, with yield maps that are coming off combines, with satellite imagery, with drone imagery and with ground-sensing technologies like electrical conductivity,” Schoenau says.

“There’s many different approaches that can be used, but really, it’s all about looking at how those various parts of the field differ, and using that (information) as a basis to make some formulations for different rates of nutrient applications….”

“And then, of course, we have the tools that have always been available to us — like soil sampling and tissue testing within those zones — that help us to determine exactly what’s going on in the soil itself.”

Although data collection and computer based modelling has become an important part of fertility management, soil sampling is the basis for any good fertility plan.

So how often should a field be sampled?

Soil samples collected after harvest can be analyzed for residual soil nutrients. Here, samples are separated for analysis at a depth of zero to six inches, and six to 12 inches. | Brian Cross photo

In an ideal world, every grower would sample every field, every year. But in reality, even the most productive and responsible land managers have their fields sampled on a rotational basis, with cores typically taken from each field every three to five years.

The number of samples drawn from a field can also vary significantly from one grower to the next.

For growers that are aggressively seeking top yields on every acre of every field, intervals in the sampling programs might be tightened and the number of samples taken from each field or each zone within a field might be increased.

Schoenau compares a sampling program to a photographic image. The greater the resolution or density of the image, the clearer the picture becomes.

Similarly, a greater number of samples taken from any given area provides a more accurate assessment of the soil nutrient levels in the soil.

“It’s always a compromise between, theoretically, what you need, and, practically, what’s possible,” Schoenau says.

In reality, the frequency and density of an individual grower’s sampling plan depends, among other things, on the grower’s yield targets and on his aversion to financial risk.

“That’s actually one of the most important things that an agronomist does out there is tailor the outcomes of a general model, specifically for an individual grower, both in terms of particular yield goals and … the associated nutrient demands and also, even the grower’s attitude toward risk,” Schoenau says.

“For example, some farmers might be risk-takers who want to go for the top end of the yield curve, or are they a bit more conservative … and tend to be looking for a lower rate where there’s less financial risk but not as much opportunity to capitalize if (growing conditions) are really good.”

Environmental conditions also need to be considered when determining a rate recommendation.

In 2019, for example, dry conditions during spring seeding were followed by frequent rainfalls in late June and July and unusually cool temperatures later in the growing season.

Soil samples are bagged, sealed and sent away for analysis at an accredited lab. | Brian Cross photo

On many prairie fields, that resulted in uneven germination, late seedling emergence, variable crop staging and delayed crop maturity.

Those conditions likely impacted the ability of crops to use available soil nutrients, including nitrogen, phosphorus, potassium, sulfur.

As a result, a more thorough post-harvest assessment of residual nutrient levels might be warranted to see what’s actually in the soil and what additional nutrients will be needed to achieve future target yields.

“We’ve had a couple of pretty strange years, with drought and in some cases, really heavy rains and so on,” says Thom Weir, a crop fertility expert and agrologist with Farmer’s Edge.

“It hasn’t been a normal year, so growers — especially in a year like this where costs are going up and returns are going down — need to be very cognizant of what they’re getting for their fertilizer dollars.”

Soil sampling is always an important element to any successful crop fertility program, Weir says.

Getting an accurate read on residual nutrient levels can save money and achieve target yields at a lower cost.

“It’s really important that producers go out and sample and find out exactly where they are with their nutrient levels.”

The economics of producing crops have also changed over the past few years, Weir adds.

Fertility decisions that a producer made when canola was worth $12 a bushel may no longer be relevant when the canola market is offering $9 or $9.50 a bu.

“When I talk to guys about variable rate applications, I often ask them what their goals are,” says Weir.

“Is it optimizing yield? Is it maximizing yield? Is it growing the same amount with less fertilizer? Or is it a quality issue (such as) … reducing protein content in malt barley?”

“Over the last five or six years, a lot of guys have been trying to maximize their yields and they’ve been quite successful doing that. We’ve seen a lot more 60 bu. canola crops in the last few years than 30 bu. canola crops.

“They’ve put the extra into it. They’ve used the best genetics that are available and they’ve been throwing everything at their crops in terms of fertility and crop protection products to try and get those maximum yields.”

But as prices go down, the costs involved in producing maximum yields may no longer be warranted.

Top-end growers might find that their economic risk can be lowered and their potential returns increased if they reduce their target yields to reflect the new economic realities.

Schoenau agrees.

In addition to changing economics, spatial variability in residual crop nutrient levels can be expected to increase in years when environmental conditions produce less uniform crops and greater yield variability in the same field.

Dry conditions early in the season can limit utilization of certain soil nutrients, he says.

Similarly, cooler-than-normal temperatures during the growing season can slow plant development, alter a plant’s nutrient needs and affect microbial activity in the soil that’s related to nutrient availability.

“Sometimes it may seem like things don’t change a lot from year to year but there’s a surprising amount of both spatial variability — changes over the landscape — and also temporal variability — changes in nutrient availability over time — that relate to the microbial processes in the soil, the affect that the weather can have, losses of nutrients and so on,” Schoenau says.

“It’s difficult to predict that. Yes, you can use a model that tries to assess those factors but it’s always good to have that verification through sampling.

“The (application) rate really relies on a solid foundation for making that rate recommendation, and certainly a soil test is a very good basis for that.”

About the author


Stories from our other publications