Precision ag can’t replace soil testing

TRUE TEST Ross McKenzie says knowing soil types in your fields and regular 
soil tests are essential in understanding its productive capacity

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Precision agriculture promises to boost the bottom line by marrying space-age technology to basic agronomics, but research scientist Ross McKenzie says farmers need to be cautious.

That’s because the science behind input prescription maps developed from GPS, remote sensing and related technologies is not fully developed, the agronomy research scientist with Alberta Agriculture told attendees at the recent Precision Agriculture 2.0 conference.

Building a variable-rate application map depends on understanding soil variation across a field and methodologies used in other regions can’t be easily adopted here because Alberta’s soils have formed more recently than soils anywhere else in North America.

“Alberta’s soils are very different from those of the U.S. Great Plains,” said McKenzie. “In the Wisconsin ice age, just 10,000 or 12,000 years ago, there was a mile of ice above the present site of Calgary. The southern edge of the glaciers was almost right along the U.S. border, so even Montana soils are very different from ours.”

As a result, variation in soil across Alberta fields is mainly linked to the landscape position and is mostly about soil texture, said McKenzie. Wind weathering has changed some areas, eroding knolls and carrying sandy particles to lower spots.

“It doesn’t take a lot of high-tech equipment to learn about the soils on your land,” he said. “You can find your soil type, slopes and the parent material of each soil polygon by looking it up on Alberta Agriculture’s soil information site. There’s zero cost and no fancy equipment required.”

After the glaciers, the next thing to affect soils was the climate. In the Brown soil zone, where soil developed under short-grass prairie, dry conditions limited soil organic matter. Slightly more organic matter developed under the mixed-grass prairie of the Dark-Brown soil zone, and even more in the Black soil zone of mixed bush and prairie. The forested northern areas developed the acidic, highly leached soils of the Dark-Grey and Grey Wooded soil zones.

The soil’s history

Whenever he’s looking at crops, McKenzie said he starts with soil cores to look at moisture levels and crop development. He always points out the horizons and their impact on the crop.

“The soil profile is its history,” he said. “It shows the physical and chemical characteristics of the soil. Soil cores are the real way to see the management zones of a field. But it does take time to get them — and you have to know what you’re looking at.”

The thickness of the dark, uppermost soil layer, the A horizon, reflects the organic matter, with low soil organic matter leading to poor soil structure that’s more prone to crusting, less water infiltration and less moisture storage. It also suggests low fertility.

“Farmer wisdom along with air photos gives you a good idea of management zones,” said McKenzie. “A topographic map that you make with an on-board GPS unit can help. You may or may not need different management zones. I see a lot of fields with no real variation. Salinity is important if it’s there, otherwise I’m not convinced remote sensing or EC mapping gives you any advantages in managing your inputs. The lushness of the crop and yields aren’t the same from year to year — they’re not well related to soil zones.”

Get a good sample

McKenzie doesn’t miss a chance to make the case for soil testing and taking time to get a good, representative sample.

“The analysis is only as good as your sample,” he said.

He advises 15 to 20 cores for each management area, from either random sites or benchmark areas taken in late fall or early spring, when soil is cool. Send them to a lab that uses analysis methods that suit Alberta soils, including the modified Kelowna method for P.

“Other methods work well for other soils, but all the soil test recommendations have been calibrated for those soils and those methods, but they are useless for Alberta,” said McKenzie. “It’s essential that the lab result has been calibrated to yield benefits on the soils where the recommendations are used.”

Get N, P, K, S, pH, EC and micronutrient analyses for all three soil depths, but only get micronutrient analyses every three to five years as they don’t change quickly, said McKenzie. Get numbers for N, S, pH and EC in the 12- to 24-inch samples. There may be very little S in the top six inches of soil but lots deeper in the rooting zone, especially after wet years, he said.

“You don’t need CEC or per cent base saturation, because our soils are so young and they don’t change over time. Texture is similar, it may be worth doing once, but not as a regular thing.”

McKenzie suggested grouping soil sample cores from the areas you think are different management zones, based on your experience, yield maps and/or their landscape position — separating upper- and lower-slope positions.

“Whether you use benchmark sites, or some random sites in each of no more than two or three management zones, you’ll probably discover as much as from high-tech mapping,” he said.

McKenzie said he isn’t against using some precision agriculture tools, but a long career of studying soils in Alberta has convinced him there is no easy substitute for soil tests. As an example, he cites soil samples he took last summer from upper and lower sites across a field. As expected, most revealed the more fertile soils were in low areas, with coarser eroded soils on high spots. But in one low spot that looked much like every other, organic matter and fertility were low. McKenzie identified that soil as aeolian, a windblown deposit.

“There’s no substitute for soil sampling and analysis,” he said.

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