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The challenges of precision ag

The technology for variable rate fertilizer is well established. But the agronomy is still catching up

Precision agriculture, including variable rate fertilizer, is commonplace in farm literature and has been for many years now. But despite the hype, the actual adoption of the practice has been slow. Ask for a show of hands at any farm meetings and the result surprises many.

GPS and autosteer is commonplace and has made a huge difference in field efficiency, reducing or eliminating overlap and operator fatigue. But after that, the winners are not as many as some lead you to believe.

In my opinion, what is missing is a framework in which to think and a fundamental knowledge of the soil resource that we are using and the natural variation that exists.

The framework to set the thinking should be the fundamental soil properties: texture, organic matter, depth of topsoil, depth to lime, pH, salt content (electrical conductivity) and depth to the water table. Soil maps are the first step in establishing the fundamental soil properties but for the most part, soil maps are not usable at the quarter section level.

But, soil maps and the soil types they map and describe can and should be the framework in which to think and share information on what worked in variable rate.

A topographic map with contours aof one meter or so is very important and can be made in many ways. Topography and landscape pattern (sloughs, etc.) is a big part of soil formation. Even a small, shallow slough can have much different soil than the uplands around it. But to capture and map those sloughs, a field scan with something like a Veris/EM38 with standard spacings of 30 feet or so may miss many important landscape features. The operator must look at what is being driven over and make extra passes to pick up those small but important differences.

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Precision ag results

I have heard many winter talks with reports on variable rate fertilizer experiments — usually nitrogen. Several site-years of data may be presented with nary a hint about the soil or landforms the experiments were on. What works on Red River clay in Manitoba is not what is going to work on Weyburn loam in Saskatchewan or a Malmo silt loam in Alberta.

In the work that Alberta Agriculture did on Lethbridge loam soils southeast of Lethbridge, soil nitrogen was not highly variable. Phosphorus and potassium were low on the high land and high on the lower land. For that soil type with that kind of cropping system, that conclusion can likely be applied to similar soil/agriculture types elsewhere.

In the current race to more technology we see many pretty colored maps that are used to “zone” a field for variable rates or other practices. Reports of an X-dollar per acre advantage are often based on limited evidence.

What works in one market area with a particular soil/landscape pattern may be of no use in a different situation. Pretty coloured maps based on past production can show the good and poor producing areas. But they do not show why production is high or low. In an area with significant saline or other problem soils, with production limited by the problem, the pretty colored maps should work. There is no point pouring fertilizer on saline acres. Veris or EM38 can do a very good job of mapping saline acres. If the fertilizer wasted on saline acres is used on non-saline acres it is easy to expect a good profit from the variable practice.

If the poor areas are poor because of a nutrient limitation, it makes sense to add extra of the limiting nutrient. Nitrogen is still the big nutrient that makes the big differences but establishing the variability and the “recipe” to deal with that variation is not simple. Nitrogen mineralization is a much bigger issue now than in the past so the nitrate test needs tweaking.

Treating the phosphorus soil test like a dipstick in a crankcase is problematic. For very low-testing soils there is a high probability of phosphorus response. For very high-testing soils there is a low probability of response. For medium-testing soils response in any given year is a coin toss. Variable phosphorus rates applied over many years would work — it would gradually raise the soil phosphorus level on very low phosphorus eroded knolls and the like. For years, I have said that phosphorus fertilizer is an investment in the land. If you own the land or farm it for many years phosphorus fertilizer will always come home in the bank.

There are examples where individual farmers/consultants have devised systems that work for a specific soil/landscape/cropping system. But with the lack of public investment in the research to evaluate what works in what situations, there will be many more false starts before variable rate becomes a standard management practice. The technology is here. The agronomy is not.

About the author


Les Henry

J.L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask. He recently finished a second printing of “Henry’s Handbook of Soil and Water,” a book that mixes the basics and practical aspects of soil, fertilizer and farming. Les will cover the shipping and GST for “Grainews” readers. Simply send a cheque for $50 to Henry Perspectives, 143 Tucker Cres., Saskatoon, Sask., S7H 3H7, and he will dispatch a signed book.



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