Guidelines To Collect The Data You Need For One Variable-Rate Strategy


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Can I get by with yield maps, a composite soil sample and simple crop removal calculations to implement a variable-rate strategy for nutrient application? Or do I need something more?


The answer comes from Warren Bills, president of GeoFarms Solutions in Calgary:

You can take this approach, but the success and sustainability of the results may not be what you hoped. You’ll want to have a couple years of yield maps under your belt, and a zone sampling approach in at least one year to consider taking this VRT approach. Many factors affect yield and yield patterns. Soil nutrients is just one of them. Using yield map data to calculate crop nutrient removal from composite soil sample results could work in theory, but leaves lots of room for error. It’s important to ask yourself some questions to make sure you are approaching the VRT decision in the proper way.


1. If I’m creating management zones based on yield data alone, what factors in my field could have contributed to the yield patterns I’m seeing? And have I seen these same yield patterns year after year?

2. Do I know where my soil sample points are in relation to the yield patterns?

3. How did I manage my straw in the years of yield data I’m analyzing?

4. What nutrients do I want to apply at variable rates?

5. How important are these nutrients to the crop I’m growing?

6. Are the nutrients mobile or immobile in the soil? In the plant?

7. Am I looking at mining, maintaining or building my soil bank?

By asking and answering these questions, you’ll begin to develop confidence that your variable rate strategy matches your goals. You may also consider some other tools to help you be as accurate as you can for calculating your application rates in the different areas/zones of the fields. Tools like remote sensed imagery, tissue tests, SPAD meter readings, slope and drainage maps, and soil texture tests are just a few more ways to reduce the amount of error in your VRT nutrient application decisions.

A lot of factors come into play when looking at making these decisions. The more you can reduce the amount of assumptions you are making, the better your results will be.

There is no better way to determine nutrient levels in management zones than to just sample for them. A typical five-zone field shouldn’t cost much to sample in relation to the potential for better accuracy in your VRT decision and improvement in yield. Minimize your chance for error and consult the experts in agronomy to help make these decisions with you. I think we may get to a point where we can develop some decent models on crop removal VRT based on yield maps, but in these early stages, with limited data sets and limited research, don’t skip over the basics to make your decisions. In fact, making drastic VRT nutrient decisions with incorrect data may lead you to “create” your own management zones within your field that have nothing to do with yield or biomass, and are simply a result of incorrect VRT decisions.

You can reach Warren Bills, president of GeoFarm Solutions, at 403.874.3848. Visit the website at



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