Tips for Understanding Macronutrient Soil Test Results

April 21, 2016

Parts per million versus pounds per acre. Olsen versus Bray. Nitrogen versus nitrate…

Some of the numbers and technical terms on the paper or email you get from the soil test lab are relevant for understanding nutrient availability for your crop and others don’t actually mean much for growing crops Western Canada.

So which numbers are meaningful? And how should you respond?

In this Canola School filmed at canoLAB ’16 in Brandon, Rigas Karamanos, senior agronomist with Koch Fertilizer, walks us through the macronutrient portion of a soil test, explaining what we should look for and expect based on N, P, K & S results:

Nitrogen:

  • Remember that nitrogen is mobile.
  • Nitrogen is available to plants when it’s in nitrate form. Fertilizer may still be in ammonium form in the soil.
  • Nitrogen measurements in Western Canada are calibrated for a 0-24 inch depth sample.

Phosphorus:

  • If you have less than 5 parts per million (or 10 pounds per acre), you have a 100 percent chance of getting a response from adding P in canola.
  • 5-10ppm = ~75 percent probability of a yield response, 10-20ppm = ~50 percent chance of response, more than 20ppm = less than 25 percent chance of getting a response from additional P.
  • Both the Olsen and Kelowna tests are calibrated in Western Canada, although Kelowna will often show a slightly higher number.

Potassium:

  • 125 ppm or 250 lbs/ac is considered the minimum soil test level for most crops. Malt barley is one of the only crops that will respond from higher K.
  • P and K rates are additive when applying with seed, so there’s usually more bang-for-your-buck from P.

Sulphur:

  • “Sulphur is one of the worst soil tests,” says Rigas. “One thing you know about soil test sulphur is if it’s low, it’s low. If it’s not low, you don’t know what it is.”
  • Always apply 10 lbs/ac.
  • Past work done with conventional, open-pollinated canola showed a ~7 to 1 ratio of nitrogen to sulphur was optimum, but research with hybrid canola shows it can utilize sulphur at any ratio. “The rooting system of hybrid canola explores the soil volume much more efficiently,” he explains. Don’t worry about the N to S ratio, says Rigas.

Canola School videos are produced by Real Agriculture.

You can find all of the episodes on Real Agriculture's Canola School page

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