The Agronomists, Ep 9: Ryan Benjamins and John Heard on interpreting soil test results


You sent some soil samples away, whether it was from your own farm or on behalf of a farmer, and now you have the results. How do you begin to fully understand the results? What you do with the results can make or break some (expensive!) decisions.

For this episode of The Agronomists, Ryan Benjamins of Benjamins Agronomy and John Heard of Manitoba Agriculture share their tips for diving into soil test results — what to look at first, and what not to get influenced by.

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  • Soil test results are an inventory of where to start, what’s achievable, and a lot of important fertility decisions ($$) are being made or were made, based on a soil test result. A fertility plan is not a black and white decision, it’s a conversation about how often the farmer thinks they should soil test.
  • How often you soil test will depend on what your rotation is, corn, soybeans; corn on corn out east?
  • Out in the west, test for soil N every year. Getting a full analysis done and the analytical cost is a minor expense.
  • 0 to 6″, 6 to 12″, 12 to 24″ are the norm, especially with the equipment out there today. A good hydraulic sampler can easily reach the 24″ mark in most fields.
  • Out east, pre-side dress nitrate test to 12″; 0 to 6″ depth for potassium, magnesium. Ontario generally isn’t set up for 24″ sampling. Too many stones.
  • Ross McKenzie clip
  • Different types of soil will have more appropriate soil extraction methods for different nutrients (in a nutshell)
  • OK, so now what? You’ve got results (assuming the appropriate method to test the soil was used), what do you do with them?
  • What’s the land tenure? What’s the financial situation? Are crop prices high or low? Do you need to build up the fertility?
  • How to dive into the results? Out east, if it’s a ‘foreign’ field: 1. soil pH (lots of nutrient availability issues arise from pH), 2. CEC (cation exchange capacity), then go from there
  • The fertilizer recommendation (if included) should be the last thing you look at in the results.
  • Western Canadian acres (about 1/3) test below 10 ppm for phosphorus, which is deficient. Eastern Canadian acres, about the same, variable.
  • Manure can have a long-lasting influence on nutrient levels!
  • Maintaining soil fertility with the yields coming off in the last few years
  • Rigas Karamanos clip
  • Phosphorus levels, different ppm (or lb per ac) mean either a huge response from the crop, a maintenance level of P needs to be applied, or very little response can be expected
  • Crop specific N, P, and K numbers
  • Canola: you can bet quite safely that S should always be applied
  • More yield with higher base fertility? 20-25 ppm P, 120+ ppm K
  • Strip-tilling research from Ryan
  • The time it takes for fertilizer to mineralize depends on the nutrient, but most are salts right, so if they’re highly soluble, they’ll dissolve fast. Don’t forget about nutrient cycling processes.
  • Base cation saturations, more research needed for Canada. Started in New Jersey. Missouri. It can be a distraction for growers.
  • Soil pH changes at depth of N fertilizer application (in no-till)?
  • Jake Munroe clip
  • Don’t forget about 4R (P and N!)
  • What are the recommendations based off of, and how did you (the agronomist) come up with this recommendation? That’s something the farmer invests in the agronomist for the answer to. It shouldn’t be ‘secret sauce’
  • The ‘hallowed’ crop removal values are currently outdated in the west. Dr. Fran Walley at the U of S, and collaborators are working on it.

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