Wheat School: Soil pH, salinity, and choosing the right soil test

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When send soil samples for testing, what do you ask for?

If you’re following Ross McKenzie’s advice, there’s likely some real value in testing more than just nitrogen, phosphorus, potassium, and sulphur (NPKS).

The former soil scientist with Alberta Agriculture says it’s not too late to test soil this year, as frozen ground may require more effort to sample, but the samples should be stable, offering a decent look at what will be there for the crop in the spring.

McKenzie says there is great value in knowing what NPKS is available, but several other soil components are also valuable to measure, as they can have a significant impact on nutrient availability or identify why you’re short yield in certain areas of the field landscape.

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If you’re soil testing, consider testing for:

  • Soil pH
    • Soil pH can change dramatically across a field and with topography. What’s more, soils can acidify over time, especially under no-till systems where the soil layers aren’t physically mixed. Acidic soils can have a negative effect on N-fixing bacteria with pulse crops in rotation.
  • Salinity
    • Measured as EC (electro conductivity), an elevated EC value could mean you’re losing  30 to 40 per cent of yield potential, but you still don’t see white on the field surface. This is “hidden salinity” and it can have a real impact on the crop.
  • Soil organic matter (organic carbon)
    • This test is not super precise, but will give you an idea of a soil’s OM and if that number is climbing or falling.

McKenzie says to double check that the lab doing the soil analysis is using the correct phosphorus extraction method when testing. Depending on the province you’re in, there are some major differences in the findings. For example, Manitoba soil samples typically use the Olson method, but that method won’t give as accurate results for Alberta samples. The Olson method works best on higher pH soils, like in Manitoba.

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