The Agronomists, Ep 45: Jeff Schoenau and Merrin Macrae on managing phosphorus losses

(Kara Oosterhuis/RealAgriculture)

Phosphorus is a key nutrient for crop production, and requires careful management to ensure there’s enough P to feed a growing crop, but not so much that the nutrient moves off the field in run-off or soil movement.

For what makes phosphorus management different versus say, nitrogen, we go to experts Dr. Merrin Macrae, professor of hydrology and biogeochemistry at the University of Waterloo, and Dr. Jeff Schoenau, professor of soil fertility and professional agrologist with the University of Saskatchewan.

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SUMMARY

  • How much phosphorus (P) should be added? How is it lost?
  • P isn’t leached or lost to water run-off easily (in Prairie soils); there are transformations in the soil that take place, over time P is less available due to these transformations
  • When the P pellet dissolves, over time the P becomes less soluble
  • Something to think about: how to use different crops to get at that aged P that’s transformed over time?
  • P can enter tile drains through macropores: worm holes, root channels, cracks in soil. Enters waterways
  • Why don’t we want P entering waterways? Algal blooms, which can be harmful. Depletes oxygen, disrupts aquatic ecosystems
  • Is there a question of the validity of soil testing for P? How reliable is a P test in a drought year?
  • Residual P bands (immobile at the best of times). Take more cores or sample across the seedbed to account for variability
  • “Legacy” P, can a cover crop access it? Legumes do a great job of scavenging residual P. Strong AMF and microbial activity, acidifying the rhizosphere
  • Are all winters created equal in the east? No. Frost kill of cover crops can release P, but a lot of it goes back into the soil or can be managed with the species of cover crops grown. Snow cover insulates cover crops
  • What about P losses through soil erosion? Is there a relationship between soil erosion and P losses through run-off? There’s a positive relation: as soil P increases, the greater your concentrations and losses of P. (Eastern Canada)
  • Spring snow melt run-off can move particulates in Western Canada, carrying soluble P with it
  • Clip #1: New tool helps minimize phosphorus loss (2017)
  • Low P, adequate, or high P; how to tackle each existing soil P level, and when to put P on? In the west, a lot of times it’s with seed at safe rates. Having P for pop-up effect is really beneficial for seedling development. Even though a soil might be good for P, we need to be thinking still about building or maintaining soil P, so that it’s not completely lost and could cost $$$
  • Liquid swine manure, with and without starter P, not much of a response on high P testing soils (Jeff has done a lot of work with manure and P too)
  • Broadcast P, sometimes incorporated, in the east. Is using broadcast as the main way to apply, opening it up to losses? Safer to incorporate when you can
  • Rooting depth. Can different crops access P at different depths, or with different root architectures?
  • If there’s the right amount of tile drainage, then it should only be an extraordinary rain event that would move P? Right?
  • In the clays, 80 per cent dissolved P in tile, 20 per cent dissolved P in surface run-off. Loams are different.
  • Taking enough soil cores is challenging, taking a slice is better. Get the area between seed rows, the band, and the seed row itself
  • Why would we want to put more P on than the crop would use?
  • Getting P into the ground is key, agronomically, and as far as stewardship goes
  • 2014 MSc work by Blake Weiseth. Soybean grain yield in a P deficient SK soil as affected by 11-52-0 placement. Run-off in simulated snow melt situation wasn’t different from unfertilized before (check out the video for more on that)
  • Which crops are more prone to releasing P than others? Oats, radish for example
  • Zombie canola, will it release P?  50 per cent of P was in soluble form in the middle of the season. A lot of that P will be back, maybe not all on the soil surface. Don’t worry about incorporating residue, the thatch layer is what’s feeding those soil microbes that are helping to cycle P. Not ideal in positional availability. Extra P will show up on a soil test (inorganic P).

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