Phosphorus Q&A – helpful tips to get the P fertilization right this spring

The agronomy team at Alberta Grains has been getting a few questions about phosphorus fertilizer products. The following are a few Q&As offering helpful tips on phosphorus:

How mobile is P in soil?

  • P is immobile in soil. It is bound strongly in soil through fixation.
  • To boost seedling growth, P starter fertilizers need to be placed close to the developing roots for easy access.
  • In extreme cases, moisture can move P down slopes or into waterways.

How do you interpret soil test P values?

  • Soil test levels are an indicator of the probability of response to a P application, they assess the accumulation or depletion of P from a field over time, but don’t predict precisely how much P to apply, or if a response will occur in a specific field and specific year. In other words, soil test P values are an indicator, not an absolute number.

What are the approaches of P management?

  • Long-term sustainability approach: aims at ensuring background levels of P in the soil are neither depleted to levels that limit crop production nor increased to levels that may pose an environmental risk. It’s feasible on the basis of long-term tenure, availability of capital, and access to P sources. It may take multiple years to build or draw down P levels in soil.
  • Short-term sufficiency approach: aims at optimizing net returns in the year of application. It is assessed based on a regionally calibrated critical threshold, measured soil test values, and the probability of response to P by the crop in the current year.

How does P fertilization change plant-available P?

  • Over the growing season, plant-available soil P will be affected by the concentration of P in the soil solution and the ability of soil to replenish the soil solution from other organic and inorganic soil P pools. With P applications, we’re feeding the soil, not directly feeding the plant except with the case of starter P.

Does starter P fertilizer provide benefits if my soil test P is high?

  • Even if soil P can provide an adequate supply to the plant throughout the growing season, with cold spring soils, a response to starter P may occur even when soil P levels are relatively high

Does the formulation of P fertilizers matter?

  • Novel P fertilizer formulations, additives, coatings, or use of microbial products have generally not shown increased effectiveness over MAP and APP under field conditions (Grant and Flaten, 2019). A pound of P is a pound of P.
  • Plants can access P in soil solution by diffusion or root interception. The availability of P is directly related to its solubility.
  • P solubilities may be impacted by soil pH and other ions. In neutral to alkaline soils, Ca and Mg will react with phosphate and form less soluble compounds. In more acidic soils, Fe and Al may have similar impact in reducing P solubility.

General solubility of common sources of P:  

References 

Grant and Flaten. 2019. 4R Management of Phosphorus Fertilizer in the Northern Great Plains https://acsess.onlinelibrary.wiley.com/doi/pdfdirect/10.2134/jeq2019.02.0061

Field Notes: So what about solubility? https://www.grainews.ca/growpro/field-notes-so-what-about-solubility/

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