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Long-term strategic management of P and K: Phase Two

Principal Investigator: David Hooker1 and Horst Bohner2

Research Institution: University of Guelph1 and Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) 2

Timeline: January 2018 – February 2022  


  • To determine the economic responses of various starter fertilizer strategies in corn, soybean, and wheat performance on side-by-side plots with inherently varied background soil test phosphorous (P) and potassium (K) levels and other soil health indicators, and to determine whether economic responses are maintained with more stabilized moderate-high background P and K fertility, compared to crop responses achieved in Phase One after relatively high applications of P and K for build up purposes.
  • To determine if soil test P and K levels stabilize at moderate-high levels following several years of high rates of P and K for buildup purposes, and how much fertilizer P and K are needed to raise soil test P and K by one ppm, based on fertilizer application rates and crop removal estimates.
  • To construct a database to improve understanding between crop removal (grain) vs. contrasting background soil test P and K and starter treatments, and to address environmental sustainability use the Phosphorus Loss Assessment Tool (updated P Index) for Ontario (PLATO) to further develop the tool and to evaluate risk for P losses.


  • Impact to Growers. Phase II will enable an economic comparison of sufficiency vs. build & maintain fertilization approaches during a ‘drawdown’ phase across various environments. This information will be used to update the official fertilizer recommendations for corn, soybean, and wheat. It will provide farmers with clear guidance on the optimal approach for managing P and K on a site-specific basis, which may also depend on soil health indicators.
  • Impact to Environment. If the results show that the build & maintain approach is superior to the sufficiency approach, technology transfer activities will emphasize the role of nutrient application timing and placement on the potential for environmental losses. Data will determine and validate the P-index through PLATO. By following best management practices with respect to timing and placement, some of the increased environmental impact of maintaining higher soil test levels can be mitigated by building up P and K and maintaining levels through sub-surface placement of starters.

Scientific Summary:

Phase One of the long-term project was established in 2010, 2011 or 2012 on 4 field sites to compare various starters (some starter treatments meet OMAFRA sufficiency recommendations) on a patchwork of various background levels of P and K, and to compare two fertilizer strategies: the sufficiency approach (current OMAFRA) vs. a build & maintain approach. Remarkable grain yields of all 3 crops were achieved with the build approach, to moderate background P and K (P<0.001). The high yields could not be achieved using any starter alone, even though some starters exceeded the OMAFRA sufficiency recommendation. This is strong evidence that the current OMAFRA recommendations are outdated. Due to the long-term nature of building soil tests over several years, crucial questions remain unanswered. Were yields responding to the “build phase”, where P and K may be more extractable with new fertilizer applications? Why did it take less P and K to increase soil tests by one ppm than was anticipated? These questions need to be addressed before a different P and K management strategies for the province are considered.

The proposed ‘Phase Two” is the next phase – a drawdown phase – which will enable an economic comparison of sufficiency and build & maintain fertilization approaches. It will provide farmers with clear guidance on the optimal approach for managing P and K. During the next phase, the only fertilizers applied will be in the starter treatments; background levels of P and K were established in Phase One. Very few datasets exist that examine the drawdown phase. Very few datasets exist where fertility responses consider the effects of soil health processes. Four field sites have been uniquely established for side-by-side comparisons. During the next 4 years, if the highest economic yields are still produced with moderate background levels of P and K, then the evidence will strongly favour the build & maintain approach, rather than simply being an artifact of the buildup phase in Phase One. This will have implications on site-specific management. Soil health influences on fertility responses will be investigated. Phase Two will also test the relationship between the amount of P and K needed to increase soil tests by one ppm (important for the economic analysis of building), test the new P-index tool (PLATO) for environmental sustainability, and update current data on crop removal and uptake. It should be noted that the opportunity to compare starters across a patchwork of various levels of background P and K in side-by-side plots is rare in North America.

External Funding Partners:

Fertilizer Canada