Agronomic Thresholds for Soil Phosphorus in Alberta: A Review

 
 
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 Executive Summary

This review was carried out at the request of the Soil Phosphorus Limits Steering Committee to identify agronomic thresholds of soil phosphorus for Alberta. This review is part of a study to identify limits for soil phosphorus limits of agricultural land in Alberta.

Phosphorus (P) is a key element in both crop production and environmental sustainability. While good agronomic management requires use of P to optimize crop growth, excessive P may degrade water quality. A major difficulty in managing soil P is knowing what levels of soil P are optimum for crop growth and what levels present an unacceptable risk to water quality. A key tool for managing crops for optimum production is soil testing; however, to achieve a balance between optimum crop production and environmental sustainability, a better understanding of how soil tests are interpreted is needed. Increased care in applying P from commercial fertilizer and manure sources will be required to achieve the desired balance between optimum crop productivity and sustainable water quality.

The Soil Phosphorus Limits Steering Committee requested the study address three areas; review the basis for soil test P recommendations in Alberta, to review the agronomic thresholds for P that apply to different crops and soil types in Alberta, and to review the implications for the application of non-commercial nutrient sources. The Soil Phosphorus Limits Steering Committee agreed that “basis for soil test P recommendations in Alberta” was to be interpreted to mean the basis for P fertilizer recommendations in Alberta. In this report, an agronomic threshold of soil P is defined as the soil P level, as determined by soil test P analysis, beyond which there is no practical economic or crop yield response to added P from either commercial (inorganic) fertilizer or organic fertilizer sources.

The review examined more than 200 references from the literature and provided an analysis of Alberta soil P data for four crops over several soil types.

Basis for P fertilizer recommendations in Alberta:

  1. Conventional soil tests for P are used to determine the amount of supplemental P required to prevent loss of crop value from P deficiency.
  2. Several philosophies can be used to interpret soil test results to make fertilizer recommendations; in Alberta the philosophy is to recommend application of only those nutrients lacking in the soil.
  3. There is no standard method for determining soil test P levels.
  4. The two versions of the Modified Kelowna extraction are most commonly used for determining soil test P levels in Alberta. One version should be chosen as the standard test.
  5. Because the processes governing crop uptake of P from the soil are different from those governing P enrichment of water, care must be taken when using soil test P data for assessing environmental risk, a purpose for which it was not intended.
Agronomic thresholds for soil P in Alberta:
  1. A moderate buildup of soil P levels is beneficial for good crop production; however, soil P must be managed carefully to prevent degradation of water quality.
  2. Based on data from field trials across the major soil groups of Alberta, an agronomic threshold of 60 ppm in the top 0.15 m of soil, based on a Modified Kelowna extraction, would provide the most flexibility for crop production should a single threshold for all crops and soils in Alberta be required.
  3. Agronomic thresholds reported in other areas are less than 60 ppm.
  4. gronomic thresholds are identified by selecting an acceptable amount of yield reduction below maximum levels. Because the relationship between yield response and soil test P levels is not linear, determining how to set the yield threshold can have a large effect on the STP value for the threshold.
  5. The relationship between soil test P and crop yield may need to be re-examined to identify the impact of lower soil test P levels on crop production if environmental limits have similar or lower values than the agronomic thresholds.
Implications for application of P from organic sources:
  1. Long-term application of P in commercial fertilizer or manure has resulted in accumulation of P in agricultural soils.
  2. There is no evidence to indicate that soil test P results from manured soils should be interpreted differently than test results from non-manured soils.
  3. In the early years of manure application, P forms in manure change rapidly from more mobile to more stable forms in the soil. The amount of mobile P forms that are stabilized decreases with time, suggesting that risk to water quality by P from manured lands is probably lower in the early years of manure application and increases with long term repeated applications in excess of crop uptake.
  4. More research is needed to understand how to manage crops to achieve both environmental and economic sustainability when manure is used as a nutrient source.
Acknowledgements

Numerous people and agencies have contributed to this report. I thank the following people for their ideas, data, review comments, assistance with the report production, and insights from their experience and knowledge:

Mohamed Amrani, Alberta Agriculture, Food and Rural Development
John Ashworth, Enviro-Test Labs
Rod Bennett, Alberta Agriculture, Food and Rural Development
Bjorn Berg, Alberta Agriculture, Food and Rural Development
Barb Clarke, Alberta Agriculture, Food and Rural Development
Don Flaten, University of Manitoba
Adrian Johnston, Potash and Phosphate Institute
Doug Keyes, Norwest Labs
Colin McKenzie, Alberta Agriculture, Food and Rural Development
Ross McKenzie, Alberta Agriculture, Food and Rural Development
David Neilson, Alberta Agriculture, Food and Rural Development
Wes Nuttall, Agriculture and Agri-Food Canada, Saskatoon
Barry Olson, Alberta Agriculture, Food and Rural Development
Jeff Schoenau, University of Saskatchewan
Gerald Stark, Alberta Agriculture, Food and Rural Development

I also thank the Conservation and Development Branch of Alberta Agriculture, Food and Rural Development and the Soil Phosphorus Limits Steering Committee for their direction and support for this project.
 
 
 
 

Other Documents in the Series

 
  Agronomic Thresholds for Soil Phosphorus in Alberta: A Review - Current Document
Agronomic Thresholds for Soil Phosphorus in Alberta: Introduction
Agronomic Thresholds for Soil Phosphorus in Alberta: Basis for Fertilizer Recommendations in Alberta
Agronomic Thresholds for Soil Phosphorus in Alberta: Implications for Management of Organic P Sources
Agronomic Thresholds for Soil Phosphorus in Alberta: Conclusions
Agronomic Thresholds for Soil Phosphorus in Alberta: References
Agronomic Thresholds for Soil Phosphorus in Alberta: Appendix - Calculated Crop Response to Add for Alberta Soils
 
 
 
 
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For more information about the content of this document, contact Ralph Wright.
This document is maintained by Laura Thygesen.
This information published to the web on June 23, 2003.
Last Reviewed/Revised on January 16, 2015.