Agronomic Thresholds for Soil Phosphorus in Alberta: Conclusions

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 Basis for fertilizer P recommendations | Agronomic thresholds for P in Alberta | Implications for application of P from organic sources

Basis for Fertilizer P Recommendations

Soil test P results ultimately form the basis for crop response and fertilizer recommendations. The most sophisticated soil fertility interpretations result from a large database of field research on specific crop responses and STP values for a variety of soil and climatic conditions. There are also several philosophies upon which fertilizer recommendations for P can be based. In Alberta, the philosophy is to only recommend application of nutrients lacking in the soil, as opposed to intentionally building up a nutrient reserve.

There is no standard method of analysis for STP. While the Modified Kelowna method is the preferred technique in the Canadian prairies, there are two versions in commercial use. It may be desirable to specify one version of the Modified Kelowna as the standard test for the agronomic threshold. Other soil P test methods can be used if the results can be compared with confidence to the standard test.

The resin membrane probe is used in situ and measures a P supply rate or flux during the growing season. The probe simulates plant uptake of P better than the conventional soil P tests. This probe is now in commercial use as a tool for soil P availability assessment and crop P requirements. It has not been used extensively with manured soils; however, it may be the future method of choice for soil P testing.

Because phosphorus (P) is a key element in both crop production and environmental sustainability, the industry must maintain a careful balance between optimum production and environmental sustainability. Monitoring soil P levels will be a key tool for assessing environmental risk Soil P tests are a practical, proven method to assess soil P levels, however they were designed to measure crop nutrient status and fertilizer requirement, not risk to water quality. The processes governing crop uptake of P from the soil are different from those governing P enrichment of water, therefore, care must be taken when using soil test P data for assessing environmental risk.

Agronomic Thresholds for P in Alberta

A moderate buildup of soil P levels is part of good crop management. There are both economic and production advantages to building up STP levels, and the optimum levels increase with increased yield potential and land tenure. Therefore, long-term optimum crop production will improve with higher STP levels. Risk to water quality also increases with increasing STP levels. Managers must maintain a balance of soil P that is high enough to provide optimum crop production but low enough to maintain environmental quality. Determining the levels of soil P that define the balance is a major challenge.

The Alberta data show that an agronomic threshold of 60 mg kg-1 for all soils and major crops would provide the most flexibility for crop production in Alberta. Exceptions, such as potato in calcareous soils in southern Alberta where the agronomic threshold is about 70 mg kg-1, should be addressed on a crop and site-specific basis.

Evidence of agronomic thresholds and crop removal rates in the literature suggest that the thresholds for annual and forage crops could be lower. Agronomic limits are based on yield response curves. There is no standard way of determining an agronomic threshold of soil P for a crop. Crop yield response to P is specific to crop type and to local conditions such as climate and soil type. Therefore, agronomic limits can be expected to have some variability.

The role of excess soil P in reducing water quality has placed a new emphasis on the importance of agronomic thresholds in nutrient management. More research is needed to expand the Alberta database of crop response to STP and to look at other methods to interpret the data for identifying agronomic thresholds.

Implications for Application of P from Organic Sources

Long-term application of commercial fertilizer or manure has gradually increased STP levels, whereas a single application of manure, at either light or heavy rates, has little effect on the labile forms of P in the soil.

In early years of manure application, the relatively high levels of soluble and labile P in manure are rapidly fixed by the soil. This rate of P adsorption by the soil decreases with repeated manure additions due to a cumulative effect that saturates fixation sites with P. Under long-term repeated applications, there is an increase in the percentage of soluble and labile forms of P, which can more readily move into runoff and groundwater. This suggests that the risk to water quality by P is less likely to occur in the early years of manure application, but that the risk will increase from long-term repeated manure applications where P is applied in excess of crop removal.

Commercial laboratories do not generally differentiate between manure P and commercial fertilizer P in the soil when recommending P rates for crops. There is no evidence to suggest that STP results from manured soils should be interpreted differently than STP results from non-manured soils.

The proportion of nutrients in organic sources is not matched to crop requirements and the P forms in organic sources are generally mineralized more slowly than the P forms in commercial fertilizer P. Research is needed to develop ways to manage cropping and soils to ensure that P supply does not exceed crop demand when organic P sources are applied to land.


Other Documents in the Series

  Agronomic Thresholds for Soil Phosphorus in Alberta: A Review
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 - Current Document
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 24, 2003.
Last Reviewed/Revised on January 16, 2015.