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New and updated recommendations for phosphorus fertilizer
Author: Nathan Slaton, Professor & Director of Soil Testing

By Nathan Slaton, Professor and Trenton Roberts, Assistant Professor – Crop, Soil and Environmental Sciences

Phosphorus fertilizer recommendations for rice and soybean were revised in December 2015 based on soil-test information and rice and soybean yield response results from nearly 150 phosphorus (P) fertilization trials.  The old and new soil-test level definitions are shown in Table 1. The primary change was that the soil-test levels (Very Low, Low, Medium, and Optimum) were revised by adjusting the soil-test P values that define each level’s lower and upper boundary. The critical soil-test P concentration that triggers a recommendation to apply P fertilizer was essentially lowered from 35 ppm to 25 ppm.  Minor adjustments were also made to the recommended fertilizer rates within each level.

About 20-25% of the rice and soybean acreage in Arkansas has soil-test P concentrations that range from 26 to 35 ppm and will no longer receive a recommendation for P fertilizer.  The research results suggest that P fertilization increases rice and soybean yields nominally and on less than 5% of the soils having soil-test P between 26 and 35 ppm.

The revised recommendations were implemented on soil test reports beginning in late December 2015 and influence only the P recommendations for rice and soybean.  Phosphorus recommendations for corn, cotton, grain sorghum, and wheat were not changed.  Remember that the soil-test based recommendations for rice and soybean are based on soil samples collected to a 4-inch depth.  Collecting soil samples from the 0-6 inch depth will usually result in slightly lower soil-test P and K concentrations as compared to samples collected from the 0-4 inch depth.

  • How can I use old soil test reports (taken prior to 2016) with new soil-test levels and P fertilizer application rates?
    Soil-test reports for rice or soybean received prior to Mid-December, 2015 will contain the old soil test levels and associated P fertilizer application rates. If you submitted samples during December and want to know if the recommendations were new, please look at the soil-test category and soil-test P concentration on your report and compare to Table 1 below. To use old soil-test reports in conjunction with the revised changes, please refer to Table 1.
  • Will lowering my P application rate or the number of acres where I apply P fertilizer influence my rice or soybean yield?
    The newly revised soil-test P boundaries and P fertilizer application rates are meant to increase the likelihood of only applying P fertilizer where there is a high probability of a significant yield response to P fertilization. By altering the soil-test boundaries and slightly lowering the associated P application rates we are increasing the chances of you receiving a positive return on you P fertilizer investment.
  • How will these changes influence my P fertilization program?
    These changes are targeted at increasing the accuracy of our soil-test-based P recommendations for rice and soybean. Overall, these changes will most likely mean that you will be applying P fertilizer to a smaller number of acres annually and that your total P fertilizer application will be less.

    • Step 1- Locate the soil-test P concentration in ppm from your old soil test report found in Section 1. Nutrient Availability Index.
    • Step 2- Using the soil-test P concentration in ppm, identify your soil NEW soil-test level under the column heading “New Soil-test boundaries”
    • Step 3- Using this soil-test level, determine your recommended P rate by following your new soil test level row to the right. For rice you will also need your soil pH, which can be found in Section 2 of your soil-test report entitled “Soil Properties”.
  • Will my soil-test P levels start to decline since I am adding less P fertilizer?
    There is a chance that with the newly revised soil-test levels and lower associated P fertilizer application rates that soil-test P levels on your farm could decrease over time. However, by maintaining a thorough set of records on soil-test nutrient levels over time and maintaining a quality soil sampling and analysis program you should not see any yield reductions.
  • Can I keep using my old P fertilizer rates?
    Yes, but please be aware that these changes in soil-test P levels and P fertilization rates for rice and soybean were made to reflect the soil-test P concentrations and rates where you will most likely see a significant yield response to P fertilization. The new changes to soil-test P levels and P fertilization rates are meant to ensure that both soybean and rice yields are maximized in regards to P fertilization, but that producer profitability and return on P fertilizer investment are optimized.

This research was funded by grants from the Soybean Check-off Program administered by the Arkansas Soybean Research and Promotion Board, Rice Check-off Program administered by the Arkansas Rice Research and Promotion Board and the University of Arkansas System Division of Agriculture.

Please feel free to contact us with any questions that you may have.

Table 1. The revised soil-test level definitions for rice and soybean with the recommended P fertilizer rates.Screen Shot 2016-01-26 at 9.57.30 AM

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