Keys to P results on a soil test; developing recommendations

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One thing that has become very clear working with understanding soil test results over the past 18 months is when it comes to phosphorus we (universities, laboratories and practitioners) have made it very complicated. There are a variety of explanations that can be given for the different reporting but the bottom line is the collective “we” need and can do better. The discussion on environmental impacts will lead to more standardized soil test result reporting that will benefit agriculture for meeting both production and environmental goals. But until those discussions occur, there are a few key questions to ask when looking at phosphorus on your soil test results.

What is the reported extractant?

There are two primary extractants used for phosphorus in Ohio, the Bray P1 and Mehlich III. Either extractant works well for our soils to generate a soil test level. But we need to remind ourselves that the only reason to do a soil test is to be able to correlate yield response to the number on the soil test report. The Tri-state Fertilizer Recommendations is developed from a Bray P1 soil test result. Fortunately there is a relationship between Mehlich III and Bray P1 and a conversion can be made. An excellent discussion on this can be found in the factsheet Understanding Soil Test for Plant Available Phosphorus. As a user of soil test results, you need to know whether the reported number is a Bray P1 or a Mehlich III result so you can appropriately use the number in relation to Tri-state Fertilizer Recommendations. It is not always clear on report forms. When in doubt call the laboratory.

What are the units used for the Phosphorus level result?

Another confusion point is the units of reporting. Check the report for pounds per acre or parts per million (ppm) which is also sometimes expressed as mg/kg on some reports. The Tri-state Fertilizer Recommendations charts provide recommendations for either of the units. The reader of the report just needs to know what the units are to get to the correct point on the chart. The conversion to ppm from pounds per acre is pounds per acre divided by 2.

What is the form of phosphorus reported?

For the most part soil test results are reported as the elemental P, expressed as P. This form is used for the Tri-state Fertilizer Recommendations chart and no conversion is needed. The other form that appears on an occasional report is oxidized form of phosphorus expressed P2O5. If both forms are on the report, just use the number expressed as P. If the only number is the P2O5 number, then it needs to be converted to P to use the Tri-state Recommendations. Just multiply P2O5 by 0.44 to get to the P number.

Tools to develop fertilizer recommendations.

Answering these three questions from the soil test report makes selecting the correct tools to get the fertilizer recommendation an easy task. If you are more comfortable with paper then download Developing Phosphorus and Potassium Recommendations for Field Crops. If soil test results are in Bray 1 Units then you can use the charts in the Tri-State Fertilizer Guide for the recommendation found here.

If you have results in Mehlich III results, then supplemental sheets with both Bray P1 and Mehlich III have been developed to avoid the need for conversion between the two tests. If units are reported as parts per million use If units from your lab are pound per acre use this table

If you would like to use a spreadsheet then check out Version 2013-4 of P, K and Lime Recommendation Spreadsheet released October, 2013.  If you have downloaded previous version please replace with this version that works with older versions of excel and correct errors in the previous releases. This can be found here. Once again check the introduction page of the spreadsheet and make sure it has version 2013-4 and 10/2013 as the release date.

Finally if you are using precision applications the equations used for the printed tables are found in the Tri-State Fertilizer Guide on page 11.

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