Phosphate Soil Sample Results

There are several methods to test for soil phosphorus levels. Each has its own advantages and disadvantages. Soil may have a much higher amount of phosphorus present as a total amount than the amount available to the plant. Most soils in fact have a much larger proportion of the soil phosphorus tied up in unavailable forms such as aluminum phosphate in the case of acid soils and calcium phosphates in the case of alkaline calcarious soils. Since phosphorus is such an important element for good crop yields, its important to know the true phosphate levels to plan your fertilizer program. Comparing results from different testing methods can be confusing because each phosphorus testing method has its own scale. The numbers that a lab can return can vary widely, even with soils with the same true available phosphate levels since the scales vary with testing method. Its important to know which method is used and why when soil testing and interpreting the results. Below are some of the most common phosphorus testing methods and then a comparison table of the different method's results. The method your lab uses may not be the best for your situation. Labs may use outdated methods since if they change methods users may have difficulty interpreting results and comparing results to past tests with a different result scale. Many labs will do a different method if you request one.

Mehlich -2

This is a double acid method used since 1978. It can be used on a wide range of soil types. It can adjust for the high levels of excess lime found in some western soils. It has a large scale that goes up to high numbers soil it can accurately assess between high and very high soil phosphorus levels.

Mehlich - 3

This test was developed in 1984 as an improvement to the Mehlich - 2 method. This method also works over a large range of soils and soil pH's. Its results have the same scale as the Mehlich - 2 method. It is currently the approved method by the Soil Science Society of America. It is a very good all around test.

Bray - 1

This test was developed in 1945. It is also referred as the "Weak Bray" method. The extractable Phosphorus is strongly related to the aluminum phosphate in the soil. It is best adapted to soils with a pH of 6.8 or less and textures of silty clay loam or finer. This test does not work well with high lime soils and high CEC soils. It also has a fairly wide scale to allow good separation of results.

Bray - 2

This test was developed in 1945 as well and is referred as the "Strong Bray" method. The difference is this test uses higher levels of acid to extract soil phosphorus. It usually results in higher test result numbers than the weak bray results. This method is very good in determining soil P levels if rock phosphate is used as a fertilizer source and the soil is acidic.. It is not a good method in neutral to alkaline soils and calcarious soils since it will over estimate the soil available phosphorus levels.

Sodium Bicarbonate or Olsen method

This method was developed in 1954 and uses the alkaline sodium bicarbonate to extract soil phosphorus. This method is best in alkaline soils with medium to high CEC levels and free lime or calcarious soils. It is a common test in the western U.S. This method has a small result scale and does not at time accurately distinguish between high and very high P levels.

Ammonium bicarbonate - DTPA

This method was developed in 1977. The phosphorus results are strongly related to soil calcium phosphate levels. This method is recommended on alkaline calcarious soils. The result scale is about ½ of the sodium bicarbonate results and as such the scale is limiting to distinguish between slight differences in soil phosphorus.

Comparison Table of Soil Test Phosphorus Result Values

Relative Level Mehlich 2 & 3 Bray - 1 at pH<7 & 0% CaCO3 Olsen or Sodium Bicarbonate Ammonium Bicarbonate - DTPA
Very Low 2 5 1 <1
4 7 2 <1
6 9 3 <1
8 11 4 1
10 13 5 1
Low 12 15 6 1
14 17 7 2
16 19 8 2
18 21 9 3
20 23 10 3
Medium 25 28 13 5
30 33 15 6
35 38 18 7
40 43 21 8
High 45 48 23 9
50 53 26 10
55 58 29 12
60 63 31 13

Note: This information should only be used as a guide. Adjustments for local conditions must always be made.

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