Alfalfa Soil Test: Olson Phosphorus

So you pulled some soil cores and now you have the results in your hand, now what? On your soil test results you will want to check out pH, potassium, phosphorus, and sulfur. Today we will focus on phosphorus recommendations, specifically for the Olson test results. 

Phosphorus has three tests that can be completed to test soil P levels: Bray-1, Olson, and Mehlich-3 are the most widely used. These are measured in parts per million (ppm) and recommendations are dependent on dryland and irrigated fields. Values will differ between Bray-1/Mehlich-3 and Olson test results, so carefully look at your soil test before making fertilizer purchases. If your soil tests are greater than 14 for Olson, you do not need to add any phosphorus for irrigated or dryland acres. 

  • 0-3 apply 60 lbs. P2O5/acre for irrigated or 40 lbs. P2O5/acre dryland.
  • 4-7 apply 40 lbs. P2O5/acre for irrigated or 30 lbs. P2O5/acre for dryland. 
  • 8-14 apply 30 lbs. P2O5/acre for irrigated or 20 lbs. P2O5/acre for dryland.

These values can be found online on the CropWatch website under the alfalfa section and include the values for Bray-1 and Mehlich-3. Also depending on your fertilization schedule, you can plan to apply phosphorus in two year increments for dryland fields; take the single year recommendations and double to calculate the two year needs.

Remember if you are still wanting to pull soil cores sample at 8 inches or historic depth, collect samples by grid, soil type, or representative area (40 acres or less). Then pull 10 to 15 random soil cores and combine in a plastic bucket to represent one soil sample. Take about a pint of soil and submit to an accredited lab.

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