Calculation of phosphorus and potassium to add as fertilizer.

Asked March 14, 2020, 12:03 AM EDT

Based upon em9179 and our analysis (attached below), I may want to add 25 ppm phosphorus and 75 ppm potassium for my blueberries to get to 25-45 ppm and 100-150 ppm, respectively. 1) Does one calculate the desired amount of chemical based upon the square footage of the area x 6 inches depth, i.e., root zone? 2) Should this be divided among multiple applications? 3) should one account for leaching? (Our soil drains very rapidly.)

Tillamook County Oregon

1 Response

Soils differ in phosphorus and potassium buffering capacity, so it’s best to consider P or K results of a soil test as an index. You can’t treat soil P or K inputs vs. soil test value like a checkbook: put so much in and you have so much to take out (in the crop). Let me recommend these publications from our neighbors to the north:

A Home Gardener’s Guide to Soils and Fertilizers (PDF-Online). This guide gives detailed information on soil structure, fertility, and irrigation in your garden. Instructions on the appropriate use of both organic and synthetic fertilizers are provided, as well as information on creating and using compost. Cogger, C. 2014. WSU Extension EM063E.

Garden Fertilizer Calculator (PDF with embedded Excel xlsx-Online). A companion publication to “A Home Gardener’s Guide to Soils and Fertilizers.” Cogger, C. 2019. WSU Extension, US Dept. Agriculture. FS324E

To answer your questions directly:

  1. Yes, ppm includes the plow depth - about 6 inches.
  2. No, I would apply all at once. Phosphorus and potassium are both very low in your soil. P especially takes time to become plant available.
  3. Phosphorus is very slowly soluble, so unless a flood carries soil away, I would not think about leaching. It's been estimated that phosphorus moves in soil about the thickness of your thumbnail per year.
  4. Potassium is more soluble than phosphorus by far but is not nearly as leachable as nitrogen.

Because you have a number of essential plant nutrients at very low levels, I recommend that you re-run the soil test with the same laboratory at the same time next year, if you can afford to do so. Ask whether they have an agronomist who can make recommendations, based on your soil test results.