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“Master gardener math”— it’s about managing fertilizer

To be fair perhaps I should say “gardener math,” as hopefully the Master Gardeners who assist homeowners in their garden preparations are already trained. So the question comes in, “How do I know what fertilizer to put on for my garden (read here also as corn crop, turf, etc. as you need).”

The first question I ask a homeowner, farmer, or golf course manager is, “what’s your soil test?” With homeowners I know to be a little careful. Because fertilizer is sold in 40 to 50 pound bags, and it’s relatively cheap, they can sometimes put on a little more than necessary. This drives the soil test results up, and may cause some strange relationships.


So here is the most recent scenario in question:


Analysis Result Optimal (by the labs recommendations)
Soil pH 7.2 6.0-6.5
Buffer pH
Organic Matter 6.9%
CEC 23.2
Phosphorus m3-ppm 337 40-60
Potassium   m3-ppm 185 220-330
Magnesium m3-ppm 907 290-470
Calcium       m3-ppm 4086 3100-4300


I would love to have this as my farm soil, but I think it has been amended a bit too much. Here are some key points to consider:

  • The pH at 7.2 may be a bit high, and probably got there by applying the whole bag of lime rather than the needed two to four pounds. For buffer pH, since the water pH is above 7, then we get no reading here, typically we would only apply lime at pH below 6.5.
  • Organic matter, yes more is always good, but again we can see that extra material was applied, as usually our native soil organic matter is 2% to 4%. This does make for a more forgiving soil, with improved water holding capacity for plant needs, than our farm soils.
  • CEC is probably higher than expected due to the organic matter. Soil organic matter can supply nutrient holding (minus charges) similar to our clays.
  • Phosphorus is shown as ppm (parts per million) by the Mehlich III test. That’s OK, I just need to convert it to something I can understand — that’s the Bray P1 test that Ohio State University used to calibrate fertilizer activity in Ohio soils. For that we multiply M3 test value times 0.832 and subtract 8.08 to equal Pray P1 in ppm.  (M3 test 337 ppm times 0.832) – 8.08 = 272 ppm Bray P1

•     With a Bray P1 of 272 ppm, we far exceed any agronomic crop need for soil phosphorus. Again, a gardener bought a bag of fertilizer and then used it all. But at this level we have a risk of water soluble phosphorus movement, and that 272 ppm P is subject to loss or movement at a level of perhaps eight to10 times more than is likely at the desired agronomic level of 15 to 40 ppm.

  • Potassium is also a little high. The Mehlich III test for potassium is OK and correlates well with the ammonium acetate test we use for field calibration studies. With a CEC of 23, our K soil test should be in the range of 125 to 155.
  • For the Magnesium and Calcium, both can come with our lime application. With this high level of magnesium, I think the gardener used “high Mag” lime. Some folks swear by high Mag lime, but in this case they may have slightly upset the systems with a bit too much magnesium. If they ever apply lime again they should apply high Cal lime.
  •  Also noted on the test recommendation notes for this soil is the suggestion that they apply sulfur. Perhaps the lab wishes to drive the pH down, I generally am opposed to this, but if you were growing a crop that prefers a lower pH, such as potato or blueberry, then you might apply sulfur.


For those of you reading this and thinking this can only happen on a small area or in a garden situation, last week I was handed a soil test from a grain farm with a phosphorus level of 373 ppm Mehlich III. When I asked how the levels got so high, I was told that it “used to be a sheep pasture.” I made some quick calculations as to how long before the levels would drop to a “normal” agronomic level.

  • Average crop removal for corn is 60 or so pounds per acre.
  • Generally we reduce our soil test P level by about 1 ppm for each 20 pounds P2O5 removed. So we may reduce our soil test level by about 3 ppm for each corn crop we grow.
  • So if my math is correct, and if we grow continuous corn, it will take 87 years to lower our soil test P in ppm Bray P1 down to the agronomic level we would like to see. As always, soil test regularly and you will see this occur and know when to start applying phosphorus again. As above at 300 ppm Bray P1, we leak water-soluble phosphorus.

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