Colby, Kansas | easymoney - 1/13/2026 18:01
I like a lot of what I'm seeing in this thread
A phosphorus soil test has nothing to do with available phosphorus it's basically a test of The acid chose in the lab.
I think this statement is partially true. No, it is not a measure of an available quantity, but it is a measure of that soil's ability to supply adequate P to the plant, and I think that's what actually matters. Acid choice is important and users should be choosing a test that is appropriate and well calibrated for their soils. For example, folks in western Kansas with high pH soils should not be using the Bray 1, Haney H3A, and in extreme cases even the Mehlich 3 for soil test P. But to say it's a test of the acid is a bit disingenuous. Have to choose the correct tool for the job.
It's a functions test, not an available test. It tells us how the soil behaves with that acid. It has nothing to do with availability to the plant
If this was true, there would be no correlation with soil test P to yield, plant response, and response to applied P fertilizer
I always like to say your P1 Bray is a wonderful function indicator of your soil. If you have to add phosphorus to maintain your P1 your soil is not cycling nutrients. Your soil is not functioning biologically.
If you have to add phosphorus to maintain P1, that means you are raising crops and removing P at a rate that exceeds the soils ability to resupply the solution P pool out of plant unavailable forms. i.e. you are mining soil P. This gets into a whole different matter, I know there is a lot of interest in what can be done biologically to make unavailable P into a plant available form. Lots of folks looking at total P in the soil and how do we access that, and being able to successfully do that would certainly open up management options. But we need to be honest, if we are removing P in excess of application and find a way to do that by unlocking the unavailable P through biology, we are still mining, we've just changed the timescale we're doing it over. There is no free lunch.
A soil test is great for finding a major deficiency. There's no correlation between a soil test and a crop uptake. There's some probability of a response But nothing correlating to what the plant is actually going to take up.
This is true as long as you are above the critical value. Corn grown on 25 ppm and 300 ppm is going to have relatively minor differences in P uptake. But I assure you there is significant differences in P uptake in corn grown on a 8 ppm vs. a 25 ppm.
Best,
Lucas
Edited by LHaag 1/13/2026 19:44
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Formula for converting p1 to available phosphorus?
