Colby, Kansas | flatlick farmer - 1/13/2026 16:58
"It's the concentration of P in the soil (ppm) that is most tightly correlated to crop response"
Can you expand on what you are talking about? My understanding is that lbs per acre and PPM are a reflection of the same number just read differently and will end up with the same concentration. I 100% agree with the fields of ours with the highest P readings tend to be our highest yielding fields and lowest P readings tend to be our lowest yielding fields. It tends to be difficult to build though.
I think the best place to start is to recall that plant uptake mechanisms of N and P are different. Recall that nitrogen is taken up into the plant by mass flow. Nitrogen, being water soluble, is dissolved in the soil water and taken up by the roots in the water by mass flow. Nitrogen also moves fairly freely within the soil when sufficient soil water is present. Contrast this to phosphorus where plant uptake occurs via interception by the roots, across the root surface into the plant. Most of this occurs at the youngest tissue of the root tips. Diffusion then resupplies solution P to the soil bordering the root surface. What do you have to have to drive diffusion? A concentration gradient. What does ppm measure? Concentration.
I think where the confusion works is when people think about N and P the same way. You are correct in that you can calculate lbs/ac from PPM and using an assumption on density of the soil. This is not a bad approach for nitrogen, because it really is a quantity, that is a quantity of N that can be taken up by the plant and our N recommendations are largely driven by some target quantity of N based on yield goal.
What soil test P is telling you is what is the concentration in the soil and is it sufficient for the soil to resupply P to the surface of the root as P is absorbed by the plant. This is driven by concentration, not quantity. Volumes of data show that the critical value (the soil test level above which a response to applied P is highly unlikely) doesn't vary by yield level. E.g. our critical value of approx. 20 ppm Bray 1 or Mehlich 3 is the critical value for 80 bu dryland corn or 280 bu irrigated corn.
One final comment, when a P soil test is converted from ppm to lb/ac, that number does not in any way represent how many lbs of P are available for uptake or removal, it is much more complicated because of soil buffering and all the transformations that occur in the soil. That's why I say it's a very misleading representation.
To the comment about soil test P and yields. What you have observed can certainly ring true if P is a yield limiting factor. In my experience I have often seen the inverse, you have areas of a field that are consistently low yielding for some non P related issue and you end up with high soil test P, because blanket rates have been applied based on field average and lower removal in those areas. Conversely, I have seen low soil test P in areas that are consistently high yielding as crop removal has outpaced blanket rate applications based on field average. There is quite possibly something at play with spatial variability the P buffering capacity of soils, e.g. soil in a particular area of a field can supply P either better or worse than expected to the plant for some reason.
Hopefully this is useful,
Lucas
Edited by LHaag 1/13/2026 19:43
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Formula for converting p1 to available phosphorus?
