Something to consider.


Negative correlation to high P

I expect the speaker has as his soil, one that is in the 10 -20 CEC range. That his soils are not calcareous with excessive levels of calcium as well as free calcium carbonate.

I say this because in a high CEC, High Shrink Vertisol Clay, potassium becomes trapped between the plates making up the clay. This is beyond the amount of potassium that has an ionic attraction with the clay particles.
This probably will distort findings for the more typical sandy loam soils.


Then we have the free lime of the calcareous soils quickly reacting with the phosphate to create rock phsophates. In a high pH soil these rock phosphates will be stable and not available to the soil biological activity.



Then 10 or so years ago a MS candidate at Perdue showed that adding higher levels of phosphate to the soil has a limited if not negative alfalfa yield response. To prevent this additional phosphate cancels out this negative yield response. This was published in the proceedings of AFGC's annual Conference held in Louisiana.

Question:
Brassica are non-host of myc fungi.
What is the significance of this statement. Reading the slick literature on Brassica ( Turnips & Rasishes ? )
suggest that the Brassica go out and find and take in numerous if not al essential elements, and when allowed to die in place these elements are easily available to the following crop(s).
Possibly suggesting that the Brassica greatly reduce the need for additional fertilizers. In fact suggesting a free lunch.