P is phosphorus, Not Potassium HayWilson.  (Edit: I think you know that but you mentioned K first)  The negative correlation to high P levels is negative to mychrorriza not to the plant.  It just means the plant can get the P directly and bypass the mychrorriza in the soil.  Without energy from plant photosynthisis the mychrorriza can not function.  Brassicas photosynthisize but do not interact with this fungus.   This reminds me of rhizobia bacteria that only interact with alfalfa.