Don't understand this... Studies in the early 90's showed that mycorrhizal infection increased phosphorus uptake by the host plant. (turf i suppose) Read some other articles that boast twice the root structure when adding VAM. vesicular arbuscular mycorrhizea Unfortunately, it's only when the phosphorus levels are at 50ppm or less.(2.5lbs/k) But not much less. No big deal, but they can't be too high either. Anywhere near or over 4lb/k would prohibit VAM from working properly. ??? Since this doesn't really matter for cool season grasses, what about you warm season grass guys. Do you watch your P levels when adding (VAM imparticular?) Just curious cause the stuff is really expensive. Is there that big a difference or just stick to humic and seaweed extracts?