Good questions axe. Ryegrass does not produce a thatch layer and I do not know if Decomposers in the sumagreen are out compeating the good guys? Clippings recycled yes. P is most available in the warm soil temps......But the P in my sandy loam soils is on the low side. What other Mineral element has such an effect on top growth besids N or P? Irrigation water is on the hard side so enough buffer there to bind some of the soluable P. Perhaps the Humic Acid in the Sumagreen is too high and knocks off too much bound Ca, Mg, P at this time?