10 months late, it is the amount of Phosphorus in Phosphorus Oxide. Which means per 1lb of Phosphorus Oxide, there is 0.437 lbs of Phosphorus. Calculation is done by stoichiometry. Phosphorus Oxide is 142 lb/lbmol and Phosphorus is 31 lb/lbmol since there is 2 lbmol P in P2O5 you multiply that by 2. 142/62 = 0.437 This is important for researchers so they can calculate the appropriate amount of phosphorus needed for their fertilizers. Excess phosphorus can lead to oversaturation in the soil making it hard for other nutrient absorption, this can lead into stunting of plant growth. I'm sure there are more reasons (eutrophication like the one talked on this vid).