There's an excess of Phosphorus in soil, the plants consume it, then the primary consumers consume multiple plants, and have a higher level of phosphorus concentration in them. Then secondary consumers consume multiple primary consumers and the phosphorus from all the things they ate are concentrated in them, and so on. Basically, even though as energy is passed along the food chain, it lessens with each trophic level, nutrients become more concentrated and the highest trophic level has the highest concentration because of how many organisms were consumed.