tissue test comes in, you should have found P Low to Deficient with a tissue test in that corn at tassle and I think you would have

Plant Phosphate is kind of a different duck. It has a steep increase in yield with added P but it also has a short optimum range before yield starts to fall off with added phosphate,
Unless you also add other essential elements that are low or marginal. Potassium is usually needed for increased yield with added phosphate. A Grad Student in Indiana said so and it proved true for me with my Texas Clay Soils..