The left chart shows how much of each nutrient plants can take up at different levels of soil acidity. Phosphorus becomes available at 6.0, but normally begins peaking at 6.5-7.5. But the different minerals containing phosphorus have their own properties and break down in different ways. The hydroxyapatite/Ca5OH(PO4)3 breaks down super easily in more acidic soil, but then stops being available at the normal target pH for phosphorus. Since the soil contains a ratio of that and iron(III) phosphate, there's a point in the pH scale where both break down at maximum efficiency. It's 1 lower than normal. You might have to supply additional phosphorus if the soil is more alkaline and the hydroxyapatite becomes unavailable.