The reaction between phosphate and reagents causes a blue tint in the sample. ... These reagents are designed to be used with samples that have an expected range of of 0.0 to 15.0 mg/L phosphorus. These reagents do not measure reactive, acid hydrolyzable or total phosphorous.
Phosphorus is a critical plant nutrient and its key use by phosphate compounds is in the manufacture of fertilizers. Even like there are bio carbon and nitrogen cycles, there is also a phosphorus cycle. Phosphorus is used in the manufacture of safety matches (red phosphorus), pyrotechnics and inflammatory shells.