NO3-N. This is a soil fertility test. This method involves the extraction of nitrate from soils using 40 g (0.02 N) CaSO4 solution. Nitrate is determined by reaction with chromotropic acid to form an azochromophore (yellow dye) which is measured spectrophotometrically at 420 nm. Soil nitrate-nitrogen can be used to predict plant response to nitrogen fertilizers. The method detection limit is approximately 1.0 mg/kg and is generally reproducible ± 6%.