How to measure NPK in fertilizer?

Measuring the NPK content in fertilizers is crucial for ensuring fertilizer quality. This is primarily achieved through targeted chemical analysis, which must be tailored to the production characteristics of NPK fertilizer granulators and other fertilizer equipment to ensure precise integration between testing and production processes.

Nitrogen content detection uses the Kjeldahl method, applicable to all types of fertilizers (covering both organic and inorganic nitrogen). The procedure involves first grinding the fertilizer sample evenly—especially granular fertilizers processed by NPK fertilizer granulators, which require crushing and mixing to avoid sampling bias. The sample is then digested with concentrated sulfuric acid and a catalyst to convert nitrogen into ammonia. After distillation and release, the ammonia is absorbed by acid, and finally, the content is calculated through titration. For field testing or preliminary screening, rapid nitrogen detection kits can be used. These kits use colorimetric reactions to compare concentrations, are easy to operate but have slightly lower accuracy, making them suitable for initial quality control after fertilizer machine production.

Phosphorus content detection commonly uses the molybdenum-antimony colorimetric method. This requires first converting phosphorus in the fertilizer into phosphate, which then reacts with molybdenum-antimony reagent to form a blue complex. The absorbance is measured using a spectrophotometer, and the content is determined by comparing with a standard curve.

Potassium content detection primarily uses flame photometry and atomic absorption spectrophotometry. Flame photometry is simple to operate and moderately costly. It calculates content by detecting the specific spectral intensity of potassium excited in a high-temperature flame, making it suitable for routine fertilizer testing. Atomic absorption spectrometry offers higher precision and is suitable for low-potassium content or complex substrate fertilizers.

For precise data (such as commercial fertilizer quality inspection), it is recommended to send samples to a third-party laboratory for testing using national standard methods. Growers or small fertilizer plants can use portable testing equipment to form a closed-loop quality control system with NPK fertilizer granulators and other production equipment to ensure that the NPK content of the fertilizer meets standards.