An optical sensor for analysis of soil nutrients by using LED light sources

A compact optical sensor based on photometric detection of soil nutrients is developed. The sensor consists of three light emitting diodes (LEDs) and a simple data I/O circuit using a PIC (peripheral interface controller) device. The wavelength of LEDs is chosen to fit the absorption band of chemical reagents whose colour develops by reaction with soil nutrients. The sensor is applied for detection of six soil nutrients: ammonia nitrogen (NH4–N), nitrate nitrogen (NO3–N), available phosphorus (P2O5), available iron (Fe), exchangeable manganese (Mn) and exchangeable calcium (CaO) from colour changes caused by addition of chemical reagent in a transparent plastic cell. The results are compared with those calculated using absorbance of solutions measured by spectrophotometer and good agreement is observed. The resolution of 1.0–20 mg/100 g has been performed for standard solutions of soil nutrients in a cell of 5.5 mm in path length. The sensor is also applied to an analysis of soil nutrients of various farmlands, and the results are compared with those obtained by a colour chart judgment.