Evaluation of sensing technologies for on-the-go detection of macro-nutrients in cultivated soils

The ever-increasing prices of fertilizer and growing ecological concern over chemical run-off into sources of drinking water have brought the issues of precision agriculture and site-specific management to the forefront of present day technological development within agriculture and ecology. Due to increases in the cost of fertilizer production inputs—predominantly nitrate (N), phosphate (P), and potassium (K) those in agriculture are looking for ways to optimize plant yield while minimizing the application and consumption of fertilizer. Since these macro-nutrients vary even on a small scale throughout a cultivated field, numerous researchers have attempted to develop an on-the-go sensing apparatus that is capable of mapping the presence of these chemicals in situ so that this map, once overlaid with parameters such as pH, electrical conductivity (EC), crop yield, and mechanical properties of the soil, can give a precise spatially varying prescription for fertilizer application. In this paper current macro-nutrient assessment technologies are reviewed and categorized by target compound (nitrogen, phosphorus, and potassium (NPK)) and by sensing approach (e.g., chemical, electrical, optical) with an eye towards the potential to create an integrated sensor capable of detecting the three macro-nutrients of primary interest. Emphasis is placed on technologies that require minimal sample preparation, limited sampling time, little maintenance and infrequent calibration. Sensors based on near- and mid-infrared reflectance, Raman spectroscopy, electrochemical methods (ISE, ISFET), biological processes and wet chemistry are explored. These technologies are reviewed with a framework that incorporates quantitative measures of performance, with qualitative indicators of technology fieldability and economics to put forward a recommendation for sensing techniques that may warrant further attention in efforts to develop an on-the-go, in situ NPK sensing system.