Indirect, non-destructive sensor-based methods of plant and soil analyses could replace many of the wet chemistry testing methods that are in place today. Over 140 years have past since Justus von Liebig first employed soil testing in 1850. Today, simultaneous analyses of moisture, organic carbon (C), and total nitrogen (N) in plants and soils using non-destructive near infrared reflectance spectrophotometry are possible. Recent work has targeted indirect measurements of the nutrient status in soils using spectral radiance data collected from growing crop canopies. The use of spectral measurements from plant canopies has heen driven, in part, by newer variable rate technologies which apply nutrients to prescribed areas. More recent work has documented significant soil variabiliry on a 1 m(2) Scale. Because of this, indirect measures are necessary to avoid the cost of chemical analyses (10,000 samples requiredper hectare) and to avoid on-the-go chemistry. Also, in order for application technologies to be environmentally sensitive, they must treat the resolution where real differences. exist in the field. Present state-of-the-art methods can sense N deficiencies in winter wheat (December - February) on a 1 m(2) scale and apply variable foliar N on-the-go at 15 kph. These indirect methods rely on indices developed using the sufficiency concept that originally applied only to immobile nutrients. Plant canopy sensing methods allow for sufficiency to be used for both immobile and mobile nutrients since intensity and capacity can be integrated into one component, total nutrient uptake.
