INDIRECT MEASURES OF PLANT NUTRIENTS

Indirect, non-destructive sensor-based methods of plant and soil analy ses could replace many of the wet chemistry testing methods that are i n place today. Over 140 years have past since Justus von Liebig first employed soil testing in 1850. Today, simultaneous analyses of moistur e, organic carbon (C), and total nitrogen (N) in plants and soils usin g non-destructive near infrared reflectance spectrophotometry are poss ible. Recent work has targeted indirect measurements of the nutrient s tatus in soils using spectral radiance data collected from growing cro p canopies. The use of spectral measurements from plant canopies has h een driven, in part, by newer variable rate technologies which apply n utrients to prescribed areas. More recent work has documented signific ant soil variabiliry on a 1 m(2) Scale. Because of this, indirect meas ures are necessary to avoid the cost of chemical analyses (10,000 samp les requiredper hectare) and to avoid on-the-go chemistry. Also, in or der 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 w heat (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 develop ed using the sufficiency concept that originally applied only to immob ile nutrients. Plant canopy sensing methods allow for sufficiency to b e used for both immobile and mobile nutrients since intensity and capa city can be integrated into one component, total nutrient uptake.