The SPAD-502 chlorophyll meter measures the transmittance of leaves at 430 nm~ which is a peak of maximum absorbance by chl, providing an indication of its chl content. Chlorophyll content of leaves or SPAD readings can be used as a nitrogen nutrition index of plants since most leafN is contained in chl molecules. In order to avoid the interference of factors besides N fertility on the interpretation of results, Peterson et al. (1993) suggest the use a N sufficiency index calculated as follows: Sufficiency index = (Average bullk readings)/(,4verage reference strip readings) x 100 %. The reference strips must be well supplied with N. A sufficiency index lower than 95 % indicates a shortage of N. In this work we study the relation between SPAD readings and laboratory measurements of chl pigments on potato and sorghum as a function of N applied. We inquire into some aspects of the use of the N sufficiency index related to the reference strips.