The method works because the spectral reflectance of the leaves varies with the amount of chlorophyll they contain, and the amount of chlorophyll corresponds to the amount of nitrogen in the soil. If the spectral reflectance were high, indicating that the chlorophyll levels were low, the farmer would want to apply a nitrogen fertilizer.
例如當氮缺乏時，葉片的反射值會因葉綠素濃度降低而增加，此時就應增施氮肥。