I'm certainly no expert on this but I used to work for a food processing company and we used refractometers all the time to test the sugar, salt, sodium hydroxide, or sulfur dioxide concentration of the solutions used in the various phases of food processing. Based on what I know, the sugar content of most plants, including green plants will swamp out any useful reading you might get as to other nutritional fractions such as minerals. A refractometer alone can't tell you what element is causing the change in density and thus the refractive index of the liquid you are measuring. Only a chemical analysis will tell you how much of what elements are in the liquid.
Fruit growers also use refractometers to determine when the sugar content of the fruit is high enough to begin picking operations. My brother in-law is a citrus rancher and he carries a refractometer with him in the late fall to check when the sugar content of the oranges is high enough to send to market. The sugar content of most fruits increases as they ripen so if all you used was a refractometer you would come to the conclusion that there was a significantly higher nutritional value in the riper fruit. The truth is that only the sugar content changed.
