Refractometers measure the refraction of light (or the amount that light rays are bent) as it travels through a liquid. It is not a sugar reading unless the substance you are testing is a solution of sugar (sucrose) in water. In fact, you can actually get brix readings for substances that have no sugar in them at all.

 There are four factors that affect the reading of refractometers:

• The quantity of atoms dissolved in the liquid
• The atomic weight of the elements involved
• The number of covalent bonds in the combinations of the atoms, (such as found in sugars and amino acids)
•  The temperature (this can be eliminated as a factor by choosing a model that is automatically temperature compensating)

 For reference, pure water has a reading of 0 brix since, by definition, there should be no substances dissolved in the water. If you were to compare two solutions made of the same compound, and the first solution had more of that compound dissolved in it than the second solution, it would have a higher reading. If for example, the first solution had 10% sucrose dissolved in it and the second had 5%, the readings would be 10.0 brix and 5.0 brix, respectively. If you wanted to compare two solutions containing an equal amount of two different substances, for example a solution of table salt compared to a solution of sugar, the sugar solution would have a higher brix reading, due primarily to the fact that it is comprised of molecules with covalent bonds (shared electrons) as opposed to the simple ionic bonds found in salt.

It can get very scientific, but to simplify how this relates to plants: if your plants have watery cells, they will have lower brix readings and not be as healthy as higher brix plants. If simple ions (such as the nitrate ion) are taken up into the plant cells instead of substances being brought into the cells and then combined into more complex substances such as sugar (by the process known as photosynthesis), the plants will have lower brix readings and will be vulnerable to diseases and insects.