I believe what they are saying is its not a good idea to actually mix the nutes together before mixing to water, i.e. you have a gallon of water and your going to use what 2-4 tbspns of one and like 2-4 teaspoons of another nute just mix the nute is the water one at a time do not mix the nutes together then mix into water as you could possibly get fumes or some sort of undesired chem reaction..