A nutrient analysis is a test of the working solution. Once a reservoir is filled with source water and charged with stock solution and pH adjuster, a sample is taken and tested. If you take the same test before the working solution is dumped (3 to 6 weeks after first charge), you get a clear snap shot of how balanced the nutrients is. Nutrient analysis test for:

Nitrate-Nitrogen, Ammonium-Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, Boron, Copper, Iron, Manganese, Zinc, Molybdenum, Sodium, and Aluminum, Nickel, Silicon, pH, Electrical Conductivity (EC)