Although the principle of mass balance is well-understood, few people understand how Hoagland and Arnon used it to develop their famous nutrient solution recipes. Here I review: 1) the application of mass balance in deriving unique hydroponic solution recipes, 2) the dangers of dumping and replacing hydroponic solutions, 3) the need to alter the silicon and chloride concentrations in Hoagland's solution based on recent advances in our understanding of plant nutrient requirements.