Patent US 4,178,716

The present invention relates to a method for operating a nutrient film plant growth technique so as to minimise the build up of one chemical in the circulating solution without a corresponding build-up of other chemicals, notably nutrient chemicals, therein. For example the feed of nutrient chemicals is in proportion to the feed of water to the solution, or the concentration of non-nutrients is maintained at a low level or discounted when assessing the amount of nutrient to add to the solution to maintain the desired nutrient level.