Investigations were carried out with the view to getting a more balanced nutrient supply in hydroponic systems and better plant resistance against influence of stress factors. Organic and biological agents based on humic and lactic acids were added to the nutrient solution. They were expected to improve the resistance of tomato plants particularly under stress conditions. Stress factors were chosen that are of major concern in hydroponic systems: suboptimal pH values and suboptimal electric conductivity (EC).
Humic acid was applied in the form of sodium humate, and lactic acid was applied as lactate (LACTOFOL). The latter has been designed as a suspension leaf fertilizer based on waste material from the dairy industry. Tomato test plants were cultivated in perlite substrate mostly until they had eleven or twelve leaves, and in one of the experiments they were investigated until they produced fruit.
The following general conclusions have been drawn: