Filling the tray with water serves two purposes: it supplies moisture to the cube so that the roots can draw up more water and it adjusts the composition of the nutrient solution.
Roots draw nutrients selectively from the solution, so the composition of the solution changes. As water and needed nutrients are removed from the solution, it becomes harder for the plant to obtain them.
Not only are there fewer nutrients to draw, but the solution becomes more concentrated as water is also removed. As the concentration increases and electrical conductivity (EC) rises, it becomes harder for the plant to draw water because the plant uses the difference in electrical conductivity to pull water from the environment.
Flooding the cubes remoistens them. At the same time it removes the nutrient build-up and lowers the EC while refueling the cube with desired nutrients. This occurs as the liquid enters the cube.
The tray should be filled to its maximum flood level. As soon as the apex is reached, the tray can be drained, leaving fresh nutrient solution for the plants.
Readers with grow questions (or answers) should send them to Ed at: Ask Ed, PMB 147, 530 Divisadero St., San Francisco, California 94117, USA. You can also email Ed at [email protected], and send queries via his websites at www.ask-ed.net. All featured questions will be rewarded with a copy of Ed’s new book, Best of Ask Ed: Your Marijuana Questions Answered. Sorry, Ed cannot send personal replies to your questions.