In the summer or in hot grow rooms, plants, in general, will take up more water than nutrients, thus causing the nutrient solution to become more salty.
In the winter time or in cooler grow rooms, the opposite will occur. Nutrient uptake will also be determined by the type of crop being grown e.g., tomatoes are heavier feeders than lettuce. It is extremely important that the grower has both a TDS meter and a pH meter and that regular testing on the nutrient solution is carried out. If the grower notices after a few days that the ppm level in the reservoir is high and the water level has decreased than the grower should top up their reservoir with either plain water or a weak nutrient solution until the optimum ppm level is reached. If the grower has noticed a drop in ppm levels then a full strength nutrient solution should be used to top off the reservoir.
Another factor to consider is the source water. You will generally find that if you are not using reverse osmosis water, you will usually have to top-off with plain water, since tap water has a lot of sodium and minerals that increase the ppm levels.
Here is an ideal scenario:
Purchase a Reverse Osmosis System, Auto Shut-off Kit and some R.O. Tubing, which can be found in the Water Treatment section of our website, and a ¼” Grommet and a ¼” Float Valve, which can be found in the Plumbing section. Also purchase a Rubbermaid trash can and a couple of cinder blocks from your local hardware store. Hook up the R.O. system and shut-off kit according to the instruction manuals. The float valve that comes with the shut-off kit should be installed in the trash can, which should be placed on the cinder blocks for elevation. Drill a hole close to the bottom of the trash can and insert the grommet. Install the second float valve in your reservoir, a little higher than where you want the water level to be. Then, run a length of R.O. tubing from the grommet to the float valve.
Now, turn on your R.O. system and go spend the time you’re going to save doing something fun!
After a period of time, both the reservoir and the trash can will be full, and the R.O. system will stop. It will only come on when the levels in either receptacle begin to fall. Once you add nutrients and enhancers to the reservoir, you will find that the PPM level actually drops each day as the plants take up nutrients and the water is replenished through the float valve (this is especially visible with healthy, actively growing plants). You will also find that you use far less pH adjusting solutions due to the improved water quality. You should only have to add small amounts of nutrients and pH adjusting solution every once and a while between reservoir changes. And, you will always have plenty of pure, fresh water available in the trash can.