Plant growth depends on water for a number of reasons, especially in a hydroponic garden. Water is the thing that drives the plant transpiration cycle. Without water, plants would not be able to uptake nutrients or transport nutrients within their tissue. Plant transpiration is discussed more in the temperature and growth page.
Water vapor also makes up the humidity in a grow room. These are both important concepts to understand. Since they have been covered in the other plant growth sections, we will spend our time here discussing the quality of the water you use to make up your nutrient solution.
Water is a carrier and transporter of nutrients, both TO your plants as well as WITHIN your plants. While H20 facilitates many plant growth processes, and despite the fact that many of these processes cannot take place without the presence of H20, most of the water itself reacts very little with the nutrient molecules in it. Of course, that is only true if you are using distilled water or water that has been passed through a reverse osmosis filter.
The biggest thing to avoid is using well water that smells like eggs (too much Sulfur) or that leaves rust colored stains in the sink or toilet (too much Iron). Always test your water before using it in a hydroponic system!
While water itself will not cause nutrient uptake problems, the dissolved salts and minerals also present in your water just might. distilled and reverse osmosis water should show a TDS of zero ppm on a properly calibrated TDS meter. As this number increases, the possibility of a nutrient uptake problem also increases. So here is what you can do to avoid any problems...
Get a glass of tap water and check it with a calibrated TDS meter. If the TDS of your water straight from the tap is greater than 200 ppm you definitely should use distilled or reverse osmosis water to avoid problems. 150 ppm should be usable, as long as no more than 50 ppm of that total comes from sodium compounds.
The majority of the dissolved solids in municipal (city) tap water come from Calcium Carbonate. Plus, sodium levels in general are not usually high enough in municipal water supplies to cause a problem. While living in Cleveland Ohio, I used the tap water regularly for several years with no problems.
To finish making my point, 100 ppm of Calcium Carbonate in your water is basically like 100 ppm of nutrients in your water that your plants can never use. Every single major hydroponic nutrient manufacturer will tell you the same thing...to get the maximum plant growth from their recommended feeding instructions, the water you begin with needs to have a very low TDS or EC reading (preferably zero).
Let me take a shot at this one...I believe plants will take up the calcium carbonate before they will take up other forms of calcium that play a vital role in the plant's ability to uptake and use some of the other nutrients efficiently. This could cause some nutrients to be used faster than others, in which case signs of a nutrient imbalance and/or nutrient deficiency would soon follow. This is a nutrition mess best avoided altogether!