[GpsFrontier]

I was wondering if there was any type of chart or guide that shows nutrient requirements for individual plants, vegetables, etc.

Well yes, but I am not sure if it is what you are thinking of. Hydroponic nutrients are made up of many elements and micro elements. There is not any way to tell exactly how much of one particular element is in a solution that I know of. But there are charts that can tell you what strength a particular plant needs in regards to TDS/PPM and pH. pH is the acidity of the solution. Where TDS and PPM can tell you how strong the solution is, but only in a total amount. Not how much of each element is in it. There are charts that can tell you how much PPM of each element a plant needs. That is only useful if you find a hydroponic nutrient that lists each and gives that measurement, or if you manufacture your own nutrients.

So if things are good , topping off seems to work , then changing the solution just because it's been 2 weeks wouldn't be needed ?

As the plants grow they pick the nutrients they need out of the solution. The problem is they only take the ones they need and leave the rest, this creates an imbalanced nutrient solution. by adding more nutrients to the solution you can bring back up the amount of nutrients that were used, the problem here is that you are adding all the element that the plants did not use also. This creates an imbalanced nutrient solution going the other way. Even if you could tell exactly what nutrients were being used (and there are charts that can tell you which these are for any particular plant), if you cant just replace the used ones you will have an imbalanced nutrient solution.

By dumping out the old nutrients and replacing them with fresh ones you can insure that they are balanced again. If these nutrients go too far out of balance the plant will wind up with nutrient lockout (kind of like going into shock), and you can have a perfect nutrient solution but the plant wont be able to absorb any of them. With plants that are heavy feeders they use the nutrients they need faster than plants that are not. So the nutrient solution will go out of balance faster also. This is where the buffer comes into play.

Say you have 4 plants feeding off one nutrient reservoir and it has a 5 gallon reservoir. Now let say you have another identical system except it has 10 gallons in the reservoir. The plants in both systems are taking the same amount of each individual elements, but the elements will not be as far out of balance as fast in the system with 10 gallons in the reservoir because of the extra 5 gallons of nutrient solution. This works the same for plants that are not heavy feeders, it's just more important to the ones that are.

Now I've read from a few places that say distilled water makes sense in a small grow situation. Surprised to hear you say this. Perhaps spring water ?

Spring water would be fine but just expensive over time. To make distilled water they evaporate it, and collect the evaporation. Because the elements don't evaporate, the evaporated and collected water is free of the elements. This water wont leave water spots because there is nothing in it to leave behind. This process also takes out all the micro elements the plants need because they don't evaporate. Spring water is filtered water and has not been evaporated. Most filters are not able to take out the micro elements because they are too small and will go right through the filters. Unless you live in the mountains or get your tap water from a well that has been filtered, tap water is usually not the best to use. I use water from the revers osmosis (RO) myself. It's highly filtered water for drinking. The RO water may even be a little too filtered. It was recommended by General Hydroponics I use 90% RO water and 10% hard water from outside (not from the water softener).

P.S. I have found that the best information on using hydroponic nutrients is from the manufactures themselves. Just either e-mail or call them, tell them what plants you are growing, what stage they are in, and ask what they would recommend. Most hydroponic nutrient manufactures want you to be successful, so they will be happy to help. After all, if you are successful you are much more likely to continue to buy their products.