Some water soluable fertilizers have a dye added to distinguish between a fertilizer mix and plain water.

Some of the dyes break down with sunlight making it questionable for use as an indicator of the amount of fertilizer in the nutrient mix.

Norm

Thanks Norm for that information. I'm going to try an experiment. Going to fill a 5-gallon bucket with nutes and expose it to sunlight, over time the color should fade out. I'll post the results in a few days.

Well here are the results of the experiment:

09/06/05 10:28 AM:

About 1600 PPM HG 5-11-26 fertilizer with CN and MG.

09/08/05 3:36 PM:

Hard to tell if there's a change. The different lighting conditions didn't help.

09/09/05 5:44 PM:

Looks definitely lighter. Had to fill the bucket with plain water to compensate for evaporation. I notice algae starting to form.

09/11/05 9:01 AM:

Green tint caused by algae but blue color still evident though much lighter that the first day.

There appear be credence to the statement that sunlight causes the blue color to fade. However, the nutrient solution in my drip system hardly gets any direct sunlight yet the blue color goes away by the 5th day for most of my plants. So, my original query and now statement that using the color of the nutrient as an indicator for nutrient change out is still valid with the caveat that exposure to sunlight in a substantial way can also cause the color to fade.