Earl, I was thinking that maybe with longer time off the nute solution dries on the roots thereby creating a higher ppm temporarily on the roots until the next spray. This might allow you to run low ppm level but still get the effect of a higher ppm. I mean think about all the nutes people have been flushing down the drain. If you could just do a low level of nutes, and use that; it would be more efficient(cheaper) than making a high ppm solution, and only using the top end of that ppm mixture only to throw the rest away. I dunno definitely not an expert just thinking this could be the case. Would explain nute burn at such low levels(of course I don't mix your nutes).