This sounds like a micro nute difficiency, like sulphur. Other micros nutes might create this appearance, but I'm pretty sure if sulphur is missing, you get a top that is very yellow. You might be able to find a sulphur fert additive you can give, but just go easy with it. In the future use a more complete fert, one that has sulphur, boron, maganese, etc. The Dynagrow brand is a good one to use, works in both hydro and soil. There are others too, just read the labels. The powder ferts like foxfarm's and dr. earth are probably about as complete as you need. You can take a look at those too. One cup of these ferts for every 3 gallons of soil should do it.