tasty :icon_smile: I don't think your soil is too alkaline. Every soil or potting mix I've bought is right around 7 . . . just like yours . . . and I don't use anything to lower the soil pH - except the nute solution I'm feeding and the natural metabolism of the plants. Also, I'm using the same analog probe as you are to measure pH at the root level - it has it's quirks and is by no means ideal, but I don't know of a digital pen that you can stick down to the bottom of a 3 gallon pot in order to measure pH at the root level, so I use it . . . I recognize its quirks, and I make it work for me. Insert, make sure needle is moving from high to low, allow about a minute to settle, and read. If it appears to give a false reading, just reinsert it and try it again.
Within the first 24 hours after feeding, it's hard to get a good reading because the plants are feeding hard . . . you will notice the needle jumps all over the place, because there's a ton of anion-cation exchange taking place at the root level . . . and that's what "pH" is - a measurement of the hydrogen ion. I don't even bother pHing at this time . . . hey, I measured the pH of my solution with my digital pen before feeding, and I know it's right. After 24 hours, things settle down and you can measure quite accurately where the pH is down at the bottom of the pot with the probe. Now you can see whether the pH is rising or falling. The soil, being near neutral, pretty much adopts the pH of the solution you put into it . . . at first. The pH at the root level is, however, influenced by the initial pH of your solution, the concentration of dissolved nutrients in it, and how fast the plants are taking those nutrients up. As HP says, when plants are very young, they are eating slower and eating less, and the pH tends to rise between feedings. When they get into say third week of flowering, they are hungry and gobbling up the nutes, so we bump it up and give them a little more . . . and the pH tends to fall as the concentration of nutes in solution quickly falls.
jm2c :48:
It is something you will see and do battle with until you learn to go with the flow of it. It happens because it is a ratio of chemical density and ballance between acidic chems and alkaline chems (both of which are needed and used by the plants). EVERYTHING affects this ballance in hydro setups. To me and my experience, the best results for my plants is when I set my PH so that it drifts across the acceptable range over a period of several days, then I reset it again. 
