High alkalinity will buffer your soil through build up, and cause pH to rise through the neutralization of acids; however, since your runoff is more acidic than your feed, we can fairly assume that acids are not being neutralized.
Drops in pH to runoff are often caused by excess salts, but there are other reasons as well; do the slurry test. Sulfur becomes less available in the lower pH range as well.
Do that foliar now, sulfur deficiency can take a while to recover from, and you won't see much growth till it's corrected. :/