It's actually not that important what the exact pH is, as long as it's between 4 and 7. A pH of 6.8 is just what is usually aimed for. The following is from a pdf entitled "Nutrient Management in Recirculating Hydroponic Culture, Bruce Bugbee Crop Physiology Laboratory Utah State University".
"pH Monitoring and Control
Is pH control important? Most people assume pH control is essential, but there is considerable misunderstanding about the effect of pH on plant growth. Plants growequally well between pH 4 and 7, if nutrients do not become limiting. This is because the direct effects of pH on root growth are small, the problem is reduced nutrient availability at high and low pH. The recommended pH for hydroponic culture is between 5.5 to 5.8 because overall availability of nutrients is optimized at a slightly acid pH. The availabilities of Mn, Cu, Zn and especially Fe are reduced at higher pH, and there is a small decrease in availability of P, K, Ca, Mg at lower pH. Reduced availability means reduced nutrient uptake, but not necessarily nutrient deficiency.
How Important is Maintaining pH 5.8? We control the pH at 4 to study root respiration (to eliminate bicarbonate in solution). We compared growth at pH 4 and pH 5.8 with wheat and were not able to find a significant difference in root growth rate or root metabolism. We now routinely grow wheat crops at pH 4 during the entire life cycle. However, although there is usually a broad optimum pH, it is still best to maintain pH at about 5.8 to optimize nutrient availability. pH levels below 4 may start to reduce root growth, in one study our pH control solenoid failed Just after seed germination and the pH went to 2.5 for 48 hours. The roots turned brown and died, but new roots quickly grew back and the plants recovered."
