PH can make or break your nutrient solution. 6.7-6.2 is best to ensure there is no nutrient lock-up occurring. Hydroponics requires the solution to be PH corrected for the medium before exposing to the hemp plants. Phosphoresic acid can make the PH go down; lime or potash can take it up when it gets too acid. Buy a PH meter for $10 and use it in soil, water, and hydroponic medium to make sure your not going alkaline or acid over time. Most neutral mediums can use a little vinegar to make them just this side of 7 ph to 6.5 or so.
Most fertilizers cause a ph change in the soil. Adding fertilizer to the soil almost always results in a more acidic ph.
As time goes on, the amount of salts produced by the breakdown of fertilizers in the soil causes the soil to become increasingly acidic and eventually the concentration of these salts in the soil will stunt the plant and cause browning out of the foliage. Also, as the plant gets older its roots become less effective in bringing food to the leaves. To avoid the accumulation of these salts in your soil and to ensure that your plant is getting all of the food it needs you can begin leaf feeding your plant at the age of about 1.5 months. Dissolve the fertilizer in worm water and spray the mixture directly onto the foliage. The leaves absorb the fertilizer into their veins. If you want to continue to put fertilizer into the soil as well as leaf feeding, be sure not to overdose your hemp plants.