Here is my take on pH:
In soil, the pH needs to stay around 6.5 for best results.
If you are growing 100% organically, having the pH at 6.5 to 6.8 is VERY VERY important because not only are the necessary nutrients all available to your plants at that pH in soil, but microbial activity that is necessary for organic matter breakdown needs to be close to 7. A truly organic soil using only 100% organic soil amendments will always drift towards 7 or just below it because this is where the soil microbes like it best and if your soil is biologically healthy, then it will adjust itself due to the microbial activity. that is the pH they like and they will help to change and regulate it.
If you are using chemical nutrients such as my 3 part in soil, or fox farms for that matter, then your nutrient solution will most likely be acidic. When mixing my 3 part at 10 ml/gallon of Grow, BLOOM, Micro, the pH will roughly be about 5.5 depending on your water profile. The soil that I have used always has a pH of about 7.2 and when I feed, the pH drops to about 6.5 which is what I want so I don't use any pH UP.
Now, If you are feeding with chemical nutrients and they are causing your soil pH to drop too far from 6.5 for whatever reason (maybe your soil has a starting pH of 6.5 and adding a nutrient solution that is 5.5 causes it to drop to 6.0), then I would use pH up or Down to adjust your nutrient solution so that your soil environment will be at 6.5. IN true 100% organics this is rarely necessarry UNLESS, your soil has too much lime in it for example or your water is extremely hard for whatever reason has a high pH and is causing your pH of the soil to rise out of the acceptable range.
I know a very large grower in Oregon who's water out of the tap was 8.5. Her soil started at about a pH of 7.5 and she never adjusted the pH of her nute solution. After some time, the pH of her soil was close to 8.0 and she was having some serious micronutrient lockout problems. Getting a RO filter, a good flush, and adjusting the pH of her nutrient solution fixed all of her problems.
Basically, what I am trying to say is that you need to keep your soil pH at 6.5. If what you are feeding your plants is causing it to move too far from that number, then buffer your nutrients so that your soil will stay at 6.5
If you are growing organically, pH adjustment is rarely a problem due to microbial activity.
If your soil is shitty and started out with a pH of 8.0 for example, then you will most likely have issues of pH drift throughout your entire grow since there is something in your soil causing the pH to want to drift up. Same goes for soil that starts out with a pH that is too low. ESPECIALLY if you are using chemical nutrients. Who cares about microbial activity when using chemicals since the nutes are already all immediately available to the plants, right?
From a nutrient manufacturer's perspective, I also have to drop the pH of my nutrients to 4.0 or below to make them stable and to keep crap from growing in them. Even most 100% organic products have a very low ph to begin with. Take Alaska Fish Fertilizer. The only way that it is able to keep on store shelves is for the manufacturers to drop the pH below 4.0 which kills the enzymes used to digest the fish. They use phosphoric acid to drop the pH. If they did not do this, the bottles would explode on the shelves. Some products, even some organics, have a starting pH of between 2-4 and sometimes these need pH adjustment before feeding...sometimes.
Main thing is to keep the pH of SOIL at 6.5 and DO NOT let it drift too far from that value no matter what.
My 2 cents....hope this helps
