Let's get this all back to the start and the correspondence that InTheShed shared:
"At one point he said this:
"It is the potential acidity or basicity of the fertilizer chemistry and the alkalinity content of your water that affects the pH of the growing medium. For the fertilizers, it is called ‘potential’ since it is determined by the chemistry and the quantity of fertilizer nutrients that are applied and the ‘potential’ they have to interact with the plant root system and influence the pH of the growing medium up or down."
All related to the medium. And the interesting use of the word "potential," but again ended it by talking about moving the pH of the growing medium.
Rather than bore you with the all back and forth, I will post this summary that I sent him in one of our last emails. He approved of this summary (italics mine):
1. Ideal pH range for mineral soil is 6.0-6.5. Soil-less growing media, such as PRO-MIX, have an ideal pH range of 5.5-6.0.
2. However, pH of nutrient water is irrelevant to the pH of any soil or growing media. It is the alkalinity of nutrient water and the potential acidity/basicity of the fertilizer(s) that influence the pH of the growing medium and root zone. For example, if the alkalinity of nutrient water is moderate or high, pH of growing medium will rise over time.
a. Plant roots are electrically charged and must maintain a neutral balance.
b. For ammonium nitrogen (NH4) fertilizers, plants release of hydrogen ions to take up NH4. Hydrogen released is essentially acid and this drives pH down.
c. For nitrate (NO3) form of nitrogen in fertilizers, plant exchange hydroxyl ions for NO3 uptake, which causes growing medium pH to rise.
d. Alkalinity (CACO3) is essentially dissolved limestone. The higher the alkalinity of water, the greater tendency to raise pH of growing medium over time.
3. It is more important to keep track of the pH of the growing medium than the pH of the nutrient solution we feed the plants.
And at the bottom of that summary I added one last direct question:
"If I’m growing in ProMix HP and I mix up the nutrient solution and it reads 7.4 pH, it is not necessary for me to adjust that number down using phosphoric acid or the like. I can pour it into the pot at 7.4 and my plants will be able to uptake those nutrients?"
His response was a direct "Yes.""
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