I hear and read a great many comments about the need to adjust the pH of nutrient solutions we use on our plants, but for the most part, it is unnecessary. Let’s start by looking at pH, itself.
” pH”, or “potential of Hydrogen”, is really a measure of the relative amount of free hydrogen (H+) and hydroxyl (OH-) ions in the solution. If hydrogen ions outnumber the hydroxyls, the solution is said to be acidic, with a pH<7. If the opposite is true, it’s basic, pH>7, and if they are in equal concentration, the solution is said to be neutral, with a pH of 7. It is important that we keep the “relative amount” in mind.
If I compare a theoretical solution containing 300 H+ ions and 100 OH- ions with another containing 3,000,000 H+ ions and 1,000,000 OH- ions, they will have the same pH, as they both contain the same relative amounts of the ions, 3:1. As you might suspect, the second solution will be a much stronger acid than the first.
Fertilizers are mostly blends of inorganic mineral salts. Once they are dissolved in water, the molecules dissociate into hydrated ions. How they break apart is what determines the solution pH.
Calcium nitrate – Ca(NO3)2 – for example, breaks down into a single Ca++ ion and two NO3– ions. The Ca++ ion attracts two hydroxl (OH-) ions and the two NO3– ions each attract one H+ ion. Those combinations are in a dynamic equilibrium, that is, the Ca++ ion bonds with the two OH- ions at the same rate that the Ca(OH)2 dissociates. In chemistry class that would be represented as Ca++ + 2OH- ⇌ Ca(OH)2.
Because the size of the calcium ion is smaller than that of the nitrate ion and has twice the charge per ion, it “holds onto” the hydroxyl ions more strongly than does the nitrate and hydrogen ions, meaning the potential of those hydrogen ions interacting with things in there environment is stronger, and we end up with a slightly acidic solution. Fertilizers, being blends of many such inorganic salts, dissociate into many, many ions, each with it’s own plus or minus charge and ionic size. It is the sum of all of those that determine the solution pH.
The fertilizer solutions we use are very dilute – a matter of a few hundred parts-per-million. To give you some perspective on that, the mass of fertilizer in a 100 ppm N solution of K-Lite will be about 1/1300 of the total mass, but will only be approximately 1/11400 of the molecules.
So with that minuscule addition to the water, it makes it easier to see how little the pH of the fertilizer solution plays on the rhizosphere (root zone) pH, but there’s another, significant controlling factor – the plant itself.
The roots of a plant must maintain charge neutrality. If the pH surrounding the roots is more acidic (i.e., contains more H+ ions) than the fluids in the root, the plant will secrete OH– ions to compensate, and that raises the pH ion the rhizosphere. The converse is also true, secreting H+ ions to compensate for a basic environment.
The bottom line is that, unless you are really overdosing your fertilizer, you need not be concerned about the solution pH.