Many folks test the pH of their fertilizer solutions and adjust them to be in the 5.5-6.5 range, because “that optimizes the nutrient’s availability and the plants’ abilities to take them up”. Unfortunately, that is not strictly correct.
The article about pH & Nutrient Availability explains the fallacy of the availability claim, and while pH certainly can affect uptake dynamics, that has much more to do with the root environment than the applied solutions. Let’s start with some basics.
pH is a measure of the concentration of free H+ ions in a solution, but can be thought of as a ratio. If the H+ and OH– ion concentrations are equal, there are no “free” H+ ions and the pH is 7, neutral. An excess in H+ results in a low, acidic pH and an excess on OH– gives a high, basic pH. As it is a ratio, if one solution had 100 H+ ions and 1 OH– ion, and another solution had 100,000,000,000 H+ ions and 1,000,000,000 OH– ions, the pH of the solutions would be identical, at 6.0. Not too surprizingly, the first solution is very weak, so won’t have an appreciable effect on plants and the pH balance can be affected by almost anything. That second solution, on the other hand, is much stronger, so can affect the plant and will be much more resistant to pH change.
When we dissolve fertilizers in water, the inorganic salts dissociate, then hydrolyze, forming equilibrium reactions with the water. Calcium nitrate, for example, can be expressed as:
Ca(NO3)2 + 2 H2O ⇋ (Ca++ + 2OH–) + 2(NO3– + H+)
As there are two H+ ions and two OH– ions “tied up” in that reaction, that is a 1:1 ratio and the resulting solution is neutral, at pH=7. However, with a complex mixture of such mineral salts in fertilizers (K-Lite, for example, has nine), each hydrolyzing into its own components, there is usually an imbalance, resulting in a pH that is either acidic or basic. Any dissolved solids in your water supply will also affect that. The story doesn’t end there, though!
Plants take up nutrients as individual ions. When it comes to orchids, the nitrogen – the most significant nutrient – is primarily in the forms of nitrate (NO3–) and/or ammonium (NH4+) ions. As a plant absorbs an ion, it must secrete and equally-charged one to maintain neutrality. If the plant absorbs a nitrate ion, a compensatory negative ion is secreted into the substrate, which pushes the pH upward, making it more basic (pH>7). Conversely, the absoption of an ammonium ion resuts in the secretion of a positive ion, pushing the substrate toward being more acidic (pH<7).
With all that under our belts, we can now look at the interaction of the fertilizer solutions and substrate with the plant.
First and foremost, understand that the solutions we apply to our plants are very dilute. A 100 ppm N K-Lite solution has 0.77 grams of powder dissolved on 1 kilogram of water. That’s a molar ratio of only 1 part fertilizer to 8800 parts water!
When using inert potting media, the plant itself controls the pH, moving it up or down over time, depending upon the fertilizer formula. Many organic media, such as bark or sphagnum, tend to be slightly acidic, while untreated rock wool, made from melted basalt, is basic, and that’s where the interactions get interesting.
The use of nitrate-based fertilizer will cause the pH of the substrate to increase over time. If your water is high in alkalinity (the resistance to pH change upon the addition of an acid), that increase will be accelerated. Ammoniacal nitrogen has the opposite effecct, lowering the pH. Either one, left uncorrected, will lead to toxicity in the long term, but I recently ran into a short-term issue.
The rock wool I use in my mixes is made from melted basalt, so is a bit alkline. I remedy that by pretreating it with an acid before use. I recently experimented with rock wool plus just enough perlite to keep the cubes spaced out and airy – but I neglected the acid wash, resulting in an alkaline substrate having excess negative ions within it. As I used K-Lite with 98% nitrate-based nitrogen, the plant struggled to secrete more negative ions into the substrate, so its uptake was stifled and I started to see yellowing, likely due to a nitrogen deficiency. I switched to K-Lite Plus, which has 15% ammoniacal nitrogen instead of 2%, and even though the pH was unchanged, the plant started greening up in a matter of a few weeks.