Soil (Soil pH Adjustment ①)

I originally bought a moisture meter to time my watering, but it happened to be a high-performance model that also included features for measuring light intensity (lux), root temperature, and pH.
Regarding Soil ①, as I wrote, my blended soil was supposed to be slightly acidic with a pH of 5.2 to 6.2, but when I measured the soil in the pots, it was leaning alkaline at 7.5 to 8.8.

▶️ Why does soil designed to be slightly acidic lean alkaline?

My blended soil is roughly composed as follows, and if I organize it along with the pH of each material, it looks like this.

The estimated pH calculated by AI was 5.2 to 6.2, so it was designed to be slightly acidic... or so it should have been.
However, as mentioned at the beginning, whether I insert the measuring device into the soil in the pot or perform a slurry measurement, it shows an alkaline reading of 7.5 to 8.8.

Why??

By the way, the pH of my tap water at home is 7.0, which is neutral, butin plant cultivation, even 'pH 7.0 = neutral',it seems to be interpreted as already somewhat high (leaning slightly alkaline).

In the end, I thoroughly investigated each material to find out who the culprit causing the shift to alkaline was.

● Kanuma/Akadama:
　・Acidic but not strongly buffered
　・Gradually pushed up if hard water or nitrate fertilizers are used continuously.

● Zeolite/Silicate clay:
　・Strong buffering on the alkaline side

Zeolite and silicate clay were one of the causes...

▶️ The cause was not just the soil materials

While investigating whether there were other causes, I found that there were other reasons besides the strong power of zeolite and silicate clay to push back against acid.
To summarize...

<Zeolite>
　・An aluminum silicate mineral with a very high Cation Exchange Capacity (CEC).
　・It firmly retains Ca²⁺, Mg²⁺, and Na⁺, and does not easily release them by exchanging with H⁺.
　・Even if acid is added, it has the power to immediately release Ca or Na to neutralize it.
　　→　Zeolite acts as a 'buffer that cancels out weak acids'.

<Silicate clay>
　・A clay mineral that is originallyslightly alkaline to neutral.
　・Slowly elutes Ca and Mg, and has the power tobring the soil toward pH 6.5–7.5.
　・The pushing-back force is stronger than the acidic contribution of Kanuma soil or Akadama.

<Tap water (hard water components)>
　・In many regions, pH is 7.0 or higher, and hardness is high (contains Ca²⁺, Mg²⁺, HCO₃⁻).
　・When watering is repeated,calcium carbonate (CaCO₃) precipitation accumulates in the soil.
　・The acidity of Kanuma and Akadama cannot counter this.

<Nitrate N in liquid fertilizer>
　・Hyponex and My Garden liquid fertilizers are mainly nitrate N.
　・When plants absorb nitrate,they release OH⁻ (hydroxide ions) from the roots,
　　alkalizing the surroundings.
　・If repeated over a long period, soil pHgradually leans toward alkaline.

It's an alkalization that happened because it was bound to happen.

▶️ Summary

The soil blend was theoretically slightly acidic, but as I used it, the measured value shifted toward alkaline. When I looked into it, it made sense, but it was a pitfall.

I don't intend to revise the soil blend for now... partly because I have a large stock of already blended soil, and dealing with what is already planted is a higher priority, so I will review that later.

I cannot change the pH of the tap water, and I have to provide liquid fertilizer, so I researched various ways to handle this.
I will cover that in a separate article.