Nearly all of us are familiar with pH as a method to quantify fluids to ascertain if they are acidic or basic. It is common knowledge the scale goes from 1 to 14 with 1 to 6 being acidic, 7 neutral and 8 to 14 basic. The pH shows the concentration of hydrogen ions, H+, within the liquid. So why is the topic of pH so basic whenever we discus live organic soils? The reason is that pH impacts what kinds of microbes live in the soil. Different microbes promote or suppress nitrification along with other organic behavior which impact the way plants develop and grow. Bacteria will increase the pH while fungus lowers it. The soil pH is effected by microbes more than the microbe is affected by the soil.
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Preferred pH levels of Different Plants

As we all know, every plant has it’s preference for a certain pH level. But ideal pH for any plant, has more to do with its preference for a specific bacteria and/or fungus  than it does with the biochemistry of pH. This is not to say certain nutrient up-takes are not effected by pH. Refer to the chart on the right.(click to enlarge) It outlines which elements are available at what pH. This is standard biochemistry. So there are two points to consider here… plant microbe preference and elemental behavior at specific pH levels. In general, woody forest plants such as trees and bushes have fungal symbiosis. Fungus thrives in low, acidic pH. So you will find acidic soils in the forests. In contrast, soft vegetative plants, such as our precious herbs, have a symbiotic relationship with bacterias. Bacterias thrive in their low, basic pH environ. So would you say herbs prefer a low pH or would it be more descriptive to say veggies grow better in a highly bacterial soil?

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Hydrogen is a Cation

The hydrogen cation is used as an exchange currency for other cations in the exchange. When you have a great deal of hydrogen ions, the pH is minimal and the liquid is said to be acidic. In a similar fashion, when you have few hydrogen ions in the liquid, it is said to have a high pH, which is alkaline or basic. I have always wondered why a pH is LOW when it has HIGH amounts of Hydrogen. This is because when calculating the pH mathematically, a negative logarithm is used. (see formula below)
We are herb growers and therefore really don’t need to be experts and learn that much more about pH. However we do need to know that each time an herb plant’s root tip interchanges a H+ cation for a nutrient cation, the amount of hydrogen ions within the liquid will increase. Because the concentration of H+ cations increases, the pH decreases, which makes the substrate progressively more acidic as nutrient up-take increases. But the pH many times,  balances out since roots (click on image right) also take up negatively charged anions. Just as plant roots use H+ as an exchange currency for cation exchanges, they use hydroxy, OH- for an anion exchange currency. More OH-, in your solution increases the pH since it reduces the percentage of H+ cations. Amazingly, fungi and bacteria are little enough to receive and shed cations and anions on their surface area, electrolytically retaining or expelling mineral nutrients from decomposition in the soil. This, also, has an influence on the pH. So with so many variables effecting the balance, being aware of the substrate’s pH is helpful in choosing what you need to add to the substrate to compliment particular kinds of microorganisms, or simply knowing what could be missing in certain pH ranges. Remember, nutrients are effected by pH availability. (yellow and green chart above – click to enlarge)
At the same time, substrate and/or soil pH is important concerning CEC because as pH increases, becoming less acidic, the amount of negative charges on the colloids increase, which increases the CEC. (cation exchange capacity)

Alkaline Alkalinity & pH

Don’t confuse the word “Alkaline” with the word  “Alkalinity”. Alkaline and alkalinity are two different measurable parameters of water. A pH below 7 is said to be acidic. A pH above 7 is said to be alkaline. So too, pH and alkalinity are two different measurable parameters of water. Alkalinity is the water’s capacity to resist changes in pH that would make the water more acidic. This is important! For example, you could have a liquid with a high lakaline pH but is very low in alkalinity. In that example it would take just a small amount of acid to change the pH.

Alkalinity is the water’s capacity to resist changes in pH that would make the water more acidic.

pH and alkalinity are two different measurable parameters of water.  Alkalinity is a measure of the buffering capacity of water – its ability to resist sudden changes in pH, a measure of the capacity of water to neutralize acids. Alkalinity is a measure of the capacity of water to neutralize acids. While pH is a measure of how acidic or alakaline the water or liquid is.

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The Math Behind pH

pH is a measure of the acidity or the base of a liquid. It is a negative decimal logarithm of the hydrogen ion.


Where an H+ is the activity of hydrogen ions in units of mol/L,  meaning molar concentration.