Soil Acidity (pH)
It is well known and well studied that succulents and cacti thrive best in neutral to slightly acidic (pH 6) soil. While it is not necessary to go out and buy a pH meter or litmus paper to find out your soil's acidity, it may be useful for you to understand how your succulent functions so you can make wise purchasing decisions. On this segment of Succulent Science, we will be exploring the concept of pH and how it relates to succulent soil.
Some key words:
acidic - like lemons, pH <7
alkaline - basic, pH >7
ion - atom or molecule with a net charge that is positive or negative.
cation - a positively charged ion, for example H+ or Na+ (sodium ion)
anion - a negative charge ion, OH- (hydroxide ion) or Cl- (chloride ion)
Basics of Water Equilibrium
When you have a glass of pure water, it isn't actually just H2O, in fact, it is a mixture of H+ cations and -OH anions. The water dissociates spontaneously, meaning that nothing has to be done to the water in order for it to "break apart" into its ions. This process is given the name autoionization and the reaction for it is this:
H2O(l) ↔ H+(aq) + -OH(aq)
When there is a dissociation reaction, we define an equilibrium constant called the dissociation constant. Because water is special, we call it Kw. The mathematical expression for the dissociation constant is Kw = [H+][OH-]. Any equilibrium constant can be found using the formula K = [products]/[reactants], but pure liquids and solids are excluded because their concentrations stay constant throughout the reaction (this is why liquid water is not in the expression).
Scientists have physically measured the concentrations of the H+ and -OH ions at 25 ℃ (about room temp.) to be each 10-7 M. The capital M is molarity and is a unit of concentration that equals moles/liters. To solve for the dissociation constant, they plugged in the concentrations and found that [10-7 M][10-7 M] = 10-14 M. Thus, Kw = 10-14 M2. Adding different amounts of either ion will not affect that equilibrium will always be established after awhile and the concentrations of the ions will multiply to give the same exact Kw. The only way to change the value of Kw is to change the temperature.
What is pH?
pH is a mathematical way of describing the acidity of basicity of a substance. All of those tiny, negative numbers can really get annoying, so we use a -log scale. The H in pH is for the hydrogen ion, H+. The pH is a measure of how many H+ ions are in solution, freely floating and unattached. The p in front of the H indicates that it is on a -log scale. So in a given room, let's say that it is 25 ℃ (room temperature) and we measure the concentration of these H+ ions to be 10-7 M. Using the equation for pH, we find that the water at room temperature is pH = 7, which is neutral. There are no units of pH, so it is just plainly "7".
pH = -log[H+]
Because of this mathematical relationship, soil with pH = 6 is ten times more acidic than soil with pH = 7. That's a lot more acidic than what it looks like!
Alternatively, you could find the pOH of a solution: pOH = -log[OH], or the way I like is that by using math, the Kw = 10-14 M2 means that pKw = 14, thus pKw = pH + pOH and 14 = pH + pOH.
For more science, go to Khan Academy.
So what does pH mean practically?
Above: everyday items with their relative pH indicated.
For succulent lovers who just want to buy the right kind of soil, buying a "succulent" soil is probably already at the correct pH. It is known that succulents grow best in slightly acidic soil and this is because of the composition of different elements in the soil that become "available" to the plant.
If the pH is too low (too acidic) then nutrients can become available to the succulent in toxic amounts. This includes manganese, aluminum, and iron. Some nutrients cannot reach the plant at all like phosphorus, potassium, magnesium, and molybdenum.
If the pH is too high (soil is too alkaline), the solubility of nutrients decreases, so some elements form solids called precipitates instead of staying in the soil. These are calcium, iron, and phosphate compounds. There is also a deficiency in the absorption of certain compounds such as manganese, phosphate, iron, copper, zinc, and boron.
You want just the right pH to reach just the right chemical cocktail. Most succulent soils are already at the correct acidity, so you never have to worry. If you're growing outdoors and are not seeing great results, check your soil's pH. It may be the culprit.
BONUS: pH and Anthocyanins
How the pH of the soil affects the color of Hydrangeas --> in a very organic science-y way. It also relates to my post on anthocyanins.
References:
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