What is Alkalinity – It May Not Be What You Think?

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Robert Pavlis

Alkalinity is used incorrectly by many gardeners and even garden writers. I’ll bet I’ve made this mistake. If you think alkalinity refers to a high pH, you should read this post.

What is Alkalinity - It May Not Be What You Think?
What is Alkalinity – It May Not Be What You Think?

What is Alkalinity?

You can describe alkalinity in two ways, which in essence mean the same thing.

a) Alkalinity is a relative measurement of the capacity of water, or soil to resist a change in pH.

b) Alkalinity is a measure of the total carbonates (CO3), bicarbonates (HCO3) and hydroxyl ions (OH) and is ususally expressed as the equivalent of CaCO3, e.g. 100 ppm CaCO3.

When acid is added to water or soil, it will react with the carbonates and be neutralized. Until all of the carbonates are neutralized, the pH won’t change. This is called buffering capacity.

Other compounds will also neutralize acids, but the carbonates exist in much higher amounts and are therefore the major ions of interest.

Compost Science for Gardeners by Robert Pavlis

Why is Alkalinity Important?

High alkalinity can be toxic to plants and it usually results in high pH, which is not good for plants. Knowing the alkalinity helps to understand the suitability of soil or water for plants.

It is important to know the alkalinity if you are trying to adjust pH, since it determines the amount of acid required to cause a change in pH.

Alkalinity vs Alkaline

The two terms sound the same and are therefore mixed up in discussions. Alkaline refers to a pH over 7. A soil can be alkaline, but the soil can’t have an alkalinity of 8.5 (pH units).

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

14 thoughts on “What is Alkalinity – It May Not Be What You Think?”

  1. Ph is log scale, that’s help? A measure of the content….but ph is log numbers, not amount of chemical to change against restance, the chemical varies big, but not log wise, nesecarly, alkininity, it’s about chemical reaction equations, log confuse that.. I expect I said that right.

  2. No that’s okay. I have no issues at my property and have ordered soil tests for several others when it involves high value trees. I’ve seen entire neighborhoods with mature 150- 200 year white oaks all develop chlorosis and begin to die back from the top down. Putting the pieces together as to why this occurs is a combination of limited mineral availability in soil, nutrient leeching from heavy rain periods, over irrigation to the point of water run off into the street, soil compaction from commercial lawn mowers and competition from turf grass roots all of these have a negative effect on the root system of trees and have a cumulative effect.
    I can green things up temporarily and reduce decline by chemical means, but more importantly I always feel torn between two sides of a quick chemical fix or a long slow process of improving the soil health by natural means.

    Most people want quick and now and don’t like it when entire limbs start dying from the top down.

    • It may be useless to the lay person and even the uninformed gardener, but for gardeners to have an intelligent conversation, they need to under the meaning of words.

      If I tell you I solve my with high alkalinity, in tap water, by diluting with distilled water, and you don’t understand the term, you may decide to try and lower your high pH the same way, which does not work.

  3. Thanks Robert,
    I’m embarrassed to admit that I’m also a retired chemist, doing a bit of gardening in retirement, and I never knew exactly what is soil “alkalinity”. The same term is also used in swimming pool maintenance.
    I presume it is just another word for pH buffering capacity? – although in the case of soils we’d just be looking at the capacity of the soil to resist becoming more acidic. Maybe this confusing word should be removed from the English language(?)

  4. Not sure about your statement that a soil cannot have an alkalinity of 8.5. I presume you mean 8.5 is the maximum pH for almost all soils. To be even more pedantic I believe certain saline soils go a little higher.
    I think this in a future post deserves more discussion such as because of high buffering capacity of calcareous soils it is hopeless to try to acidify them

    • The reason soil can’t have an alkalinity of 8.5 is that Alkalinity is not measure in pH units.
      But on reflection I guess it could have a alkalinity of 8.5 ppm. I made a small change in the article to clarify.

  5. It wasn’t until about a year ago that I learnt that Alkalinity was a thing. I knew that a high pH is termed Alkaline, but didn’t realize that the high pH in my seedling potting mix was being caused by high Alkalinity of my reticulation water. The bicarbonate level in the water was about 200ppm, which is OK for plants in the ground, but is harmful for seedlings in small cells. Around 50-80 ppm is better for seedlings. Once I found this out and treated the water with acid, all my seedlings started growing as they should have.

  6. No you are incorrect, Cation Exchange Capacity is the soils resistance to changes in ph.

    I did not have to look this up as it’s memorized in my mind.

      • Ok I guess I misread your statement. Explain to me this then. Where I live we have neutral to alkaline soil. The areas where ph is the highest are clay subsoil and organic top soil layer stripped when homes were built. There is basically no organic(carbon) matter in the soil. Clay has a very high water holding capacity and very high CEC ratio. I wouldn’t say the soil is toxic but many plants suseptible to chlorosis become chlorotic and die. Additionally phosphate levels in the soil are through the roof and I have also learned high phosphates can block uptake of minerals.
        What I’m getting at is you wrote carbonates neutralize acids, but these alkaline soils have little to no carbon. Please explain

        • Carbonates are NOT carbon, although they contain a carbon atom. When you say your soil is low in organic matter you are referring to dead and recently dead living tissue. This does have a higher CEC and one reason soil should have this increased to reasonable levels.

          Carbonates are ions, minerals if you like, that come from things like limestone, which is mostly made up of calcium carbonate.
          In alkaline soil plants can have trouble getting enough iron and other nutrients, which can lead to chlorosis. Unless your pH is really high, this normally only happens in acid loving plants. High P levels are toxic to plants, and also interfere with nutrient availability.

          I would suggest getting a soil test done and get some help from the lab on how to fix things.

  7. Great information as always! Thank you Robert. I have a question: Have there been any experiments with Iguanas and marigolds? People swear by it in the south but I don’t get it. If an animal is hungry is there any deterrent? It doesn’t make logical sense to me but if you know anything please….thank you!


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