Organic soil biology and PH

Discussion in 'Growing Organic Marijuana' started by MI Wolverine, Nov 27, 2011.

  1. Oh my, here we go lol. Not really, just thought i'd share a good write up that helped me in the past.
    It's a write up on ph in organics I found interesting----

    Soil Biology and pH by Jeff Lowenfels

    The success of the AeroGarden, the first plug-and-grow aeroponic kitchen appliance, is testament to the fact that ordinary people do not understand the concept of pH and don't want to deal with it in their growing situations. Make it so you can practice hydroponics without this chemistry barrier and they will come, apparently.

    Frankly, the concept of pH also confuses soil gardeners. Heck, the definition of pH was inadvertently reversed in my book "Teaming With Microbes: A Gardener's Guide to the Soil Food Web." (Yes, some readers noticed; I received two "you made a mistake" notes. But that's not as many as I thought I'd receive.) Fortunately, the mistake was corrected in time for the second printing.

    In any case, soil gardeners have been told certain plants require acidic conditions- for example, rhododendrons and azaleas- or else they won't grow. The solution advocated by most experienced gardeners is not dissimilar from what a hydroponics grower would do: adjust the pH with chemicals, such as agricultural lime, to make the soil more alkaline. To make alkaline soil more acid, we are told to add sulfur. Because they are chemical changes, these solutions work for a short time. But to me pH is a biological matter.

    A bit of quick pH review is in order (if only to make amends for the mistake in my book). You may remember that pH is a measure of the acidity or alkalinity of a solution on a scale of 1 to 14; 1 being most acidic and 14 being most alkaline. A more technical description is that pH is the measurement of the concentration of hydrogen ions, H+. If you have lots of H+, the pH is low, or acidic. If you have few of them, the pH is high, or alkaline.

    If you are adding fertilizers and using chemicals, you are stuck in the chemical realm. Organic gardeners, soil food webbies in particular, realize that pH has more to do with biology than it does with chemistry. That's because of the way plant roots take up nutrients. Root hair surfaces are covered with positive electrical hydrogen cations. Think of these charges as ping-pong balls. If soil particles are small enough, their surfaces are covered by these ping-pong ball charges, both positive (cation) charges and negative (anion) charges. These cations are not limited to hydrogen; they also include calcium, potassium, sodium, magnesium, iron, and ammonium. All are important plant nutrients.

    When a root encounters a clay or organic particle, it can exchange one of its hydrogen cation for another positive one from the particle. It can choose from calcium, potassium, sodium, magnesium, iron, ammonium and hydrogen, as these are all cations carried by clay and silt and are all, as luck would have it, major plant nutrients.

    This is known, incidentally, as cation exchange capacity, or CEC. Sand and silt have low CECs, because they comprised of particles that are too large to hold electrical charges. This is why humus and clay are needed to make soil good. They are extremely small particles and can carry cations.

    So, back to pH. Every time a plant root exchanges a hydrogen ion for a nutrient ion, it increases the concentration of hydrogen ions in solution. Thus, the pH goes down and things should become more acidic.

    Ah, but things usually balance out because the positive cations on the root surface also attract negative charges. Here, hydroxy ions (OH-) are the exchange ping-pong balls, and addition of hydroxy ions lowers the concentration of hydrogen ions in the solution, and pH goes up.

    I know this still sounds like chemistry and not biology. However, each plant has an optimum pH requirement. What soil growers need to know (and hydroponics growers don't) is that the type of bacteria and fungi attracted to a plant's rhizosphere by the plant's exudates has a lot to do with setting this optimal pH. Bacteria produce a slim that raises the pH, and fungi produce acids that lower the pH. Since the plant is in control of the biology it attracts, in a natural system, it is the plant that determines the pH, and not some chemistry teacher.

    So, while you may forget the chemistry of pH, at least remember there is a biological side. Do no harm to it, and you shouldn't have to worry much about pH when you grow plants in soil. Moreover, the nutrient exchanges that occur above also have a lot to do with what kind of bacteria and fungi are attracted to the root zone as some like higher pH and others lower pH.
  2. Nice thread to link people trying to go organic and still freaking out about pH.
  3. I agree....nice resource MIW.

  4. Great info MIW, I see you spreading much knowledge. Thanks I am totally new to organics and am really thankful for all knowledge.
  5. Thank you for this writeup. Helps my understanding of ph and organics
  6. I simply dont even think about ph anymore - or lets put it this way: I dont check it or try and alter it, and it does indeed seem that the plants and microbes know what they want and need and take care of themselves.

    Good stuff MIW. Thanks.

    I was thinking, though...

    Our soil mixes are probably pretty even ph, or at least close? to begin with. I wonder how plants will do if the soil is in an extreme acid or alkaline soil mix...will it still be able to balance itself and be happy?

  7. Thanks MIW, I was looking for answer to this question.
  8. Great article, thanks for sharing!
  9. MI W

    Thank you for your thread.

    Once the CeC is understood even on a elementary level it becomes easy to understand the adjustments one has to make when choosing different materials in a potting soil.

    For example, Coir has a CeC of 70 whereas organic Sphagnum Peat Moss has a CeC of 100 meaning Coir is less efficient. Then consider that Coir contains no measurable levels of Sulphur (Su) requiring some consideration if a person is going to use a liming agent.

    The one thing that I wished the author had touched on is why soil biologists write elemental Calcium as Ca++ to note the 'extra' exchange sites.

    The other thing that might have been mentioned is that the plant's roots exude Hydrogen (H) as an exchange medium - he kinda went there but the role of Hydrogen is a bit more complicated.

    Then consider that the compounds that plants create are primarily molecular formula of CxHxOx with Hydrogen usually having the highest level. THC for example has a molecular formula of C21•H30•O2


  10. you had me til thread. haha

    good info man
  11. [quote name='"LumperDawgz2"']
    The other thing that might have been mentioned is that the plant's roots exude Hydrogen (H) as an exchange medium - he kinda went there but the role of Hydrogen is a bit more complicated.[/quote]

    LD could you please explain or point me to some education on the (H) role. You got me curious.
  12. #12 LumperDawgz2, Nov 29, 2011
    Last edited by a moderator: Nov 29, 2011

    See how this explanation works for you:
    More info at link that should help you out on your question.

  13. [quote name='"LumperDawgz2"']StickyFiskers

    See how this explanation works for you:

    More info at link that should help you out on your question.


    Thanks LD2.

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