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  • From: "Jeff Lowenfels" <jeff@gardener.com>
  • To: "gardenwriters-on-gardening" <gwl-g@lists.ibiblio.org>
  • Subject: Re: [gwl-g] soil pH
  • Date: Wed, 14 Apr 2004 14:07:12 -0800

Jeff, there are a few different ways to answer this question, but let's
start with what happens when a plant absorbs nutrients. These are
electrically charged cations and anions.
There is a balance or equilibrium between electric charges in the soil.
Then along comes a root hair with its own surface electrical charges and
exchanges or absorbs one of the cations (a positively charged nutrient) from
one of the clay or humus particles.

Two things of significance happen. First, the plant exchanges or releases a
hydrogen cation in return for every nutrient cation it adsorbs. This keeps
the balance of charges equal but increases the amount of H+ in the soil
solution, a factor that increases the pH, which is, in fact, just a measure
of the H+ in solution. Similarly, if it adsorbs an anion, it releases (OH)-.

Compost contains lots of humus and clay which have large surface areas
full of Cation Exchange Sites. When these materials come into contact with
the root, bingo. So you get a change in pH in the rizoshpere.Go beyond the
root and you don't get as much exchange if any. Hence the pH stays the same.

This is an extremely simplistic explanation, never a good idea on a list
like this one, as I am sure I am about to find out!



Cheers,

Jeff L





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