Agriculture Reference
In-Depth Information
pH
Soil pH is talked about a lot in the gardening world, but most people don't understand it
and the number garnered from a pH test is generally misused. After this short section,
you'll know how to use pH data better than most garden experts. Now, pH can get very
technical, but I'm going to stick to what is relevant to us in the garden, yet try not to over-
simplify too much for the scientists out there.
For our purposes, pH is basically a measurement comparing how many positively
charged hydrogen ions we have in our soil versus the other cations — calcium, magnesium,
potassium, sodium and aluminum. The more hydrogen ions we have, the lower our pH is
and the more acidic it is. The more of the other cations we have, the higher our pH is and
the more alkaline it is. The scale goes from 0 to 14, with 7 being neutral, but most soils are
between 4 and 9. At 4, all cation exchange sites are occupied by hydrogen, so there's not
much in the way of nutrition for plants.
At just under pH 7, all cation exchange sites have cations other than hydrogen, which
doesn't necessarily mean the soil is highly fertile because we still don't know which
cations are there. In theory, it could be all magnesium, which would not be a very balanced
soil. Also, what if you're gardening on a soil with 98% sand and silt, both of which have no
CEC? In this example, even if all of your clay and organic matter exchange sites are occu-
pied by cations, your soil is probably very low in cations because you don't have many ex-
change sites. A pH below 4 involves additional organic acids and above 7 or 8 involves ad-
ditional carbonates. Usually the pH is somewhere in between, which means we have some
hydrogen and some of the other cations.
Most nutrients, particularly the most essential nutrients, are most readily available to
plants somewhere in the 6-7 pH range, gradually decreasing as the pH gets further up or
down the scale. Some micronutrients become more available outside this range, especially
in low pH soil, potentially to toxic quantities. So it's not that the acidity of a 4.5 pH soil is
harmful to the soil, it's that most nutrients aren't as available to plants, and a few may be
too available. Further, many microbes can't live at an extreme pH, so the soil food web will
be lacking.
So we can see it's best to have a pH somewhere in the middle. Actually, between 6 and 7
is generally considered ideal, which may be true, but this is where a mistake is often made.
Let's say we take a soil sample and determine the pH is 5.5. We will be told to add lime to
raise the pH of our 5.5 soil, usually this is dolomite lime.
The reason we are told to do this is because, as discussed earlier, cations can knock each
other off exchange sites. All things being equal, the hydrogen ion is the most attracted to a
cation exchange site. If we look at the major cations in decreasing order of their affinity for
cation exchange sites — that is, how strongly attracted they are to the cation exchange sites
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