Agriculture Reference
In-Depth Information
— it goes hydrogen (H
+
), calcium (Ca
2+
), magnesium (Mg
2+
), potassium (K
+
) and sodium
(Na
+
). There are many other micronutrients in the full list, but these are the major ones.
Hydrogen has the greatest affinity for a cation exchange site, but there's one other
factor that influences which cations will bind to exchange sites. If there's an abundance of
some other cation, such as calcium (Ca
2+
), some of it can knock off some of the hydrogen.
This is called the “mass effect.”
Back to our example of adding dolomite lime to raise pH. Dolomite is calcium carbon-
ate and magnesium carbonate. The calcium and magnesium in the lime will knock some
of the hydrogen off the cation exchange sites. Some of that hydrogen will combine with
the carbonate, and some of it will go elsewhere. That will give us less hydrogen and more
cations, therefore raising the pH. This may happen in the short term, and even in the long
term if done annually for a number of years, although the soil will tend to move back to-
wards its starting pH. Still, it works for now.
So the problem is not that dolomite lime won't raise the pH, but that our pH test did not
tell us if we actually needed calcium and magnesium. Perhaps we already have too much
magnesium, or too much calcium. It's almost certain that we don't need both in the ratio
that dolomite lime gives us. On the other hand, some
high
pH soils are due mostly to sodi-
um and potassium, and they actually still need calcium and perhaps magnesium. We
wouldn't know that if we just used the pH number as our basis for liming. Adding more of
the wrong nutrient is just going to make things worse, as we'll see later.
It is the pH that gives us a clue that we may have a nutritional and microbial imbalance
in our soil, but this gives us no information as to why that may be so. As such, it's of very
little use to us. If we do regular pH testing over a number of years, making sure we always
take our soil sample from the same place, at the same time of day and year, in the same
conditions, the one thing it can help tell us is if our soil management practices are work-
ing, since pH will move towards neutral when we balance the nutrients in our soil and in-
crease the organic matter content. Besides, plants seem to grow well in soils of various pH
levels given plenty of organic matter.
It's not that pH isn't important to plants and microbes. For the most part, we're happy
to have it be between 6 and 7 to have the healthiest plants. Remember, hydrogen is used in
cation exchange and for mining certain minerals from the soil, so a slightly acidic pH is
ideal. It can be difficult for plants to get phosphorus out of an alkaline soil. Knowing the
pH value, however, doesn't help us much with soil management decisions, and it certainly
shouldn't be used to determine how much lime to add to the soil. pH is the result of the
elements in our soil, not the cause. From Charles Walters'
Eco-Farm: An Acres U.S.A.
Primer
, “excess acidity is nothing more than the reciprocal of fertility depletion.”
Plants that are considered “acid-loving” may just need certain trace minerals in abund-
ance, and those trace minerals are more available in acidic soil. Rhododendrons, for ex-
