Geoscience Reference
In-Depth Information
mention, we must say that the decomposition of organic matter is slow and produces
alcohols, aldehydes (e.g., methanal, ethanal (or acetaldehyde), propanal,
2-methylbutanal), and other compounds that are generally toxic to the roots of most
cultivated plants. Additionally, free iron and manganese as well as their soluble
forms released from inorganic compounds and minerals are detrimental to the phys-
ical and chemical properties of soil and to plant roots. Moreover, the soil reaction is
changed with its pH indicative of acidifi cation. Cultivated plants suffer from these
unfavorable conditions stemming from decreased diffusion rates. If diffusion
remains nil, conditions eventually deteriorate and lead to the destruction of all cul-
tural plants.
Although the term or expression soil pH is commonly used to designate a spe-
cifi c property of an entire mass or volume of soil, its use actually only characterizes
the reaction expressed as pH of the water solution contained in soil pores. Even this
restriction is not an exact description of what we measure since we mix the soil with
an excess of pure water. The reader already experienced in the chemical term pH
can skip the following two paragraphs because we next explain pH in a popular
manner.
A very small portion of water molecules H
2
O is dissociated (it means “split”)
into ions of hydrogen H
+
(i.e., the positively charged H) and of hydroxyl OH
−
(nega-
tively charged OH). The concentration of both (H
+
and OH
−
) is the same in pure
water. Both values equal 10
−7
eq/l (a unit describing the concentration). Written in
more understandable units, the amount of H
+
is 1 × 10
−7
g in 1 l and the amount of
OH
−
is 17 × 10
−7
g in 1 l (or 0.0000001 g/l and 0.0000017 g/l). These concentrations
are really very small. The value of the water solution reaction does not depend upon
the mutual ratio of weights - or more exactly - upon the ratio of masses of H
+
cat-
ions and OH
−
anions. Instead, the most important conclusion is that everything
depends upon the number of positive and negative charges.
When we write pH, we mean the negative value of the logarithm to the base 10
of the H
+
concentration in the liquid. For a value of 10
−7
eq/l, we obtain log
0.0000001=−7 that indicates a neutral reaction of pH=7. Similarly, if we were
quantifying the concentration of OH
−
for a neutral reaction in pure water, we would
write p(OH) = 7. When salts of acids or bases are present in soil water, pH values
change. If an acid is present in the solution, the H
+
concentration is increased and
the value of pH sinks below 7. The value of pH becomes smaller, not larger, with
increased acidity because of the negative exponent, i.e., 10
−5
eq/l is a larger value
than 10
−7
eq/l. Generally, a pH value below 7 means an acid reaction, the presence
of acids, and a dominance of H
+
. When pH is above 7, then the reaction is alkaline,
bases are present in the solution, and it means a dominance of OH
−
.
In soils we accept any value of pH within 6.5-7.2 as a characteristic value of a
neutral reaction. If pH is above 7.2, the reaction is considered alkaline. If pH is
below 6.5, the reaction is acid. Both kinds of reaction are graduated from weak to
strong. We should not forget that the pH number is the exponent of 10. Consequently,
if we compare soil one having a pH of 6 with soil two having a pH of 7, we must
remember that the concentration of H
+
ions in the fi rst soil is ten times higher than
that in the second soil. In addition to acid, neutral, and alkaline reactions, we distin-
