Geoscience Reference
In-Depth Information
pH, plants suffer from a shortage of both. Similar situation exists with zinc under
high pH. On the other hand, the increased solubility of some elements such as alu-
minum and manganese under low pH yields toxic conditions for many plants.
It is a relatively rare situation when we precisely measure an exactly neutral soil
reaction value of pH = 7. Typically, a neutral soil reaction is denoted whenever an
individual measurement of pH falls anywhere between values of 7.2 and 6.5 (see
also Sect.
7.4
)
. Traditionally, after we have mixed soil with pure water and mea-
sured its pH, we obtain a value designated as its active pH. On the other hand, if we
chemically push out (extract) all exchangeable cations (all bases together with the
adsorbed H
+
) from the soil, a pH measurement of the extract quantifi es the soil's
exchangeable acidity. The higher the difference between active pH and this
“exchangeable pH,” the lower is the saturation of CEC by bases and the higher is the
need of liming the soil.
7.6
A Carnival with Change of Masks Again?
The change of chemical composition of soil water fi lling soil pores does not simply
mean that the presence of one type of adsorbed cations has been replaced by another
type of cation. Some cations have a very thick hydration envelope. For example, Na
+
has an exceptionally large hydration envelope. If these highly, individually hydrated
cations are adsorbed on the solid surfaces of fi ne soil particles, their surrounding
envelopes of water do not allow them to get intimately close to the negatively
charged surfaces of clay minerals. Inasmuch as water within soil pores is in constant
molecular motion regardless of how fast the soil water moves or does not move
through the soil, positively charged exchangeable cations remain next to the stable
solid surfaces of clay minerals balancing their negative charge. As a result, each
solid negatively charged particle is surrounded by its balancing cations and remains
unaffected by any water movement. If, however, there is a distinct ratio of monova-
lent Na
+
present among the exchangeable cations, the solid surface continues to
display a certain amount of its negative charge without compensation owing to the
great distance of many Na
+
cations away from its surface. The very thick hydration
envelopes of Na
+
cause this substantial distance. Consequently, the solid particle
surfaces appear negatively charged from the point of view of any moving water.
These persisting negatively charged particles do not attract each other. Quite the
opposite, they repulse each other. And as a result, they cannot form coagulated units
nor is the genesis of microaggregates possible. Owing to this absence of formation
of micro-fl akes and other types of mutually bound soil clay minerals and silt parti-
cles, micro- and macroaggregation do not exist.
If the soil pores in a naturally developed well-aggregated structured soil having
both micro- and macroaggregates are penetrated by water containing solutes of Na
minerals, the initially present exchangeable bivalent cations within the pores start to
be expelled and replaced by Na
+
ions that each surrounded by their own hydration
envelopes. Retaining their negative charge, the previously neutral behavior of the
