Geoscience Reference
In-Depth Information
soil particles is lost, and they repulse each other while the aggregation disintegrates.
The higher is the concentration of Na
+
in the solution penetrating into the pores, the
stronger is its impact until the original structure is suffi ciently transformed to that of
a typically behaving structureless soil that becomes slushy and swells when wetted
by rain or irrigation water. Its pore-size distribution has been changed substantially
with macropores disappearing and microspore distributions displaying a high abun-
dance of very fi ne pores. When the transformed soil begins to dry, it shrinks and
deep, wide cracks appear instead of the earlier original natural macropores, and the
abundance of very fi ne micropores remains or even increases. Soil scientists classify
soils according to values of exchangeable sodium percentage, ESP. Because a value
of ESP above 10 usually causes already the mentioned problems, soils are denoted
as being sodic - or in greater detail, with low, medium, or strong sodicity. However,
the sodic problems start at lower ESP if the concentration of salts in the water in soil
pores is low. High concentrations of salts in soil water act upon exchangeable Na
+
through something like osmotic pressure that presses the cations closer to the solid
surface. As a result, the excessive negative charge is reduced, and the disruption of
micro- and macroaggregates is less intensive. A sodic soil having a high concentra-
tion of salts is called a saline sodic soil. Nevertheless, the actual action of exchange-
able Na
+
in a saline sodic soil also depends on the composition of all of the salts
dissolved in its pore water (Fig.
7.10
).
The destruction of soil structure changes the frequency of different sized pores
from a distribution of two peaks to only one peak. Earlier, we compared the pore-
size distribution curve of a well-aggregated soil to the two humps of a Bactrian
camel. Now another similarity becomes apparent - the destruction of soil structure
changes the pore-size distribution to a curve having only one peak, comparable to
the lonely hump of a dromedary camel. Is not the analogy similar to the change of
Fig. 7.10
Left
: exchangeable calcium cations completely compensate the negative charge of a soil
particle surface at the shortest distance possible.
Right
: exchangeable sodium cations do not fully
compensate the negative charge of the soil particle surface
