Agriculture Reference
In-Depth Information
+1to+3andalsodiferinsizeaccordingtothenumberofwatermoleculesintheir
hydration shells.
1
Depending on their hydration energies, cations tend to release
these water molecules when attracted close to a clay surface. For example, K
+
has a
low hydration energy and readily sheds its water molecules, Ca
2+
has an intermedi-
atehydrationenergyandonlypartiallyshedsitswatermolecules,andNa
+
has a
high hydration energy and remains hydrated. Thus the force between layers and
the distance separating layers varies according to the dominant cations present.
Because cations accumulate in the spaces between clay layers, water molecules
try to diffuse into these spaces in response to a concentration gradient between
the ambient solution and the interlayers. In so doing, they create a pressure that
pushes the layers farther apart. This swelling pressure, which changes with the
type of cation and the ionic concentration of the soil solution, can have a disrup-
tive effect on aggregate stability. For example, three times as many exchangeable
Na
+
ions than Al
3+
ionsarerequiredforchargebalanceinaclaycrystal,andthe
Na
+
ions are highly hydrated; thus the tendency for water to diffuse into the crys-
tal,andhencetheswellingpressure,ismuchgreaterinaNa-claythananAl-clay.
AggregateStability
Apartfromanaggregate'ssize,shape,andcolor,akeycharacteristicisitsabilityto
resist“slaking”asitwetsup.Whenwaterisrapidlyabsorbed,trappedairexertsa
disruptive pressure, augmenting the swelling pressure that develops between the
clay layers and around the clay particles themselves. Altogether, these pressures
may exceed the forces holding an aggregate together, causing it to collapse or slake.
Because of their particular chemical and biological properties, the aggregates shown
in figures 4.3 and 4.4 do not slake. A further change that may occur when a dry
aggregate wets and slakes is that the swelling pressure is sufficient to push the clay
particles so far apart that they disperse and go into suspension. Box 4.3 describes
theconsequentefectsofswellingpressureinmoredetail.Figure4.5showsasimple
test for clay dispersion that can be done in the vineyard.
Soil Strength
Soilconsistencereferstoaggregatestrength,whichdependsontheforcesholding
an aggregate together. The resilience of a soil's structure is affected by aggregate con-
sistence, which is very dependent on the water content. In the vineyard, aggregate
consistenceisassessedbytheforcerequiredtobreakdownaclodorlargeraggregate,
about20mmindiameter,bysqueezingbetweenforeingerandthumborbypres-
sureappliedunderfoot.heconsistencescalerangesfromloose(zero)—aggregates
1
A hydration shell is an ordered sheath of water molecules surrounding a cation.
