Agriculture Reference
In-Depth Information
Box 4.3 What Causes Clay to Disperse?
WhenCa
2+
ions are the predominant exchangeable cations, the clay layers
within particles, and whole clay particles, come close together in roughly parallel
alignment. The reason is that the attractive force between the flat clay surfaces
(negativelycharged)andthepositivelychargedcationspredominates,andtheclay
is said to be flocculated. However, as Ca
2+
ionsareprogressivelyreplacedbyNa
+
ions, the weaker negative-to-positive attraction and greater tendency for water
molecules to diffuse between the flat surfaces cause the particles to swell and move
farther apart. Also, the swelling pressure resulting from the influx of water increases
when the soil solution becomes more dilute, as happens when the soil is very wet.
The net effect is that clay particles separate to the point where the weakened forces
of attraction are overwhelmed and the clay deflocculates or disperses.
The clay suspension shown in the middle jar in figure B4.3.1 came from
acreekininlandQueensland.WaterinmanyAustralianinlandstreams
remains “cloudy” because of dispersed clay that is eroded from soils containing
exchangeableNa
+
.hecriticalamountofNa
+
, expressed as a percentage of the
Dispersed sediment
Flocculated sediment
Figure B4.3.1
Asampleofdispersedclay(middlejar)andthesameclay(rightjar)
locculatedwith0.1Mcalciumchloride(CaCl
2
)solution.Comparetheclearsupernatant
above the flocculated clay with the clear CaCl
2
solution in the left jar.
(continued)
