Agriculture Reference
In-Depth Information
Box 4.3
(continued)
soil'scationexchangecapacity,abovewhichdispersionoccursrangesfrom6%to15%,
depending on the type of clay mineral present and the soil solution concentration.
Dispersed clay is easily transported in runoff water, which worsens the tendency for soil
toerode(see“heInteractionamongCoverCrops,Mulches,andSoilWater”inthis
chapter).Furthermore,asthesoilsurfacedries,thedispersedclayformsahardcrust
that inhibits water infiltration and the emergence of young seedlings.
The jar on the right-hand side of figure B4.3.1 shows the same suspension
after a small volume of concentrated CaCl
2
solution(takenfromthejaronthe
left-handside)wasaddedandmixed.hedispersedclayhaslocculatedandsettled
tothebottom,andthewaterisnowclear.Flocculatedclayisaprerequisitefor
the formation of small stable aggregates, which in turn clump together to form
largeraggregatesthatarestabilizedby“cementingagents”(e.g.,CaCO
3
, Fe and Al
oxides)andbiologicallyproducedgumsandmucilage.
heNa-Cainteractionisimportantforthelocculationofmontmorillonite,illite,
and vermiculite clays. For kaolinite, flocculation that depends on an attraction between
positive charges on the clay edges and negative charges on the flat surfaces is more
important.SuchlocculationprevailsatapHlessthan6whentheedgesarepositively
charged but breaks down as the pH increases and the edges become negatively charged
(see“RetentionofNutrientsbyClayMineralsandOxides,”chapter3).
Air-dry aggregate
Rainwater
Complete dispersion
Partial dispersion
No dispersion
Remolded
moist aggregate
Rainwater
Dispersion
No dispersion
Figure 4.5
A simple dispersion test for soil aggregates, based on the test devised by Emerson
(1991),modiiedbyCass(1999).(RedrawnfromWhite,2003.)
easilyfallapartintoconstituentparticles(e.g.,sand)—torigid(point7)—aggre-
gatescannotbecrushedunderfoot.hebestconditionisweaktoirm(points2to
3)whenclodsorlargeaggregatesbreakdowneasilyintoamixofsmallaggregates—
a friable “tilth.” Figure 4.6 shows a sample of clay soil under vines where any surface
clodsbreakdowneasilyintoirmaggregates10to30mminsize.
