Agriculture Reference
In-Depth Information
kind of aggregation predisposes to not only good drainage and aeration but also
adequatewaterstorage.
Although grass roots and organic compounds are the key to good surface
soil structure, their effect is less significant in the subsoil because they are less
abundant there. Good subsoil structure consists of aggregates that are orientated
vertically and are longer than they are broad, with well-defined cracks between
and within them. Ideally, the texture range is loam to clay loam to light clay, and
the color is golden brown to red, without mottles, showing that the subsoil is
well drained and not waterlogged when wet. The dominant exchangeable cation is
Ca 2+ ,withiron(Fe)andaluminum(Al)oxidesfrequentlyactingasbridgingcom-
pounds between clay particles. Figure 4.4 shows an example of a well-structured
subsoilinavineyardintheCoonawarraregion,SouthAustralia.
ForcesatClaySurfaces
The amount and type of clay minerals have an important effect on aggregate for-
mationandstability.Asdescribedin“RetentionofNutrientsbyClayMinerals
and Oxides,” chapter 3, layers within the clay crystals have an overall negative
charge that is just balanced by the positive charge of exchangeable cations attracted
to the surfaces. The cations available in the soil solution have charges ranging from
Figure 4.4 The well-structured subsoil
of a Terra Rossa in the Coonawarra
region,SouthAustralia.hescaleis
10cm.(White,2003)
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