Agriculture Reference
In-Depth Information
Si
4
O
6
O
4
(OH)
2
(Mg Al
3
)
-
O
4
(OH)
2
Si
4
O
6
H
2
O
Na
+
Na
+
ca.4 nm
Na
+
Na
+
Variable
spacing
H
2
O
Structure of Na-montmorillinite crystals. The basal spacing is variable because of the Na
+
ions in the interlayer spaces (White 1997). Reproduced with permission of Blackwell
Science Ltd.
Figure 2.8
Soils with such clays show pronounced shrinkage and cracking when dry and
swelling on rewetting (fig. 2.9)
2.2.4.3
Accessory Minerals
Accessory minerals are predominantly
free oxides
and
hydroxides
, including various
forms of silica, iron oxides, aluminum oxides, manganese, and titanium oxides.
Of these, the oxides of Fe, Al, and Mn are the more important.
Iron oxides
accumulate in soils that are highly weathered, especially those de-
rived from more basic rocks. They occur as discrete particles or as thin coatings
on clay minerals. Iron oxides have strong colors ranging from yellow to reddish-
brown to black, even when disseminated through the soil profile. Commonly,
when solutions rich in Fe
2
seep from waterlogged soils or rock fissures, a volu-
minous rusty-red precipitate of
ferrihydrite
forms. If conditions remain moist and
cool, ferrihydrite slowly transforms to the yellow mineral
goethite
(
-FeOOH).
This is the most common and stable iron oxide in the soil. Under hot and dry
conditions, ferrihydrite transforms to
hematite
(
-Fe
2
O
3
). Hematite is a bright
red oxide characteristic of soils that are highly weathered and subjected to peri-
ods of intense drying.
