Agriculture Reference
In-Depth Information
Box 2.2
Soil Consistence
Consistence is measured by the resistance offered to breaking or deforming a
soil when a compressive, shear force is applied. In the field, consistence is assessed
on aggregates 20-30 mm in diameter as they are pressed between the thumb and
forefinger. Usually the test is done on both air-dry and moist soil (at the “field
capacity,” as defined in section 3.3.3). The resistance of the aggregate to rupture or
deformation is expressed on a scale ranging from loose (no force required, separate
particles present) to rigid (cannot be crushed under foot by the slow application of
a person's body weight).
A soil that has a high resistance to deformation is said to be strong. High soil
strength may be associated with compaction, which is generally undesirable.
Detection of compacted layers in vineyard soils is discussed in chapter 7.
2.2.2.2
Texture and Soil Temperature
Texture has a pronounced effect on soil temperature. Clays hold more water than
sandy soils. Water modifies the heat required to change a soil's temperature be-
cause its specific heat capacity is three to four times that of the soil solids, and
considerable latent heat is either absorbed or evolved during a change in the phys-
ical state of water, for example, from ice to liquid water or vice versa. Thus, the
temperature of a wet clay soil responds more slowly than that of a sandy soil to
changes in air temperature during spring and autumn (section 3.5.2).
The temperature regime of vineyards can be influenced by the stoniness of
the soil surface. Large stones act as a heat sink during the day and slowly reradi-
ate this heat energy during the night, creating a more favorable microclimate in
the vine rows. A good example of this effect is in the famous Chateauneuf-du-
Pape appellation of the Southern Rhone Valley in France.
2.2.2.3
Texture and Tilth
Texture should not be confused with
tilth
, which refers to the surface condition
of cultivated soil prepared for seed sowing—how sticky it is when wet and how
hard it sets when dry. Tilth therefore depends on the soil's
consistence
, which re-
flects the influence of both texture and structure, modified by the water content
(box 2.2).
Minerals of the Sand and Silt Fractions
2.2.3
2.2.3.1
Basic Crystalline Structures
Sand and silt particles are mainly resistant residues of
primary
rock minerals, to-
gether with small amounts of
secondary
minerals (salts, oxides, and hydroxides)
formed by rock weathering. The primary minerals are mainly silicates whose crys-
tal structure is based on a simple molecular unit, the
silicon tetrahedron
, SiO
4
4
(fig. 2.4a). Another basic molecular unit found in the aluminosilicate minerals is
the
aluminum octahedron
, Al(OH)
3
(fig. 2.4b). The electric charge properties of
these units are discussed in box 2.3. Note that the edges of these molecular units
measure only a fraction of a nanometer, which is one-billionth of a meter, so we
are examining here the detailed fine structure of a crystal lattice.
