Agriculture Reference
In-Depth Information
Figure 3.5
An ironstone gravel layer at the top of the B horizon in a duplex soil on sandstone in the
Yarra Valley, Australia. Photograph by the author. See color insert.
tralia. Similarly, soil layers cemented by secondary CaCO
3
deposits are called cal-
crete. A
fragipan
(found in soils subjected to present or past freezing to depth) is
not cemented, but the size distribution and packing of the soil particles produces
a layer of a high bulk density.
Plow pans
are formed in cultivated soils at the max-
imum depth of cultivation (20-25 cm), as a result of the shearing force applied
by the implements and compression by tractor wheels. These pans can be partic-
ularly bad in vineyards where the inter-rows are rotary-hoed (section 7.4.1).
Soil Porosity
3.3
Porosity is important because it determines the total amount of water the soil can
hold. For a given soil volume, the porosity is defined as
Volume of pores
Porosity
(3.1)
Volume of soil
It follows that the pore volume is equal to the product of the porosity and
the soil volume. This equation is most appropriate for nonswelling soils. In a soil
with much expanding-lattice clay, such as montmorillonite, the total soil volume
and the porosity may change appreciably as the water content changes.
3.3.1
Bulk Density
The simplest method of estimating porosity is from the soil's bulk density and
particle density.
Particle density
p
(rho p) is discussed in box 2.1.
Bulk density
