Agriculture Reference
In-Depth Information
Box 4.2
(continued)
A
θ
value of 0.25 m
3
/m
3
is the same as 2.5 mm per cm depth of soil. Thus the
equivalentdepth
d
(inmillimeters)ofwaterinanysoildepth
z
(incentimeters)is
given by
d
=××
10
θ
z
(B4.2.3)
TableB4.2.1givessomeexamplesoftheuseofthisequation.
Table B4.2.1
Some Examples of Equivalent Depths of Water in Soil
Soilvolumetricwatercontent
θ
(m
3
/m
3
)
Equivalentdepthofwater
d
in
soil depth
z
(mm)
Soildepth
z
(cm)
0.25
20
50
0.25
40
100
0.40
20
80
0.40
40
160
Atanygiventime,thesumofthewater-illedandair-illedporositiesequals
thetotalporosity.Whenthesoilisatits
FC
, the value of air-filled porosity is
called the air capacity, or sometimes the drainable porosity. The volume of avail-
ablewaterina1-m-deepproile(seebox4.2)iscalledtheavailablewatercapacity
(
AWC
).hesetwovariables—aircapacityand
AWC
—can together be used to
classifyasoil'sstructuralquality,asshowninigure4.2.
Soildroughtinessincreasesastheamountofavailablewaterdecreases,whereas
its susceptibility to waterlogging increases as the air capacity decreases. An air
capacityof15%togetherwithan
AWC
of20%orgreaterisregardedasbeingvery
good. The simple classification of figure 4.2 is useful for vineyard soils because it
sets practical limits for satisfactory aeration and available water. Additionally, by
taking into account the effect of soil strength on root penetration, it is possible
toreinethissimpleclassiicationofsoilstructuralquality,asdiscussedin“Soil
Strength”laterinthischapter.
Soil Aggregation
he size, shape, and consistence of aggregates vary considerably between soil
types and often between the topsoil and subsoil of one soil type. For example,
figure 4.3 shows a surface soil that is well aggregated, with aggregates mainly
