Agriculture Reference
In-Depth Information
Table 4.1
Relationship among Pore Size, Formative Forces, and Pore Function
Pore
diameter(μm)
Biotic or physical agent
Pore function
5000-500
Cracks resulting from drying;
small animals, earthworms,
primary plant roots
Aeration and rapid drainage
500-30
Grass roots, small insects
Normaldrainageandaeration
30-0.2
Fine lateral roots, fungal hyphae,
and root hairs
Storageof“available”water(see
“Aeration and Drainage” in this
chapter)
<0.2
Wettinganddryingofwater
associated with clay mineral
surfaces
Retention of residual or
“nonavailable” water
CompiledfromCassetal.(1993)andWhite(2006).
atmosphere.However,inmostsoilstherearealso“dead-end”pores(upto5%of
thetotal)thatremaingas-illedevenwhenthesoilappearstobesaturated.he
O
2
in these pores is soon depleted, with the result that respiration switches from
aerobic(inthepresenceofO
2
)toanaerobic(intheabsenceofO
2
)inthispore
space, and CO
2
begins to accumulate. Poorly drained soils and those affected by
a high water table can be predominantly anaerobic so that gases such as nitrous
oxide(N
2
O)andmethane(CH
4
)maybeproduced.Consistentwithtotalporos-
ity,theair-illedporosityisexpressedasafractionofthesoilvolume(m
3
/m
3
)
according to the ratio
Volume of air-filled pores
Volume of so
=
A
ir-filled porosity
il
Water Storage
About two days after a soil has been thoroughly wetted by rain or irrigation, drain-
agebecomesslowandthesoilissaidtoattainitsieldcapacity(
FC
).Tomeasure
the water content at
FC
accurately, the soil surface, which should be bare of plants,
must be covered after wetting to prevent evaporation. The water content at
FC
sets the upper limit for stored water available to the vines and any mid-row cover
crop. Gradually, as plants extract this water, narrower and narrower pores become
filled with air until the water remaining is too difficult to extract and plants wilt.
If this condition persists, the permanent wilting point (
PWP
) is reached (see
“RedistributionofWaterintheSoil”inthischapter).heamountofwaterheld
between
FC
and
PWP
—the soil's “available water”—is an important soil property.
Box4.2summarizesdiferentwaysofmeasuringasoil'swatercontent.