Agriculture Reference
In-Depth Information
Figure 4.1
A vineyard soil with swelling clays showing cracks on drying.
Total porosity is important because it determines how much of the soil vol-
umewater,air,androotscanoccupy.Equallyimportantaretheshapeandsizeof
the pores. The pores created by burrowing earthworms, plant roots, and fungal
hyphae are roughly cylindrical, whereas those created by alternate wetting and
drying appear as cracks (igure 4.1). Overall, however, we express pore size in
termsofdiameter(equivalenttoawidthforcracks).Table4.1givesaclassiication
ofporesizebasedonporefunction.
Aeration and Drainage
Asoil'sporespaceisnormallyoccupiedbywater(thesoilsolution)andair.When
all the pores are filled with water, the soil is saturated. As water drains out or evap-
oratesfromthesurface,gasesoftheair,predominantlyoxygen(O
2
)andnitrogen
(N
2
),entertheporespaceandthesoilbecomesunsaturated.hegasesenterthe
largest pores first, primarily by mass flow, accompanied by diffusion into smaller
pores.NormalrespirationofsoilorganismsandplantrootsconsumesO
2
and pro-
ducescarbondioxide(CO
2
),andtheexchangeofthesegasesbetweentheatmo-
sphere above and a soil's atmosphere is called aeration.
Provided there are some continuous pores in the soil, gas exchange through
theseporesproducesrespectiveconcentrationsofN
2
and O
2
of78%and20%
(by volume) in the soil air, similar to the atmosphere. he concentration of
CO
2
insoilairnormallyrangesfrom0.1%to1%,comparedto0.04%inthe
