Agriculture Reference
In-Depth Information
Table 2.4 Diffusion coefficients
of chelated ions at 25 C
D( dm 2 s 1 )
Zn 2 +
×
10 8 a
8 . 8
10 8 a
ZnEDTA
6 . 8
×
Fe 3 +
6 . 2 × 10 8 b
3 . 9 × 10 8 b
FeEDTA
×
10 8 b
FeDTPA
4 . 2
10 8 b
FeEDDHA
5 . 4
×
Source :
a Elgawhary et al . (1970).
b O'Connor et al . (1971).
Table 2.5 Diffusion coefficients of gases
in air and water at 25 Cand1atm
D( dm 2 s 1 )
in air
in water
O 2
2 . 05
×
10 3
2 . 26
×
10 7
10 3
10 7
CO 2
1 . 55
×
1 . 93
×
2 . 20 × 10 3
1 . 73 × 10 7
CH 4
1 . 66 × 10 3
2 . 00 × 10 7
H 2 S
10 3
10 7
N 2
2 . 04
×
2 . 02
×
2 . 19 × 10 3
2 . 49 × 10 7
NH 3
N 2 O1 . 55 × 10 3
1 . 98 × 10 7
×
10 3
×
10 7
NO
2 . 04
2 . 55
Source : Lerman (1979).
near zero at the boundary between the soil and overlying water and gradually
increases over the upper 0.5 to 1 cm of soil or deeper. The cross-sectional area
for diffusion and ion interchange with the soil solid are altered correspondingly.
The Impedance Factor for the Liquid Phase, f L
The impedance factor is strictly empirical, accounting primarily for the geometry
of the soil pore network but also for ion exclusion by negative adsorption from
narrow pores, and for the increased viscosity of water near charged surfaces. It
is similar for all simple ions and molecules. It can be measured by following the
self diffusion of a nonadsorbed ion, such as Cl ,forwhich C = θ L C L and hence
D = D L f L .
Discontinuities in the liquid pathway and the effects of anion exclusion from
narrow pores and of viscosity near charged surfaces are important at low moisture
contents. The value of f L therefore approaches zero in very dry soil and the
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