Agriculture Reference
In-Depth Information
ruptures the living organisms. The dissolved organic C (DOC) in the extract is
measured and the quantity per kg soil is calculated. An unfumigated soil is also
extracted to act as a control. Biomass C is then calculated from the equation
(DOC in unfumigated soil)
Biomass C
K
ec
where
K
ec
is an empirical factor measuring the efficiency of C extraction from the
microbial population.
K
ec
values are between 0.3 and 0.5, depending on the soil
type, but the generally accepted value is 0.35. Measurements are usually made on
the top 15-25 cm of the soil profile, where the C substrates and organisms are
concentrated.
(DOC in fumigated soil)
pK Values and Buffering
Appendix 3
Acids are defined as potential proton (H
ion) donors, and bases as potential pro-
ton acceptors. In aqueous solutions, acids dissociate H
ions that are accepted by
H
2
O. For example, the dissociation of an acid HA in water is written
HA
H
2
O
H
3
O
A
(A3.1)
The ion H
3
O
is called the hydronium ion (normally written as H
). The
degree of dissociation of HA determines the strength of the acid, which is mea-
sured by the dissociation constant
K
a
, defined as
a
H
a
A
K
a
(A3.2)
a
HA
a
H
2
O
where the terms on the right-hand side are
activities
(see box 4.4). The activity of
water (
a
H
2
O
)
1, and in dilute solutions, the activities of the other species can
be replaced by concentrations. Thus, equation A3.2 is normally written as
[H
][A
]
K
(A3.3)
[HA]
or converted to the negative log form (cf. pH), which is
[A
]
pK
pH
log
10
(A3.4)
[HA]
The smaller the value of pK, the stronger the acid. Most soil acids (associ-
ated with organic matter and soil minerals) are weak acids.
If equation A3.4 is rearranged as
[HA]
pH
pK
log
10
(A3.5)
[A
]
we see that when there are equal concentrations of undissociated acid HA and
base A
, the pH
pK (this is the half-neutralization point). At this point, the
addition of small amounts of acid or base causes the least change in pH. This is
therefore the point of maximum buffering of the solution. The
buffering capacity
