The Makeup of Soil - Soils for Fine Wines - page 48

O O

C C

O -

O

+ Cu 2+

Example of the formation of a Cu-

Cu 2+

Figure 2.16

OH O

chelate (White 1997). Reproduced
with permission of Blackwell Science Salicylic acid Stable 6-membered

Ltd.

chelate

Other Properties of Humus 2.3.4

Cation Exchange Capacity
Humus is an organic colloid of high specific surface area and high cation exchange
capacity ( CEC ). Of the several functional groups containing O, those that disso-
ciate H ions—the carboxylic and phenolic groups—are the most important. The
former have pK values between 3 and 5, whereas the latter begin to dissociate pro-
tons only at pH 7. The meaning of pK values and buffering is explained in ap-
pendix 3. Thus, the CEC of soil organic matter increases as the pH increases, and
SOM has buffering capacity over a wide range of pH. The CEC is between 150
and 300 cmols charge ( ) per kg dry matter, which is very significant in a soil
with a low clay content. For example, if the organic matter content of a soil's A
horizon is 5%, its contribution to the CEC of this horizon would be 7.5-15 cmols
charge/kg soil. CEC is important for the retention of nutrient cations such as
Ca 2 , Mg 2 , and K (see chapter 4).

2.3.4.1

Chelation
Humic compounds form coordination complexes with metallic cations by dis-
placing some of the water molecules from the cation's hydration shell. The sta-
bility of the resultant complex is enhanced through the “pincer effect” of the co-
ordinating groups, giving rise to a chelate compound (fig. 2.16). Chelates formed
with divalent and polyvalent cations are the most stable, with the stability con-
stants decreasing in the order
Cu Fe Al Mn Co Zn
Metal complexes formed with the HA fraction are largely immobile, but the
more ephemeral Fe 2 -polyphenolic complexes are soluble and their leaching con-
tributes to soil profile development in Podzols and Podzolic Soils (box 1.5).

2.3.4.2

Factors Affecting the Rate of Organic Matter Decomposition

2.3.5

Type of Substrate
Because various organic materials are decomposed in soil, we can expect a range
of decomposition rates. The rate of substrate decomposition is assumed to follow
simple first-order kinetics, according to the equation

2.3.5.1

dC
kC

(2.1)

dt
dC/dt is the differential calculus expression for the rate at which the amount of
substrate C changes with time t . The parameter k (the reciprocal of time) is the