Soil fertility and sustainability - Fundamentals of the Physical Environment - page 471

Table 19.7 Cation exchange values for various soils

Exchangeable cations (me 100 g -1 or cmol c kg -1 )

Soil Ca Mg K Na H CEC % BS pH

Cambisol (Scotland) 3·0 27·5 0·2 0·1 3·6 34 90 6·5

Chernozem (Russia) 30·5 1·8 0·5 0·2 0 33 100 7·3

Podzol (Scotland) 0·6 0·7 0·2 0·1 37 39 4 4·3

Ferralsol (Kenya) 4·8 1·2 0·3 0·2 3·5 10 65 5·5

Luvisol (Canada) 22·8 0·6 0·3 0·1 3·8 28 86 6·2

material to study, and its chemistry varies, depending
upon the prevailing soil conditions and the nature of the
original plant material. During humification the original
plant material quickly loses the most readily decom-
posable fractions (sugars, polysaccharides, amino acids),
but it takes longer to break down the more resistant
carbohydrates (celluloses). The most resistant fraction,
lignin, accumulate in humus.

(cmol c kg -1 ) to a high of about 150 me 100 g -1 (cmol c kg -1 ),
depending on the type of clay mineral ( Table 19.7).
Organic or humic colloids have much higher activity
values in the range 150-300 me 100 g -1 (cmol c kg -1 )
( Figure 19.8 ). The reasons for this high activity are not
fully understood, but it appears to derive from the negative
charges of phenolic (OH) and acid carboxyl groups
(COOH) which occur in both humic and fulvic acids.
Soil organic matter (SOM) is that fraction of the soil
which is derived from plant and animal remains added to
the soil surface, and subsequently decomposed by soil
organisms. The organisms range from the larger soil fauna
(earthworms, ants) to soil micro-organisms (bacteria,
fungi, actinomycetes). Decomposition of the fresh organic
material is the process of humification , and results in a
dark-coloured amorphous material known as humus
which gives the surface soil its dark colour. It is not an easy

Types of soil humus

The many organisms and micro-organisms in soil are also
synthesizing organic molecules during humification, and
microbial proteins and micobial polysaccharides are
added to SOM on their death. Therefore SOM is not
simply a residual product of freshly decomposed the dead
plant matter, but much consists of synthesized microbial
products which are influential in giving humus its
important properties. Figure 19.9 illustrates how an active
organic cycle can rapidly break down large quantities of
organic matter by the activities of bacteria and fungi.
Larger organisms such as earthworms act to mix the
organic matter throughout the topsoil. The end result is
a relatively stable mull humus (pH 5.5).
If decomposer organisms are less active or absent,
slow decomposition gives a build-up of only partially
fermented litter. Under acid soil conditions, due to acid
parent material, or excessive leaching or acid-tolerant
vegetation, the population of bacteria is reduced, and
slow decomposition by fungi will produce raw humus or
mor . This consists of three layers: litter (L), fermentation
(F) layer and very thin humus (H) layer. Mor is not well
decomposed, or intermixed with mineral material, in the
absence of bacteria and earthworms (pH

200

150

100

50

4.0). An
intermediate type is moder humus (pH 4·0-5.5). The
nature of the soil, climate and vegetation will determine
which of the three types is formed ( Plate 19.5a , b , c ). In
poorly drained, waterlogged situations the lack of oxygen

0

Humus Montmorillonite Illite Kaolinite

Figure 19.8 Cation exchange capacities (CEC) of humus and
common clays.