Soil Carbon Dynamics and Nutrient Cycling - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 99

capacity due to the action of carboxyl groups.
Thus, soils with a higher C content gener-
ally have the ability to retain cations such as
calcium (Ca), magnesium (Mg), zinc (Zn),
iron (Fe) and many others that are important
for plant growth. Organic C has an indirect
effect on nutrient availability to agricultural
crops through its influence on soil physical
conditions, especially structural attributes
such as the formation and stabilization of
aggregates. This in turn has a positive influ-
ence on the infiltration and retention of
water and the growth of plant roots - factors
that are of central importance for agricul-
tural productivity and food security. These
factors, and others, are considered indi-
vidually.

authors found a general correlation between
C and organic P, though much weaker than for
N or S. The mean ratio C:organic P was 133:1.
The relatively constant values for the
ratio C:N:S:organic P in the stable fraction of
soils (i.e. excluding fresh plant material if
this is present) across a very wide range of
soils may be evidence that this material is
derived largely from microbial sources, be-
cause the ratios in plant material are much
more variable. The main input of organic
matter to soil is plant material, but it is
thought that it is the products of its bio-
logical transformations by soil fauna and
microorganisms that constitute the more sta-
bilized forms of organic matter accumulating
in soil. There is evidence from spectroscopic
studies that about 85% of the organic N in
soil is in the form of amide groups, consist-
ent with being in proteins. However, proteins
are among the most labile natural macro-
molecules, and when added to soil under
experimental conditions, normally persist for
no more than 2 or 3 days, so it is surpris-
ing to find evidence of their dominant pos-
ition within soil organic matter (SOM). It is
thought that this unexpected persistence is a
result of stabilization processes that include
chemical entrapment by association with
humic substances and physical adsorption
on clay surfaces.
Table 7.1 shows that even a soil with the
relatively low organic C content of 1% (at the
low end of the range for arable soils in tem-
perate regions) contains over 2000 kg N ha − 1
in the cultivated layer. Even if only a few
per cent of this stock of N becomes available
to plants each year, it makes a significant
contribution to plant nutrition. For example,
crops growing in unfertilized treatments of

Organic Matter as a Reservoir of
Plant Nutrients

In addition to C, the organic moieties that
constitute soil organic matter contain sub-
stantial quantities of elements that are highly
significant as plant nutrients, especially N,
P and S. The C:N ratio of soils is surpris-
ingly constant, with values almost always
in the range between 10:1 and 12:1. Higher
values in the range 15:1-20:1 occur in peat
soils. In a global data set analysed by Kirkby
et al . (2011), the mean C:N ratio was 11:1.
The measured C:N ratio can vary somewhat
according to the amount of fresh plant ma-
terial in a soil at the time of sampling; this
may come from recently added crop res-
idues or, especially in the case of soils under
grass, be due to dead plant residues. In one
set of soils examined by these authors, the
C:N ratio of the 'light fraction' organic matter
(plant-like material separated by either siev-
ing or density separation) had a C:N ratio
ranging from 13:1 to 21:1, more similar to
values typical of plants. The C:S ratio in soil
is somewhat more variable than the C:N
ratio, but generally fairly constant, and aver-
aged 79:1 in a subset of soils where S had
been analysed (Kirkby et al ., 2011). P content
is linked less strongly to C, mainly because a
substantial fraction of soil P is present in in-
organic forms, in contrast to N or S. If only
organic forms of P were considered, these

Table 7.1. Quantities of nitrogen (N), sulfur (S) and
organic phosphorus (P) contained in the organic
matter of a typical soil in arable agriculture in the
temperate region. Values refer to the 0-20 cm
depth and assume a soil bulk density of 1.3 g cm - 3 .

Approximate elemental content in 1 ha of soil
(0-20 cm depth) under arable cropping (kg ha -1 )
C

N S Organic P

26,000 2,360 330 200