Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 20

and Db in a cultivation experiment extending over 18 years and a range of
tillage practices:

Db = − 0.102(SOM%) + 2.07 ( r 2 = 0.93; P < 0.001)

This reinforces a major point in these investigations. With the excep-
tion of the study by Kemper and Koch (1966), there is little or no evidence
for sudden or increasingly marked change in soil properties in relation to
SOM or SOC. The relationships are generally linear, and do not indicate a
catastrophic change; or the possibility of identifying clearly whether there is
a critical value of SOM or SOC for a specific soil type, whether or not in
relation to a specific land use.
Investigations by Hollis et al . (1977) of the relationships between SOC
and volumetric water content (THV5) at 5 kPa tension in topsoils of soils

from the West Midlands of England gave the relationship below for all land
uses:

−

THV5 = 23.88 + 7.85 (OC%) − 0.43 (OC%) 2 [ n = 77; 73.5%

variance explained]

Expansion of this data set ( n = 99), and its subsequent stratification
according to land use, is illustrated in Fig. 1.1.4. The main point is that
expansion of the data set does not greatly affect the form of the relationship,
although this clearly differs slightly between land uses. However, the
differences between the land use groups are clearly small and are not
great enough to suggest significantly different soil behaviour, or a marked
difference in the contribution to volumetric water content from SOC.
We have investigated the contribution of SOC and other soil factors,
for a larger number of topsoils, to the soil volumetric water content at
different tensions. The data are summarized in Table 1.1.1.
It can be seen that SOC explains ~13% of the variance at low tensions,
falling to 1% at wilting point. Examination of similar data for sub-soils
shows that SOC contributes < 2% to the variance of volumetric water
content at all tensions. Stratification of the topsoil data set in terms of clay
content and major agricultural land uses is summarized in Table 1.1.2.
This table illustrates the complex nature of these relationships, even
where relatively large amounts of data are available for analysis. It also
highlights a further difficulty. As already said, SOM or SOC do not
represent a single entity, but a complex mixture which changes in time and
space. Clay content also represents a very wide range of mineralogy,
which influences soil water release and retention in complex ways. There
are additional factors which, although we cannot demonstrate their effects
numerically, will also contribute to soil behaviour. Fe and Al have been
mentioned, but calcium carbonate content, the composition of the
exchangeable cation population and their interactions are others - many of
which we have insufficient data for.