Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 16

substantiated, how do they differ between soil types and management
practices; and what is the magnitude of any decline in soil properties if these
critical levels are not maintained?

The Evidence

There are thousands of papers, reports, etc. which assert that a change in
SOM status is associated with an improvement or deterioration in the
behaviour of agricultural soils . However, assertion is not enough, nor
should it be enough - against a background of increasing regulation based
on quantitative approaches - to say that soil scientists know this to be true.
We were seeking robust , numerical evidence to support these assertions for
soil types relevant to the UK, e.g. equations of state, regression relation-
ships and graphical demonstration of relationships, supported by properly
designed experiments with acceptable statistical treatment. We were also
seeking work in which the authors were careful in the use of terms such as
SOM, SOC, 'humus', and the like, and not using these interchangeably
without adequate explanation. We examined about 1500 such papers
(back to ~1938), whose abstracts led us to believe that the paper contained
quantitative evidence relevant, although not exclusively so, to temperate
agriculture and soils types in the UK.
The vast majority of the papers examined, despite the claims to
the contrary, did not contain quantitative data which met our criteria for
robustness, good experimental design or careful use of terminology (often
all of these).
Of those papers which did contain 'hard' evidence, the majority
were concerned with aggregate stability. Apart from the difficulties about
SOM versus SOC, etc., the literature suffers from different definitions of
aggregate size, and a plethora of methods for determining aggregate
strength or stability, with almost no inter-method or inter-laboratory
comparisons. This is clearly an area which would benefit enormously from
some standardization. There is an increasing move in the more modern
literature to quote aggregate size in multiples of 250 m, although many

papers quote aggregate size in terms of the mean weight diameter (MWD),
which does not give aggregate size directly.
Probably the most comprehensive investigation of the relationship
between total SOM and aggregate stability is still that of Kemper and Koch
(1966), who examined 519 samples of topsoils and sub-soils from the
western USA and Canada. They derived a number of curves showing that
aggregate stability declined increasingly sharply as SOM contents fell below
~4% (Fig. 1.1.1). It is interesting to note that the rate of decline is steepest
when all the data are combined (and does seem to give some credence to
the 2% threshold, albeit for SOM, not SOC), but is less marked when the

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