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In-Depth Information
P.J. Lovelandet al.
Critical Levels ofSOM
1.1
Critical Levels of Soil Organic
Matter: the Evidence for
England and Wales
P.J. L OVELAND 1 , J. W EBB 2
AND P. B ELLAMY 1
1
Soil Survey and Land Research Centre,CranfieldUniversity,
Silsoe,BedfordshireMK45 4DT; and 2 ADAS Consulting Ltd,
Woodthorne,WergsRoad, Wolverhampton WV6 8TQ, UK
Introduction
There is a widespread perception that the amount of organic matter (SOM)
or organic carbon (SOC) in soils needs to be kept above a minimum level in
order to prevent, or at least minimize, irreversible decline in a range of soil
properties. Soil structure, ease of cultivation, improved water retention,
better seedbed establishment, reduced erosion risk, more effective use
of nutrients (especially N and P) have all been commented on (see, for
example, Carter and Stewart, 1996), although it is known that, in certain
soils, other factors such as the 'free' iron content and exchangeable sodium
content may have a marked effect on aspects of soil structural behaviour.
Greenland et al . (1975) proposed a 'rule of thumb' that soils in England
and Wales should be regarded as structurally unstable if the SOC content
fell below 2%; in conventional terms, this equals 3.4% SOM. This value
seems to have become a reference point or benchmark in the soil science
community. The UK Royal Commission on Environmental Pollution, in
its report on 'Sustainable Use of Soil' (RCEP, 1996), commented on the
undesirability of allowing SOM to decline too far, although avoiding a
specific recommendation as to any limiting values. The RCEP report thus
raised some fundamental questions in relation to SOM in the soils of
England and Wales and the long-term, sustainable use of UK soils for
agricultural production, against a background of the need to manage
soils in an environmentally sensitive manner. We sought to address three
of these questions: what is the quantitative evidence for critical levels of
SOM in relation to specific soil properties; if such critical levels can be
 
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