Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 23

Table 1.1.3. Number of National Soil Inventory samples in soil organic carbon classes (% in
< 2 mm oven-dry soil) in different years.

Arable/ley grass (n= 899) Permanent grass (n= 767)

% Organic carbon 1980 1994 1980 1995

< 2 317 350 43 27

2-4 387 444 313 404

4-6 121 66 231 246

> 6 74 39 180 90

χ

2 value with 3 df

31.2 45.6

P< 0.0001 P< 0.0001

There are relatively few data for 'active' carbon for soils in England and
Wales. Griffiths and Jones (1965) and Griffiths and Burns (1972) found
strong evidence that microbially produced polysaccharides contributed
considerably to soil strength, whilst Reid and Goss (1980) attributed
similar increases to 'root exudates'.
None of this work found a relationship between the increase in soil
strength and the initial amount of total SOC, or the ratio between total
SOC and the amount of polysaccharide/exudate. More recently, Ball et al .
(1996) have investigated the effects of differences in soil carbohydrate
chemistry in relation to soil structure, but the data are insufficient to draw
firm conclusions in a national context.

Conclusions

From the forgoing analysis, we have concluded that:

•

there is little consistent evidence that there are critical thresholds of SOC
above or below which soil physical properties change significantly;

•

there is a growing body of evidence that the 'active' fraction of SOM is
a more important factor in controlling change in soil properties than
total SOM, but there are insufficient data for the amounts and kind of
'active' SOM for soils in the UK to enable firm conclusions about the
magnitude of its affect on soil behaviour;

•

there are insufficient data to explore thoroughly the relationships and
interactions between other factors which affect soil physical behaviour;

•

it would be inappropriate to set thresholds for critical levels of SOM or
SOC for particular soils; and

•

a single value for a critical level of SOM cannot be set for the soils of
England and Wales on the basis of the data available.