Soil organic matter
– a fundamental but often forgotten aspect
ROTHAMSTED
RESEARCH
Johnny Johnston, Paul Poulton & Kevin Coleman
Department of Soil Science, Rothamsted Research, Harpenden, Herts, AL5 2JQ
Soil organic matter (SOM) has a vital role in soil fertility. It is
considered to be an indicator of soil quality and it is often suggested
that the amount of organic matter in soil should be increased.
However, in most soils SOM reaches an equilibrium level
so cannot be increased indefinitely.
Hoos Barley
The equilibrium level, expressed as %C or t C ha-1, depends on farming system, soil type and climate so
that in any climatic region:
Fosters
with the same farming system, the equilibrium level will be larger in a clay than in a sandy soil.
on any one soil type, the equilibrium level will be larger with permanent grass than with
continuous arable cropping.
In temperate climates, SOM changes slowly and only in long-term experiments can changes be reliably
monitored, explanations sought and carbon turnover models developed and validated.
2.0
1.8
Rothamsted
1.6
% Org C
1.4
Effect of soil type on SOM with continuous arable cropping
1.2
1.0
Woburn
0.8
0.6
0.4
0.2
0.0
1930
1940
1950
1960
1970
1980
1990
2000
2010
With continuous arable cropping, the equilibrium level is smaller in the sandy loam soil at Woburn than
in the silty clay loam at Rothamsted. Initially, the soil at Rothamsted contained more SOM than that at
Woburn although arable crops had been grown for many years on both sites. As arable cropping
continued, SOM declined slowly in both soils towards a slightly lower equilibrium level. At Woburn a
treatment with a 3-yr ley followed by 2 arable crops ( ), increased SOM slowly towards a new
equilibrium level but, even after 60 years there was less SOM in this soil than in the continuous arable
soil at Rothamsted.
Effect of different farming systems on SOM in a silty clay loam at Rothamsted
100
90
Highfield
Ley-arable experiments were started in 1949 on sites previously in long-term grass (Highfield) or arable
(Fosters). On Highfield some plots remained in permanent grass, some were ploughed. On Fosters some
plots were sown to grass others remained in arable cropping.
SOM increased with improved grassland management on Highfield towards a new equilibrium while on
Fosters it increased under newly sown grass but, after 50 years there was still less than in the permanent
grassland soil on Highfield. With arable cropping, large amounts of SOM were lost on Highfield but it took 50
years to decline to that on Fosters. The similar level of SOM with continuous arable crops on both fields
suggests that this is probably the equilibrium level for this farming system on this soil type.
Where 3 arable crops followed a 3-yr ley on Fosters, SOM increased by c. 7 tha -1 after 36 years (data not
shown). SOM built-up under leys is partly lost during subsequent arable cropping.
O rg an ic C in s o il , t ha -1
80
70
60
50
40
Fosters
30
20
10
0
1940
1960
1980
2000
Y ear
0.350
Effect of establishing permanent grass on SOM
0.300
Total N %
0.250
At Rothamsted, there are fields with known histories of being in permanent grass and records of changes
in %N in soil. On this silty clay loam, the relationship shows that when a soil with very little SOM is sown
to grass it takes about 100 years to reach the equilibrium level for SOM under permanent grassland and
about 25 years to get to the “half-way stage”.
0.200
0.150
0.100
0.050
0.000
0
50
100
150
200
250
300
350
400
Years in grass
100
100
Data on the long-term changes in SOM in Rothamsted experiments have
made it possible to develop and validate the model, RothC-26.3, for carbon
turnover in surface soils.
The fit of the model to the observed changes in SOM in the Hoos Barley
experiment is good. SOM has built-up where FYM is added annually and
now approaches the equilibrium for this treatment. SOM has long been at
equilibrium in the NPK fertilised and unmanured soil.
The model predicts the slow build-up of SOM well on Geescroft where
woodland has developed since 1881, and there is an even better fit to the
decline in SOM where an old grassland was ploughed in 1959 and no crops
or weeds have been grown since.
80
FYM annually
-1
80
70
70
-1
Or gan ic C i n Soil, t C h a
Modelling changes in SOM
90
Hoosfield
O rg a ni c C in S o il, t C h a
90
60
FYM 1852-1871
nothing thereafter
50
40
30
20
Estimated
60
50
40
30
Unmanured
Geescroft
10
Bare
fallow
20
0
1840
1860
1880
1900
1920
1940
1960
1980
2000
2020
10
Year
0
1870
1890
1910
1930
1950
1970
1990
2010
Year
Reference: A E Johnston, P R Poullton & K Coleman (2009) Advances in Agronomy, 101, 1-57.
Acknowledgement: Rothamsted Research is an Institute of
the Biotechnology and Biological Sciences Research Council
(BBSRC) of the UK.