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Prout, J. M., Shepherd, K. D., McGrath, S. P., Kirk, G. J. D. and Haefele,
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on organic carbon to clay ratio. European Journal of Soil Science. pp. 1-
11.
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O R I G I N A L A R T I C L E
What is a good level of soil organic matter? An index based
on organic carbon to clay ratio
Jonah M. Prout
1,2
|
Keith D. Shepherd
3
|
Steve P. McGrath
1
|
Guy J. D. Kirk
2
|
Stephan M. Haefele
1
1
Department of Sustainable Agricultural
Sciences, Rothamsted Research,
Harpenden, Hertfordshire, UK
2
School of Water, Energy & Environment,
Cranfield University, Bedford, UK
3
Land Health Decisions, World
Agroforestry (ICRAF), UN Avenue,
Nairobi, Kenya
Correspondence
Jonah M. Prout, Department of
Sustainable Agriculture Sciences,
Rothamsted Research, Harpenden,
Hertfordshire AL5 2JQ, UK.
Email: jonah.prout@rothamsted.ac.uk
Funding information
This research was funded by a PhD
studentship for JP through the
Rothamsted-Cranfield Soil Agriculture
Research and Innovation Accelerator.
Rothamsted Research receives grant-aided
support from the Biotechnology and
Biological Sciences Research Council
(BBSRC) of the United Kingdom. SPM and
SMH are funded by the Institute Strategic
Programme (ISP) grant
“
Soils to
Nutrition
”
(BBS/E/C/000I0310). The
Rothamsted Long-term Experiments
National Capability (LTE-NC) is
supported by BBSRC (BBS/E/C/000 J0300)
and the Lawes Agricultural Trust.
Abstract
Simple measures of appropriate levels of soil organic matter are needed for soil
evaluation, management and monitoring, based on readily measurable soil
properties. We test an index of soil organic matter based on the soil organic
carbon (SOC) to clay ratio, defined by thresholds of SOC/clay ratio for speci-
fied levels of soil structural quality. The thresholds were originally delineated
for a small number of Swiss soils. We assess the index using data from the ini-
tial sampling (1978
–
83) of the National Soil Inventory of England and Wales,
covering 3,809 sites under arable land, grassland and woodland. Land use, soil
type, annual precipitation and soil pH together explained 21% of the variance
in SOC/clay ratio in the dataset, with land use the most important variable.
Thresholds of SOC/clay ratio of 1/8, 1/10 and 1/13 indicated the boundaries
between
“
very good
”
,
“
good
”
,
“
moderate
”
and
“
degraded
”
levels of structural
condition. On this scale, 38.2, 6.6 and 5.6% of arable, grassland and woodland
sites, respectively, were degraded. The index gives a method to assess and
monitor soil organic matter at national, regional or sub-regional scales based
on two routinely measured soil properties. Given the wide range of soils and
land uses across England and Wales in the dataset used to test the index, we
suggest it should apply to other European soils in similar climate zones.
Highlights:
•
We assess the use of SOC/clay ratios as guidelines for soil management in
England and Wales.
•
We use data from 3,809 sites to assess thresholds based on work for Polish,
French and Swiss soils.
•
SOC/clay threshold values can indicate degraded and good soil structural
condition.
•
The thresholds show the effect of land use and provide an index for use in
England and Wales.
Received: 11 February 2020
Revised: 8 June 2020
Accepted: 9 June 2020
DOI: 10.1111/ejss.13012
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
© 2020 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science.
Eur J Soil Sci.
2020;1
–
11.
wileyonlinelibrary.com/journal/ejss
1
Page 3
K E Y W O R D S
land use, soil clay content, soil organic carbon, soil structure
1
|
I N T R O D U C T I O N
What is a good level of soil organic matter? Maintaining
and, if possible, increasing the level of soil organic matter
is generally a good thing for most functions expected of
soils, including carbon sequestration, and increased
levels improve soil structure. Farmers, food producers
and governments need to know their soil status in rela-
tion to a critical value of soil organic matter. However, as
soil organic matter varies with land use, soil type, loca-
tion and other variables, an index for gauging the level of
soil organic matter under given conditions needs to
account for these variables.
Verheijen, Bellamy, Kibblewhite, and Gaunt (2005)
derived indicative ranges of soil organic carbon (SOC)
content for arable soils of England and Wales that are
potentially attained under different types of management
and environmental conditions, and they found that clay
content, precipitation and depth of topsoil could explain
25% of the variation in SOC content. Clay soils under wet-
ter conditions had higher values than more-sandy soils,
and grassland soils had higher values than arable soils
with similar clay content. Clay content is a key factor
because of its effects on SOC protection, including adsorp-
tion on mineral surfaces and within soil aggregates
(Dungait, Hopkins, Gregory, & Whitmore, 2012; Six,
Conant, Paul, & Paustian, 2002). Under constant land
management and organic matter inputs, soils tend towards
a steady-state SOC content, with a capacity for stabilizing
SOC modelled as a function of clay content (Hassink, 1997;
Hassink & Whitmore, 1997; Six et al., 2002; Stewart,
Paustian, Conant, Plante, & Six, 2007).
Dexter et al. (2008) found that soil physical properties
(bulk density, water retention characteristics and clay
dispersibility) could be better explained by the relative
amounts of SOC and clay content to each other than by
their total contents. In their analysis of data on French
and Polish arable and grassland soils, maxima of correla-
tions between the mass of clay per unit mass of SOC and
soil physical properties corresponded to SOC/clay = 1/10,
the SOC/clay content ratio was a good indicator of soil
physical conditions, and this ratio gave a general separa-
tion between the different land uses. These findings were
subsequently supported by others in Denmark (de Jonge,
Moldrup, & Schjønning, 2009; Schjønning et al., 2012)
and a study in England (Jensen et al., 2019). Johannes
et al. (2017) developed the approach further and, in an
analysis of Swiss soils, defined SOC/clay thresholds of 1/
8, 1/10 and 1/13 as indicating the boundaries between
“
very good
”
,
“
good
”
,
“
suggest improvement
”
and
“
poor
”
levels of structural condition.
In this paper, we assess the three SOC/clay thresholds
of Johannes et al. (2017) for soils across different land
uses and climates in England and Wales. We use data
from the original sampling of the National Soil Inventory
(NSI), which contains information on soils at 5,662 sites
under agricultural and non-agricultural land uses across
the two countries (Bellamy, Loveland, Bradley, Lark, &
Kirk, 2005). This is a far larger dataset with greater varia-
tion in soils, environments and land use than the datasets
used by Dexter et al. (2008) and Johannes et al. (2017),
and so provides a more comprehensive test of the SOC/
clay ratio. We have three objectives. First, to assess the
variation in SOC/clay ratio and its drivers across the NSI
dataset. Second, to test its ability to delineate soils of dif-
ferent structural quality. Third, to illustrate the use of the
SOC/clay index for mapping soil carbon across England
and Wales, and for gauging changes in a long-term exper-
iment with contrasting organic and inorganic fertiliser
treatments.
2
|
M A T E R I A L S A N D M E T H O D S
2.1
|
National-scale data
The NSI was first conducted between 1978 and 1983.
Topsoil (0
–
15-cm depth) samples were collected at the
intersections of an orthogonal 5-km grid over the entire
area. A full description of the survey methods, analyti-
cal methods and available data is given in the LandIS
database (www.landis.org.uk; Proctor, Siddons, Jones,
Bellamy, & Keay, 1998). We considered only arable, ley
grassland, permanent grassland and woodland sites,
and excluded sites without measurements of soil clay
content, pH or depth of topsoil, or that were classified
as
“
peat
”
. To reduce the impact of sites with very high
SOC content relative to clay content, we excluded 290
outliers with SOC/clay > third quartile +1.5
×
inter-
quartile range. This gave 3,809 sites. Figure 1 shows the
distribution of the sites across the two countries and
Table 1 gives summary statistics for SOC and clay
contents.
Soils at each site were classified by major soil group
(Avery, 1980). Data on soil carbonate content were
obtained from field observations of fizzing on addition of
2
PROUT
ET AL
.
Page 4
HCl to samples on a 5-point scale from non-calcareous to
very calcareous.
Soil structural quality was characterized using the
Agricultural Land Classification of England and Wales
(MAFF, 1988), which gave scores of good, moderate or
poor structural quality according to the texture and
shape, size and development of aggregates, and friability
of subsoil. The NSI contains values for each of these
except friability; therefore, we made estimates based on
the shape and size criteria (and where possible develop-
ment of aggregates was taken into account) (Table S1).
Monthly average precipitation was obtained from
the UKCP09 dataset (Met Office, 2017). Mean accumu-
lated annual precipitation was calculated for the years
1910
–
1983 and values at each NSI site were intersected
using ArcGIS version 10.4. (ESRI, 2015). Ranges for
precipitation
classes
were
taken
from
Verheijen
et al. (2005): < 650, 650
–
800 and 800
–
1,100 mm year
−
1
,
with the addition of
“
very wet
”
for annual precipitation
>1,100 mm year
−
1
.
2.2
|
Data analysis
Statistical analyses were performed in R version 3.5.0 (R
Core Team, 2017). Random forest analysis (package:
randomForest; Liaw & Wiener, 2002) was used to analyse
the variance of SOC/clay with land use, average annual
precipitation, major soil group, pH, lower depth of top-
soil, calcareous score and risk of flooding. A square-root
transformation was applied to SOC/clay to reduce the
skewness of the data. Three-quarters of the data
(
n
= 2,857) were used as a training set, and the RMSE
and R
2
values of predictions of the remaining set
(
n
= 952) were calculated. Training and sample sets were
randomly selected. Spatial or other correlations across
training and validation sets were unlikely because only
topsoil samples were used and the minimum distance
between sites was 5 km.
Chi-squared tests were used to compare numbers of
sites within SOC/clay ranges under different land uses
and precipitation classes and to test the relationship
between the SOC/clay thresholds and soil structure.
We used the results of statistically significant chi-
squared tests to interpret interactions between vari-
ables, with contributions from specific combinations of
variables to the chi-squared statistic inferred from the
differences
between
observed
frequency
and
that
expected if there was no interaction between the
variables.
We tested SOC/clay thresholds of 1/8, 1/10 and 1/13
as indicating the boundaries between
“
very good
”
,
“
good
”
,
“
moderate
”
and
“
degraded
”
levels of structural
condition, following Johannes et al. (2017).
Figures were produced using R package ggplot2
(Wickham, 2016) and maps were produced using QGIS
3.0.1-Girona (QGIS Development Team, 2020).
F I G U R E 1
Map of arable, ley grass, permanent grass and
woodland sites in the National Soil Inventory sampled between
1978 and 1983 (
n
= 3,809)
T A B L E 1
Soil organic carbon (SOC) and clay contents by land-use class in the National Soil Inventory
SOC content (g kg
−
1
)
Clay content (g kg
−
1
)
n
Mean
Median
Min.
Max.
Mean
Median
Min.
Max.
Arable
1,661
25
22
4
126
262
247
26
879
Ley grass
602
34
31
7
109
267
257
60
756
Permanent grass
1,277
42
39
6
138
281
260
47
795
Woodland
269
40
37
1
158
251
242
10
606
All land uses
3,809
34
30
1
158
268
252
10
879
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2.3
|
Field-scale data
We assessed the effects of field-scale soil management
on SOC/clay ratios relative to the threshold values
using data from a long-term organic manuring exper-
iment at Woburn, Bedfordshire, UK (Mattingly, 1974).
The experiment had eight treatments with four repli-
cates: (a) peat for 6 years then ley, (b) farmyard
manure (FYM), (c) grass ley plus nitrogen, (d) grass-
clover ley, (e) green manure (GM) for 6 years then
ley, (f) straw, and (g) and (h) two inorganic fertiliser
treatments (details in Mattingly, 1974). Treatments
were applied in two cycles (1965 to 1972 and 1979 to
1987); the second cycle of treatment is denoted by
“
then
”
above if different from the first. We calculated
SOC/clay ratios for each plot and then averaged
the values for each treatment. The plot-level soil
clay content ranged from 78 to 131 g kg
−
1
and the
initial SOC content ranged from 5.7 to 8.6 g kg
−
1
(Table S2).
F I G U R E 2
Soil organic carbon (SOC) content as a function of clay content for different land uses. Lines are SOC/clay thresholds:
Solid = 1/8, dashed = 1/10, dot-dash = 1/13
4
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3
|
R E S U L T S
3.1
|
Variation in SOC and clay contents
and SOC/clay ratio
Mean SOC contents increased in the order arable <<
ley
grass
<
permanent
grass
≈
woodland
soils
(Table 1). Mean clay contents and their ranges were
similar across land uses, except those of woodland
soils were smaller. The dominant soil types in all land
uses were brown soils and surface-water gleys; the
proportions of other soil groups varied (Table S3).
Arable sites tended to have smaller average annual
precipitation than the other land uses (Tables S4
and S5).
The proportions of sites above and below the three
SOC/clay thresholds differed between land uses, par-
ticularly for the SOC/clay = 1/13 threshold (Figure 2
and Table 2). A greater proportion of arable sites had
SOC/clay <1/13 (i.e., depleted in SOC for their clay
content) and a greater proportion of permanent grass-
land and woodland sites had SOC/clay >1/8 (i.e.,
enriched
in
SOC
for
their
clay
content;
X
2
(9) = 681.3,
p
< .001).
Analysis of the influence of land use, soil and other
variables on SOC/clay ratio by random forest analysis
showed that 21.0% of the variance was explained by the
variables examined (Table 3). Land use, average annual
precipitation, major soil group and pH were more impor-
tant than carbonate score, flood risk and depth of topsoil.
When the model was run with just the top four variables,
the variance explained did not change; however, the
importance of land use increased relative to the other
variables.
3.2
|
Effects of land use and
precipitation
The effect of land use was clear, with lower SOC/clay
ratios observed for arable land and predominantly higher
SOC/clay ratios observed for grassland and woodland
(Table 2). As there was some geographical relationship
between the distributions of land use and precipitation,
the effects of each on numbers of sites relative to the
SOC/clay
thresholds
were
considered.
Verheijen
et al. (2005) suggested that dry sandy soils were more at
risk of lower SOC content than wetter clayey soils and
that grassland soils would have higher SOC content than
(ley-) arable soils. Comparing SOC/clay threshold ranges,
land uses and precipitation classes (< 650, 650 to 800, 800
to 1,100 and > 1,100 mm year
−
1
; Table S6), two questions
were asked. (i) Were arable soils under dry climate condi-
tions more likely to have SOC/clay <13 than arable soils
under wetter climate conditions? (ii) For soils under dry
climate conditions (< 650 mm year
−
1
), were arable soils
more likely to have SOC/clay <13 than other land uses?
In answer to the first question, chi-squared analysis
showed that precipitation class was not independent of
SOC/clay ratio for arable soils (
X
2
(9) = 78.9,
p
< .001).
Comparing the contributions of each combination to the
chi-squared statistic showed that a larger number of soils
receiving less than 650 mm year
−
1
and smaller numbers
of soils receiving more than 650 mm year
−
1
than
expected had SOC/clay <1/13. Also, a smaller number of
dry soils and larger number of soils with greater than
800 mm year
−
1
had SOC/clay >1/8. This suggests that
T A B L E 2
Percentages of sites
above, below and between soil organic
carbon (SOC)/clay thresholds of 1/8, 1/
10 and 1/13 for each land use and land-
use precipitation class combination
Percentage of sites with indicated SOC/clay ratio
n
≥
1/8
<1/8
≥
1/10
<1/10
≥
1/13
<1/13
Arable
1,661
28.8
14.0
19.0
38.2
Ley grass
602
50.2
20.3
14.6
15.0
Permanent grass
1,277
66.9
15.4
11.1
6.6
Woodland
269
67.7
16.0
10.8
5.6
T A B L E 3
Contributions of indicated variables to variance in
SOC/clay ratio analysed using random forests. Training data were a
random selection of 75% of the data (
n
= 2,857). With all seven
explanatory variables, root mean square error (RMSE) for training
data = 0.06, R
2
= 0.21; RMSE for remaining data = 0.07; R
2
= 0.21.
With only the top four variables, RMSE for training data = 0.06,
R
2
= 0.21; RMSE for remaining data = 0.06; R
2
= 0.22
Increase of mean square error (%)
Land use
32.7
39.8
Annual precipitation
28.0
26.0
Major soil group
26.4
20.3
pH
22.5
20.3
Depth of topsoil
10.4
Carbonate score
10.0
Risk of flooding
5.2
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5
Page 7
lower precipitation conditions were related to SOC/clay
<13 for arable soils.
Chi-squared analysis to answer the second question
showed that land use was not independent of SOC/clay
ratio for soils under dry climate conditions (
X
2
(9) = 94.0,
p
< .001). A larger number of arable soils and smaller
number of grassland and woodland soils than expected
had SOC/clay <1/13 than if the land use was independent
of
SOC/clay
ratio
range
for
soils
receiving
<650 mm year
−
1
annual precipitation. For soils with SOC/
clay >1/8, the reverse was true (i.e., arable < grassland or
woodland). This suggests that land use was affecting the
number of dry climate soils with SOC/clay <1/13.
The relative effects of land use, precipitation and soil
type were evident from the distribution of the 820 sites
with SOC/clay ratio < 13 across England and Wales (Fig-
ure S1). These sites were predominantly arable, and their
distribution across eastern and central England con-
firmed the lesser statistical effect of precipitation and
major soil group observed. Northwest England and Wales
had notably few degraded sites, although soils sampled
there were mostly under non-arable land uses.
F I G U R E 3
Box plots of soil
organic carbon (SOC)/clay ratio for
each major soil group. Horizontal lines
are SOC/clay thresholds: Solid = 1/8,
dashed = 1/10, dot-dash = 1/13.
Abbreviated major soil groups: Terr.
Raw, terrestrial raw; Lith.,
lithomorphic; SW gley, surface-water
gley; GW gley, ground-water gley
F I G U R E 4
Box plots of soil
organic carbon (SOC)/clay ratio for
each structural quality score. Horizontal
lines are SOC/clay thresholds:
Solid = 1/8, dashed = 1/10, dot-
dash = 1/13. Numbers of samples in
each group were
n
= 2,250, 1,111, 229
and 208 for good, moderate, moderate-
degraded and degraded, respectively
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3.3
|
Effects of soil type and soil pH
The statistical effect of major soil group appeared to be
driven by two of the soil groups and some of this
might already have been accounted for by land use
(Figure 3). Podzolic soils tended to have SOC/clay >1/
8 and were mostly not arable, whereas clay-rich
pelosols were more likely to have SOC/clay <1/13 and
a higher proportion were arable. The lower importance
of soil group might be linked to the smaller sample
sizes of the podzolic and pelosol soils compared with
brown and gley soils, for which SOC/clay ratios
showed similar variation.
As pH decreased below pH = 5, the SOC/clay ratio
tended to increase (Figure S2). Above pH = 5 there was
less of a trend when considering permanent grass and
woodland soils; however, arable and ley grass soils
showed decreasing minimum SOC/clay ratio particularly
above pH = 7, although sites with SOC/clay >1/8 were
still observed.
F I G U R E 5
Maps of soil organic carbon (SOC)/clay ratio across England and Wales under (a) arable land, (b) ley grass, (c) permanent
grass and (d) woodland coloured by SOC/clay index
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3.4
|
Relation between structural quality
and SOC/clay ratio
Structural quality, classified as good, moderate, moder-
ate-degraded and degraded, tended to improve with
increasing SOC/clay ratio, as shown by the box plots
in Figure 4 and the chi-squared test result for the rela-
tion between SOC/clay range between the thresholds
and structural quality (
Χ
2
(9) = 129.3,
p
< .001). Most
(82%) of the relationship between SOC/clay and struc-
tural quality was explained by (a) a larger than
expected frequency of sites with SOC/clay <1/13 and
moderate-degraded or degraded structure, (b) a smaller
than expected frequency of sites with SOC/clay >1/8
and degraded structure, and (c) smaller than expected
frequency of sites with SOC/clay <1/13 and good
structure.
3.5
|
Variation in SOC/clay ratio across
England and Wales
Mapping the index across the two countries (Figure 5)
showed the effect of land use and geography at the time
of survey. For any land use, degraded sites were not lim-
ited to a particular region. But, as previously mentioned,
there were fewer degraded sites towards the northwest
and in Wales. Calculating summary values of SOC/clay
by land use (Table 4) showed that the minimum value
increased slightly in the order: arable = ley grass < per-
manent grass < woodland. The median results showed a
stronger difference between arable sites and the other
land uses, with arable in the moderate category and the
other land uses equal to or above the very good threshold.
The different land uses had similar upper SOC/clay
values as a result of excluding outliers.
3.6
|
Changes in SOC/clay ratio with
field management
Figure 6 shows changes in SOC/clay ratios over 30 years
of the Woburn organic manuring experiment. Leys and
treatments with organic matter application (straw and
manures) showed similar trends of increasing SOC/clay
ratio during the application period and a decreasing ratio
after the treatment was stopped, but with differing mag-
nitudes. Peat and farmyard manure gave the largest
increases, followed by the ley treatments and then straw.
T A B L E 4
Summary of soil organic carbon (SOC)/clay ratio
decimal values calculated for each land use in the NSI subset
SOC/clay ratio
Mean
Median
Min.
Max.
Arable
0.109
0.090
0.018
0.357
Ley grass
0.139
0.125
0.018
0.359
Permanent grass
0.165
0.154
0.022
0.360
Woodland
0.174
0.160
0.025
0.355
F I G U R E 6
Changes over
time in soil organic carbon
(SOC)/clay ratio in the Woburn
long-term manuring experiment.
Points are treatment means.
Horizontal lines are thresholds
separating degraded (SOC/clay
<1/13), moderate (SOC/
clay = 1/13
–
1/10), good (SOC/
clay = 1/10
–
1/8) and very good
(SOC/clay >1/8) soil conditions.
Treatments were applied in two
cycles (1965 to 1972 and 1979 to
1987); peat and green manure
(GM) treatments were replaced
by grass ley for the second cycle.
Fert. 1 = (PKMg)
straw plus
P; Fert. 2 = (PKMg)
FYM
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Inorganic fertiliser-only treatments showed a general
trend of decreasing SOC/clay ratio and consistently occu-
pied the degraded class.
4
|
D I S C U S S I O N
4.1
|
Variation in SOC/clay ratio with
land use and soil type
In agreement with Dexter et al. (2008) and Johannes
et al. (2017), arable soils had a larger proportion of sites
with SOC/clay ratios below 1/10, and permanent grass-
land soils had a larger proportion above 1/10. Dexter
et al. (2008) did not consider soil group or structural condi-
tion of the soils in their study, and Johannes et al. (2017)
chose only one soil type. Based on the agreement of their
results with previous studies on the importance of the
SOC/clay = 1/10, Johannes et al. (2017) suggested it
should apply to a range of soils. Our finding that few
grassland and woodland sites had SOC/clay <1/13 sup-
ports the use of SOC/clay = 1/13 as an indicative thresh-
old for degradation, as grassland and woodland soils are
not generally subject to major disturbance and are close to
semi-natural systems. Our analysis shows that many ara-
ble soils were depleted in SOC compared with the more
natural systems. Ley grassland soils were intermediate
between arable and permanent grassland soils. The NSI
survey did not include information on the length of leys
nor the time under ley at sampling, but typically this is
between 3 and 8 years. Some proportion of arable sites will
have been part of ley rotations at the time of sampling.
The large variation of SOC/clay ratio within each
land use and soil group demonstrates that clay content is
not the only determinant of SOC dynamics, especially
considering that land-use history before the sampling will
have big effects too. As discussed above, despite the scat-
ter, the thresholds show differences between soils under
different land managements.
The variance of the SOC/clay ratio explained by ran-
dom forest analysis was similar to the variance of SOC
content explained by Verheijen et al. (2005) with stepwise
general regression modelling, using similarly derived pre-
cipitation data, and the same soil dataset (although a dif-
ferent subset). We would expect the variance explained to
increase with more specific measures of land manage-
ment within land-use classes (crop type, residue treat-
ment, land-use history and, for grassland systems,
grazing management). Interpolated precipitation data is
another estimation which could be improved; however,
this is what is generally available at this scale.
The effect of the major soil group on the SOC/clay
ratio suggests some consideration should be given to soil
type, as highlighted by Johannes et al. (2017). Comparing
the variation in the SOC/clay ratio between major soil
groups, similar variation and medians were found for
lithomorphic, brown, gley and man-made soils. The ten-
dency for a higher SOC/clay ratio of podzolic soils might
be attributed to concentrated organic horizons in the top-
soil. The tendency for lower SOC/clay ratios of Pelosols
might be attributed to higher clay contents combined with
a higher proportion (62%) of them being arable compared
with most major soil groups. Although acidic soils had a
tendency for higher SOC/clay ratios, there appeared to be
little relationship between pH and SOC/clay ratio in agri-
culturally productive pH ranges (circa. pH = 5.5 to 7).
4.2
|
Significance of the threshold values
The fact that the empirical threshold values found by
Johannes et al. (2017) for Swiss soils also hold for the
wide range of soils and land uses across England and
Wales in our study suggests that they have some funda-
mental basis, and that they may apply in soils in similar
climate zones across Europe. An association of soil struc-
tural quality with the SOC/clay = 1/10 ratio was expected
from
physicochemical
considerations
(de
Jonge
et al., 2009; Jensen et al., 2019). Intuitively, there will be
some minimum range of SOC/clay ratio below which soil
structure is impaired, and some maximum range above
which the capacity of soil clays of given mineralogy to
bind SOC is exceeded. However, there are no obvious rea-
sons why the precise threshold values indicated by our
and the Swiss study should be absolute.
The observed decrease in soil structural quality with
decreasing SOC/clay ratio was statistically significant,
although there was overlap of the boxplots of SOC/clay
ratio between structure classes. Our analysis was limited
by the quality of the available data on structure. This was
based on the scheme defined for the Agricultural Land
Classification of England and Wales, which includes a
measure of friability. Because friability was not recorded
in the NSI, we had to estimate structural quality without
it, introducing error.
The mechanistic link between structural quality and
SOC/clay ratio should reduce errors due to cross-correla-
tion with spatial and temporal variations in the data. We
found, as did Verheijen et al. (2005), that SOC content
tended to decrease with decreasing precipitation across
England and Wales, partly in interaction with land use.
However, low SOC/clay ratios were not limited to partic-
ular combinations of land use and precipitation; there-
fore, we would not consider precipitation to limit the
SOC/clay ratio in this dataset and geographical range.
Land management was shown to affect proportions of
PROUT
ET AL
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9
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very
good
and
degraded
soils
under
dry
(< 650 mm year
−
1
) climate conditions. So, SOC/clay
ratios of at least 1/10 should be attainable in such soils.
4.3
|
Practical usefulness of the index
The SOC/clay index is a simple measure to evaluate the
SOC status of any given soil in England and Wales, inde-
pendent of the land use. It will therefore be meaningful
for experts and non-experts and has consequences for
many soil functions beyond agricultural uses. It could
allow farmers to identify degraded soils on their farms
and adjust their management accordingly. It could also
be used to monitor and understand the state of soils at a
national scale to inform decision making and policy.
Application of the index to data from the long-term
Woburn experiment showed it was consistent with expec-
tations, with an improving index in treatments favouring
organic matter accumulation, and a deteriorating index
in soil-degrading treatments. This illustrates the time
taken for the various contrasting managements to change
SOC and the index. The soil in the Woburn experiment is
a sandy loam; the results show that the index can be used
for soils with low clay content, despite the narrowing of
the SOC/clay thresholds with decreasing clay content,
and the relatively small changes in SOC content between
the treatments. It should be noted that, to be useful for
monitoring purposes, measurements of SOC and clay
over time and between sites need to be consistent.
It would be interesting to look at other longer-term
studies to explore a wider range of clay contents, treat-
ments and time periods. Saturation concepts suggest that
a soil closer to steady state or saturation limit should
accumulate carbon more slowly than one further from
saturation (Six et al., 2002; Stewart et al., 2007). Hence,
whether sites with lower index values (higher degrada-
tion) improve more quickly could be tested.
5
|
C O N C L U S I O N S
An index of soil organic matter with threshold SOC/clay
ratios of 1/8, 1/10 and 1/13 satisfactorily separates the
soils of England and Wales into very good, good, moder-
ate and degraded classes of SOC content and physical
structure condition. In agreement with previous publica-
tions, grassland and woodland soils mostly had an SOC/
clay ratio > 1/10, indicating that their SOC contents are
close to or above the capacity for protection of SOC by
interaction with clay particles. That these more natural
systems tend to have an SOC/clay ratio > 1/10 supports
this as a suitable threshold for good condition.
Arable soils and soils receiving less annual rainfall
were most likely to be physically degraded, although
rainfall was a less important factor determining the SOC/
clay ratio. Very-good status soils (SOC/clay >1/8)
occurred in low rainfall areas, even under arable manage-
ment, suggesting that rainfall does not fundamentally
limit SOC content in this climate.
The SOC/clay ratio index allows the evaluation of soils
on a scale from degraded to good soil conditions. It there-
fore gives a ready metric for communication to experts and
non-experts, enabling users to adjust their practices and
decision makers to develop adequate policies. An SOC/clay
ratio greater than 1/10 should be achievable for all managed
soils of different textures. Many arable soils in England
and Wales evidently have a substantial SOC deficit,
suggesting a significant opportunity to increase SOC storage
to both improve soil conditions and sequester carbon.
Being based on two routinely measured soil properties,
the index provides a suitable means of monitoring SOC at
national, regional or sub-regional scales. Given the wide
range of soils and land uses across England and Wales in
the dataset used to test the index and agreement with liter-
ature using French, Polish and Swiss soils, it should apply
to other European soils in similar climate zones.
A C K N O W L E D G E M E N T S
We thank John Hollis, formerly of Cranfield University,
for his advice on the structural quality assessment using
the NSI dataset. We thank the e-RA data managers and
Rothamsted Research for access to data and information
on the Rothamsted Long-term Experiments. We also
thank Paul Poulton and Andy Macdonald for their help-
ful comments and additional information. Mapping
derived from soils data © Cranfield University (NSRI)
and for the Controller of HMSO 2019.
D A T A S H A R I N G A N D D A T A
A C C E S S I B I L I T Y S T A T E M E N T
The NSI dataset is held by Cranfield University and
accessed via LandIS (www.landis.org.uk). UKCP09 data are
available from the Centre for Environmental Data Analysis
(CEDA) archive (http://catalogue.ceda.ac.uk/uuid/94f757d
9b28846b5ac810a277a916fa7).
Data
for
the
Woburn
Organic Manuring experiment can be obtained via the
Electronic Rothamsted Archive (era.rothamsted.ac.uk).
C O N F L I C T O F I N T E R E S T
The authors have no conflicts of interest related to the
work presented in this manuscript.
A U T H O R C O N T R I B U T I O N S
Study concept and design: all the authors. Analysis and
interpretation of data: JMP, KDS and SMH. Drafting of
10
PROUT
ET AL
.
Page 12
the manuscript: JMP. Critical revision of the manuscript
for important intellectual content: GJDK, SPM, SMH and
KDS. Statistical analysis: JMP and KDS.
O R C I D
Jonah M. Prout
https://orcid.org/0000-0003-3448-0818
Guy J. D. Kirk
https://orcid.org/0000-0002-7739-9772
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S U P P O R T I N G I N F O R M A T I O N
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article:
Prout JM, Shepherd KD,
McGrath SP, Kirk GJD, Haefele SM. What is a
good level of soil organic matter? An index based
on organic carbon to clay ratio.
Eur J Soil Sci
. 2020;
1
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11.
https://doi.org/10.1111/ejss.13012
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