Page 1
European Journal of Soil Science
, January 2018,
69
, 39–47
doi: 10.1111/ejss.12475
Limited effect of organic matter on soil available water
capacity
B . M i n a s n y
& A . B . M c b r a t n e y
Sydney Institute of Agriculture, The University of Sydney, 1 Central Avenue, 2015 Eveleigh, New South Wales Australia
Summary
Soil water-holding capacity is an important component of the water and energy balances of the terrestrial
biosphere. It controls the rate of evapotranspiration, and is a key to crop production. It is widely accepted that
the available water capacity in soil can be improved by increasing organic matter content. However, the increase
in amount of water that is available to plants with an increase in organic matter is still uncertain and may be
overestimated. To clarify this issue, we carried out a meta-analysis from 60 published studies and analysed large
databases (more than 50 000 measurements globally) to seek relations between organic carbon (OC) and water
content at saturation, field capacity, wilting point and available water capacity. We show that the increase in
organic carbon in soil has a small effect on soil water content. A 1% mass increase in soil OC (or 10 g C kg
−
1
soil mineral), on average, increases water content at saturation, field capacity, wilting point and available water
capacity by: 2.95, 1.61, 0.17 and 1.16 mm H
2
O 100 mm soil
−
1
, respectively. The increase is larger in sandy soils,
followed by loams and is least in clays. Overall the increase in available water capacity is very small; 75% of
the studies reported had values between 0.7 and 2 mm 100 mm
−
1
with an increase of 10 g C kg
−
1
soil. Compared
with reported annual rates of carbon sequestration after the adoption of conservation agricultural systems, the
effect on soil available water is negligible. Thus, arguments for sequestering carbon to increase water storage are
questionable. Conversely, global warming may cause losses in soil carbon, but the effects on soil water storage
and its consequent impact on hydrological cycling might be less than thought previously.
Highlights
•
We investigated how available water capacity can be increased with a 1% increase in soil organic carbon.
•
We analysed data from 60 published studies and global databases with more than 50 000 measurements.
•
The increase in organic carbon in soil has a small effect on soil water retention.
•
A 1% mass increase in soil OC on average increased available water capacity by 1.16%, volumetrically.
Introduction
Soil water-holding capacity is one of the most important soil factors
for plant growth, influencing carbon allocation, nutrient cycling
and the rate of photosynthesis. Studies have shown that in many
parts of the world, soil water-holding capacity controls yield and its
volatility (Yang
et al
., 2014; Williams
et al
., 2016), and increasing
soil water-holding capacity by the addition of organic matter (OM)
was suggested as a means to buffer yields against future variable
weather conditions. In addition, sequestering carbon in the soil
by the addition of OM has been reported to contribute to the
mitigation of climate change (Lal, 2004, 2006; Williams
et al
.,
2016; Minasny
et al
., 2017). Furthermore, enhancing soil organic
Correspondence: B. Minasny. E-mail: budiman.minasny@sydney.edu.au
This paper was invited for the 70th Anniversary of the British Society of
Soil Science and was received on 8 March 2017; revised version accepted
12 July 2017
carbon (OC) can improve soil quality, for example by increasing
nutrient retention, promoting aggregation and improvement in soil
structure, and enhancing soil biotic activity and improvements in
soil moisture and temperature regimes (Lal, 2004). Increasing soil
OC is also suggested as a means of drought management (Lal,
2004).
The positive effect of OC on soil water retention has been much
studied (Hudson, 1994; Rawls
et al
., 2003) and widely promoted,
but there is still no clear consensus on its quantitative effect.
Some studies have reported positive effects (Bauer & Black, 1992),
whereas others showed a limited effect (Bell & Van Keulen, 1995).
Some reports even present huge effects, for example a report by
Maynard (2000) cited that the addition of leaf compost to soil in
an inner city garden increased the soil’s water-holding capacity by
7.5 mm 100 mm
−
1
soil. The validity of such an estimate is yet to be
verified.
© 2017 British Society of Soil Science
39
Page 2
40
B. Minasny and A. B. Mcbratney
Given the paucity of such quantitative evidence-based infor-
mation, we wished to answer the question: how many millime-
tres increase in available water capacity can be expected with an
increase in soil organic carbon (OC) of 10 g kg
−
1
mineral soil (or
1%)? We carried out a meta-analysis on the reported quantity of
water retention at saturation (SAT), field capacity (FC) and wilting
point (WP) in relation to an increase in OC content. Consequently,
we calculated the amount of water that is available to plants (AWC)
as the difference between FC and WP. We also analysed various
local, national and global soil databases and derived empirical rela-
tions that enable direct estimation of the effect of OC. We calculated
the change in water content (expressed in either per cent mass or
per cent volume,
Δ
𝜃
) with an increase in OC (expressed in per cent
mass,
Δ
(OC)), or
Δ
𝜃
/
Δ
(OC). Finally, we compiled this information
to determine the best evidence-based estimates of OC effects.
Methods
Literature review
We conducted a literature review for studies that relate water
retention and organic matter content; in particular, water content
at saturation (SAT), field capacity (FC) and wilting point (WP). In
some studies, SAT was assumed to be the same as total porosity.
Field capacity is taken as water content at
−
5 kPa for studies from
the UK,
−
10 kPa for studies from Australia, and
−
33 kPa for studies
from the USA. There are also field-measured values equivalent
to FC known as drained upper limit (DUL), the water content
at which drainage from a pre-wetted soil had become negligible
(Ratliff
et al
., 1983). Wilting point is measured more consistently
at
−
1500 kPa. Wilting point in the field is also known as crop lower
limit (CLL), the soil water content at which plants were almost dead
or dormant as a result of the water deficit. Available water capacity
(AWC) is the difference between FC and WP. We recognize that
there are differences in the definition of field capacity from different
studies that may affect the calculation of AWC.
The criteria for the inclusion of material for this study were:
•
Studies that examined the effect of application of organic amend-
ments (e.g. sludge, compost, mulch, manure or management
[e.g. no till]) on soil hydraulic properties (e.g. Kladivko & Nel-
son, 1979; Jordán
et al
., 2010). The studies needed to report OM
or OC content with one of the following water retention proper-
ties: SAT, FC, WP or AWC.
•
Studies that compared areas with different soil OC contents
(e.g. Bauer, 1974; Ankenbauer & Loheide, 2017).
•
Pedotransfer function (PTF) studies, which derived empirical
relations between soil water content at different potentials as a
function of soil texture and OC (e.g. Gupta & Larson, 1979;
Rawls
et al
., 2003).
We differentiated studies that measured gravimetric water content
(
w
in per cent mass g 100 g
−
1
) and those that measured volumetric
water content (
𝜃
in per cent volume mm 100 mm
−
1
). Reported
organic matter content was converted to organic carbon (OC) using
the van Bemmelen factor of 0.58. We then calculated the change
in water content (expressed in either per cent mass or per cent
volume) with an increase in OC (expressed in per cent mass), or
Δ
𝜃
/
Δ
(OC).
For studies on the application of organic amendments to the soil,
we selected only studies that showed a difference in treatment of at
least 10 g kg
−
1
OC. We then fitted a linear regression between OC
and
𝜃
and estimated the slope of the line as
Δ
𝜃
/
Δ
(OC).
For databases with different soil texture and mineralogy, the
following linear relation was established to include the effect of soil
texture:
𝜃
=
b
0
+
b
1
Clay
+
b
2
Sand
+
b
3
OC
.
(1)
The response of OC to
𝜃
was estimated from the coefficient
b
3
.
For published pedotransfer functions where the relationships are
linear, similar to Equation (1) above (e.g. Gupta & Larson, 1979),
we took the regression coefficient of OC as estimates of
Δ
𝜃
/
Δ
(OC).
For published PTFs with complex nonlinear equations (e.g. Rawls
et al
., 2003) or parametric PTFs (e.g. Wösten
et al
., 1999), which
predicted parameters of a water retention function, we calculated
the increase in
𝜃
at SAT, FC and WP with an increase in OC from
0.5 to 1.5% for each percentage of sand and clay content of a texture
triangle. This increase in OC was used because it represents the
most reasonable change in OC in agricultural soil from the addition
of organic matter.
We also grouped the response of OC based on soil texture into
three groups, following the USDA soil texture classes:
•
Coarse texture (clay 0–35%, sand 44–100%), which includes:
sand, loamy sand, sandy clay loam and sandy loam.
•
Medium texture (clay 0–40%, sand 0–52%), which includes:
loam, clay loam, silt, silt loam and silty clay loam.
•
Fine texture (clay 36–100%, sand 0–64%), which includes:
clays, sandy clay and silty clay.
Here we define sand according to the USDA classification as
particles between 50 and 2000
μ
m.
Analysis of soil databases
We took advantage of databases that have been compiled for
various purposes, such as national collation for mapping, or for
deriving PTFs. We analysed local, national and international data
that contain measured water retention, particle-size distribution and
OC content. These include:
•
The International Geosphere-Biosphere Programme Data and
Information System (IGBP-DIS) soil database (Tempel
et al
.,
1996), which contains a uniform set of global soil data for the
development of pedotransfer functions. The dataset contains data
from ISRIC and USDA NRCS. To avoid overlap with the NCSS
data described below, which are also used in this study, we used
only data from ISRIC. Field capacity was measured at
−
33 kPa
gravimetrically.
© 2017 British Society of Soil Science,
European Journal of Soil Science
,
69
, 39–47
13652389, 2018, 1, Downloaded from https://bsssjournals.onlinelibrary.wiley.com/doi/10.1111/ejss.12475 by Hayley Peter-Contesse - Oregon State University , Wiley Online Library on [19/10/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Page 3
Limited effect of organic matter on soil water retention
41
•
The UNSODA (UNsaturated SOil hydraulic Database) (Nemes
et al
., 2001), which contains measured soil water retention,
texture and OC content of soil samples from around the world.
We used samples from undisturbed cores only, with FC set at a
volumetric water content of
−
33 kPa.
•
GRIZZLY (Haverkamp
et al
., 1997), which is a database that
contains 660 soil samples collected from laboratories and field
experiments in different countries. Field capacity (FC) was set at
a volumetric water content of
−
33 kPa.
•
Ritchie
et al
. (1987) compiled a database of field-measured soil
water limits and laboratory-measured properties from the USA.
Drained upper limit (DUL), equivalent to FC, has the largest
measured water content of a soil after it had been wetted and
allowed to drain until drainage rate is negligible. Crop lower limit
(CLL) represents the smallest water content of a soil after plants
have stopped extracting water (Ratliff
et al
., 1983).
•
The APSRU (Agricultural Production Systems Research Unit)
project, which contains measurements of DUL and CLL through-
out Australia, including the southwest of Australia, Victoria,
New South Wales, Queensland and some areas in Tasmania (Dal-
gliesh
et al
., 2006).
•
Forrest
et al
. (1985) contains measurement of soils from eastern
Australia (mainly in southern Queensland to southern Australia),
where FC was measured from undisturbed cores at the potential
of
−
10 kPa.
•
Prebble (1970) contains soil profile measurements from Queens-
land, where water content at a potential of
−
33 kPa was used to
represent FC.
•
Geeves
et al
. (1995) includes soil data from wheat-growing areas
in southern NSW and northern Victoria, Australia; water content
at a potential of
−
33 kPa was used to represent FC.
•
The National Soil Site Collation or NSSC (Searle, 2014) is
a compilation of Australian soil data collected by research
and government agencies. The NSSC database contains soil
measurements from survey and research projects throughout the
country. We selected two sets of data from this database, which
contains measured water retention, soil texture and OC.
For each database, we fitted a linear model as in Equation (1),
and we calculated the response of OC to
𝜃
from the slope of the
regression. For databases with a large variation in soil texture, we
fitted a separate regression for each of the soil texture groups.
The neural network model
To model and understand the effect of OC on water retention more
consistently, we derived point PTFs to predict water content at satu-
ration (SAT),
−
33 kPa (FC) and
−
1500 kPa (WP) from the National
Cooperative Soil Characterization (NCSS) database. The NCSS is
a national soil characterization database available from the USDA
at the Natural Resources Conservation Service (Soil Survey Staff,
1995) that contains more than 100 000 samples with measurements
of soil texture, organic carbon content, and water retention at sat-
uration (SAT),
−
33 kPa (FC) and
−
1500 kPa (WP) from the USA.
We filtered the data for outliers, and excluded histosols and soil with
andic properties because they behaved differently from mineral soil.
Water content was measured gravimetrically and was converted to
a volumetric basis by the corresponding bulk density. We used only
data with OC content less than 10%, which is usually more com-
mon in agricultural soil. In addition, we split the data into topsoil
(observed depth of 0–30 cm) and subsoil (30–100 cm); this resulted
in 27 638 data.
We fitted a feed-forward neural network model (Hornik
et al
.,
1989) to the data. The network has five hidden units with inputs:
sand, clay, OC and output of water content at SAT, FC and WP. To
account for variation in the data we ran a bootstrap, a sampling with
replacement method (Efron, 1979) with 100 realizations. Therefore,
we had 100 neural network models and we used the average of the
models as a prediction.
Results
Gravimetric water content
We needed to differentiate between studies that measured gravimet-
ric and volumetric water content. We found 25 studies in which
gravimetric water content (
w
) had been measured with estimates
of
Δ
w
/
Δ
(OC) (Table S1, Supporting Information). The data in
Figure 1 show that the average increases in gravimetric water con-
tent (g 100 g
−
1
) with an addition of 10 g kg
−
1
(or 1%) OC are: 4.6,
3.7, 1.4 and 2.1 for saturation, FC, WP and AWC, respectively.
Table 1 lists the summary of the analysis. This result is in contrast
with the early conclusions of Riley (1981) and Jong (1983), who
indicated that increased OC contents resulted in a shift of the whole
water retention curve, or increased water contents at all potentials.
With the van Bemmelen factor set to 0.58, an increase of 1% OM
would increase the saturation level by 2.7% only, of which 1.2% is
considered to be available to plants.
Volumetric water content
For volumetric water content (
𝜃
), we found 19 studies that derived
empirical relations between water retention and OC (PTF studies)
and 16 studies on the effect of OM addition on water retention, and
analysed 12 datasets to estimate
Δ
𝜃
/
Δ
(OC) (Table S2, Supporting
Information). The results for volumetric water content (Figure 2)
are similar to those for gravimetric water content, and we can see
that the average increases in volumetric water content (
𝜃
) with
an addition of 1% (10 g kg
−
1
) OC are: 2.9, 1.6, 0.17 and 1.2 mm
100 mm
−
1
for SAT, FC, WP and AWC, respectively (Table 2).
The increases in
𝜃
when we considered all textures (general
relations) are modest. However, if we make a distinction based on
texture, the increase is much larger in coarse-textured soil, followed
by medium-textured soil and least in fine-textured soil. In general,
the largest increase in volumetric water content is at saturation or
total porosity, followed by FC and a small increase in WP (Table 2).
The increases in
𝜃
in coarse-textured soil for SAT, FC and
WP are: 4.6, 2.3 and 0.9 mm 100 mm
−
1
, respectively. For
medium-textured soil
𝜃
increased by 3.6, 2.1 and 0.7, respectively,
© 2017 British Society of Soil Science,
European Journal of Soil Science
,
69
, 39–47
13652389, 2018, 1, Downloaded from https://bsssjournals.onlinelibrary.wiley.com/doi/10.1111/ejss.12475 by Hayley Peter-Contesse - Oregon State University , Wiley Online Library on [19/10/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Page 4
42
B. Minasny and A. B. Mcbratney
Δ
w
/Δ(OC)
Figure 1
A violin plot of reported
Δ
w
/
Δ
(OC), rate of increase in gravimetric water content (g 100 g
−
1
) with a 10 g kg
−
1
increase in OC. A violin plot is similar
to a boxplot, where each side represents the density plot of the variable. The point in the middle of the violin represents the mean of the data. OC, organic
carbon; SAT, saturation; FC, field capacity; WP, wilting point; AWC, available water capacity.
Table 1
The rate of gravimetric water content increase (g H
2
O 100 g
−
1
soil)
with an increase of 10 g C kg
−
1
mineral soil.
SAT
FC
WP
AWC
Mean
4
.
61
3
.
71
1
.
36
2
.
13
Standard deviation
3
.
43
2
.
93
0
.
77
2
.
35
n
9
32
33
30
SAT, saturation; FC, field capacity; WP, wilting point; AWC, available water
capacity;
n
, number of samples.
and for fine-textured soil by 3.2, 1.3 and 0.5 mm 100 mm
−
1
. The
increase in AWC is modest, with an average of 1.9 mm 100 mm
−
1
in coarse-textured soil to 1.4 mm 100 mm
−
1
in fine-textured soil.
The average values are similar to those of Hudson (1994), who
reported that
Δ
(AWC)/
Δ
(OC) was 1.28 for sands and 2.51 for
silt loam. Seventy-five per cent of the studies reported increases
in AWC of between 0.7 and 2.1 mm 100 mm
−
1
with a 10 g kg
−
1
increase in soil OC. Negative values of
Δ
𝜃
/
Δ
(OC) also occur in
25% of the data for FC and 2.5% for WP and AWC, especially in
fine-textured soils.
Discussion
The results from various studies and databases showed consistently
that the increase in water content with the addition of organic
matter is greatest for saturation (potential of 0 kPa), followed by
Table 2
A summary of the rate of increase in volumetric water content
(mm H
2
O 100 mm
−
1
soil) with an increase of 10 g C kg
−
1
mineral soil.
Texture
SAT
FC
WP
AWC
General
n
14
23
20
19
Mean
2
.
95
1
.
61
0
.
17
1
.
16
SD
1
.
53
1
.
54
1
.
55
1
.
01
Coarse
n
12
19
18
19
Mean
4
.
59
2
.
33
0
.
86
1
.
94
SD
2
.
17
2
.
96
1
.
74
1
.
28
Medium
n
12
22
21
21
Mean
3
.
59
2
.
11
0
.
68
1
.
79
SD
1
.
65
1
.
74
1
.
19
1
.
5
Fine
n
9
12
11
11
Mean
3
.
23
1
.
28
0
.
54
1
.
41
SD
2
.
09
2
.
05
1
.
74
1
.
95
SAT, saturation; FC, field capacity; WP, wilting point; AWC, available water
capacity; SD, standard deviation;
n
, number of samples.
field capacity (
−
10 or
−
33 kPa) and wilting point (
−
1500 kPa).
Organic matter can increase soil’s water retention directly and
indirectly. Many reports, either from laboratory or field studies,
have shown consistently that bulk density decreases (and thus
porosity increases) with increasing OM, irrespective of texture
(Adams, 1973). A hypothesis of this increase is that it results from
dilution of the soil matrix with the less dense organic material
(Soane, 1990; Kay, 1998). The mixture model of Adams (1973)
© 2017 British Society of Soil Science,
European Journal of Soil Science
,
69
, 39–47
13652389, 2018, 1, Downloaded from https://bsssjournals.onlinelibrary.wiley.com/doi/10.1111/ejss.12475 by Hayley Peter-Contesse - Oregon State University , Wiley Online Library on [19/10/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Page 5
Limited effect of organic matter on soil water retention
43
(OC)
Figure 2
A violin plot of reported
Δ
𝜃
/
Δ
(OC), rate of increase in volumetric water content (mm 100 mm
−
1
) with a 10 g kg
−
1
increase in OC grouped by texture
class. A violin plot is similar to a boxplot, where each side represents the density plot of the variable. The point in the middle of the violin represents the mean
of the data. OC, organic carbon; SAT, saturation; FC, field capacity; WP, wilting point; AWC, available water capacity.
estimated that the total porosity increases 2.1 mm 100 mm
−
1
with
1% (10 g kg
−
1
) increase in OC. This model implied that organic
matter does not interact with the mineral framework (Honeysett &
Ratkowsky, 1989).
Other proposed mechanisms include improvement of aggregation
and structure, and the large surface area of organic matter (Sharma
& De Datta, 1994; Franzluebbers, 2002). The formation of stable
soil aggregates could create larger volumes of macropores, meso-
pores and micropores (Kay, 1998). Giusquiani
et al
. (1995) show
by an examination of thin sections that plots treated with composts
had more elongated pores (
>
50
μ
m) than the control. There is also
a hypothesis that OM increases a soil’s surface area; however, the
small increase in water content at WP (equivalent to a pore diam-
eter of 0.2
μ
m), which is controlled mainly by adsorption forces,
does not seem to support this idea.
Experiments on the addition of organic matter to soil do not
always show positive results for an increase in water retention
at field capacity and wilting point. Earlier research suggested
that OM affected the AWC for coarse-textured soils only with
less than 20% clay content (Jamison & Kroth, 1958; MacRae
& Mehuys, 1985). There is another possibility that increased
aggregation does not necessarily translate into an increase in AWC
because increased aggregation can decrease the micropore volume
(MacRae & Mehuys, 1985). Such speculation is yet to be verified,
however.
Despite all these inconsistent findings and controversy, our
meta-analysis from many studies and empirical relations derived
from a large number of samples show consistently that there is a
positive effect of OC on water retention; however, this effect is
mainly on water content at saturation and decreases with increasing
water suction. The large effect of OC on porosity and bulk density
leads to the assumption that its effect on soil water retention can be
substituted by bulk density in pedotransfer functions (Zacharias &
Wessolek, 2007).
To visualize the effect of OC, we developed a neural network
model using a large database (
n
=
27 638) from the USDA NRCS,
which predicts
𝜃
at SAT, FC and WP from inputs of clay, sand and
OC. The results were consistent with our meta-analysis showing
that the increase is largest for saturation, followed by FC and WP
(Table 2). Interestingly, our model revealed that the increase in
water retention
Δ
𝜃
/
Δ
(OC) for subsoil was larger than for topsoil.
The average
Δ
𝜃
/
Δ
(OC) values for SAT, FC and WP for topsoil
were 2.68, 1.04 and 0.27 mm 100 mm
−
1
, respectively, and for
subsoil 5.41, 1.36 and 0.55 mm 100 mm
−
1
, respectively. A possible
explanation for the more pronounced effect on subsoil is that
topsoil values are affected by management and cultivation practices.
Nevertheless, the larger increase for both FC and WP made the
increase in AWC smaller in subsoil.
Figure 3 shows the response of water retention with increasing
OC. As expected, the largest increase is in SAT, followed by FC
© 2017 British Society of Soil Science,
European Journal of Soil Science
,
69
, 39–47
13652389, 2018, 1, Downloaded from https://bsssjournals.onlinelibrary.wiley.com/doi/10.1111/ejss.12475 by Hayley Peter-Contesse - Oregon State University , Wiley Online Library on [19/10/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Page 6
44
B. Minasny and A. B. Mcbratney
0
10
20
30
40
50
60
70
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
θ
/mm 100mm
-1
Clay Content/g 100g
-1
Saturation
1% OC
-33 kPa(FC)
2% OC
-1500 kPa(WP)
3% OC
4% OC
5% OC
Figure 3
Modelled response of water content at SAT, FC and WP with increasing clay content (on the
x
-axis) and OC content (lines on the graph). The line
for each water content represents OC at 1, 2, 3, 4 and 5%, successively. The model was trained on the NSSC dataset. SAT, saturation; FC, field capacity, WP,
wilting point; OC, organic carbon.
and WP. The largest increase is from 0 to 1% OC, and the increase
tends to become smaller with increasing OC content. There are
diminishing returns associated with adding more and more organic
carbon. The increase in water content is significant for sandy soils
with 0 to 20% clay content, and becomes smaller from 20 to 40%
clay and insignificant when clay is greater than 40%. As both FC
and WP increase with increasing OC content, the largest gain in
AWC occurs in sand–sandy clay loams (Figure 4). The result for
FC is similar to those presented by Rawls
et al
. (2003), except that
our result did not show a negative effect with an increase in OC for
clay content larger than 60%.
We are aware of the limitations of this study, and we list some of
those to be considered.
•
The analysis used data with a definition of field capacity based
on a potential of either
−
10 or
−
33 kPa. These differences
might affect the results for AWC, and in addition field capac-
ity could be more dynamic depending on a soil’s internal
drainage (Assouline & Or, 2014). Nevertheless, our analysis also
included field-determined FC data, the DLL. However, such data
(e.g. Ritchie
et al
. (1987)) did not show the significance of OC
for predicting DUL.
•
Our analysis was based on a compilation of studies with the
addition of or amendment with organic matter, management
effects and soils with varying organic matter contents. Treat-
ment studies also vary in terms of short- and long-term addi-
tions of different organic amendments and management. Soils
with different mineralogy might have different responses to
organic carbon contents. The types of organic matter can also
be an important factor in determining aggregation. Neverthe-
less, results from controlled studies of OM addition (such as
Daynes
et al
., 2013) showed similar results to PTF studies
(e.g. Hudson, 1994). We analysed the difference between esti-
mates from PTFs and treatment-effect studies and showed that,
except for FC, both groups of data showed similar median values
(Table S3, Supporting Information). The
Δ
(FC)/
Δ
(OC) for treat-
ment studies had a slightly smaller median value (1.32) than that
for PTF studies (2.01).
•
This study does not explicitly consider the effect of plants (such
as a cover crop) on soil water storage (e.g. Basche
et al
., 2016).
© 2017 British Society of Soil Science,
European Journal of Soil Science
,
69
, 39–47
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Page 7
Limited effect of organic matter on soil water retention
45
SAT
FC
WP
AWC
mm 100mm
-1
mm 100mm
-1
Sand/%
Sand/%
Sand/%
Sand/%
Clay/%
Clay/%
Clay/%
Clay/%
Figure 4
Modelled response of the rate of increase in water content (mm 100 mm
−
1
) at SAT, FC, WP and AWC with an increase in OC content from 5 to
15 g kg
−
1
as a function of sand and clay contents. The model was trained on the NSSC dataset. Sand is defined as particles between 50 and 2000
μ
m. SAT,
saturation; FC, field capacity: WP, wilting point, AWC, available water capacity; OC, organic carbon.
Daynes
et al
. (2013) demonstrated that including plants further
enhanced the effect of OM on soil water retention.
•
Pedotransfer functions using clay, sand and OC contents
(Equation (1)) might not resolve all of the variation in water
retention. Structural effects such as compaction, biopores, other
cementing agents (such as Fe oxides and CaCO
3
) and shrinking
and swelling clays are not resolved by this model. We chose a
simple multiple linear model (Equation (1)) rather than more
complex data mining models because it is more straightforward
for deriving the effect of OC. The accuracy of the linear model
for various databases is also quite good (Table S2), with average
R
2
values increasing from SAT, FC and WP: 0.4, 0.6, 0.7,
respectively.
•
We used the van Bemmelen factor of 0.58 for converting between
OM and OC; the actual value might vary with texture.
These limitations do not detract from our conclusions.
Climate-smart farming practices such as conservation agricul-
ture have been advocated to build up soil carbon, and thus
improve water-holding capacity. However, the rate of soil carbon
sequestration and resulting increase in available water capacity
do not correspond. Application of OM to soil does not trans-
late directly to its build up in soil. Studies on the effect of best
management practices resulted in an annual sequestration rate of
0.1–1.0 Mg C ha
−
1
(Minasny
et al
., 2017) or 0.005–0.05% C for
topsoil (0–15 cm), which translates to a negligible annual increase
of 0.01–0.1 mm 100 mm
−
1
for a coarse-textured topsoil. The ben-
efits of conservation agriculture might be through indirect effects
of mulching that reduce evaporation and runoff, improved soil
aggregation and enhanced biogeochemical functioning. Studies
that showed a large increase in AWC applied organic matter at
© 2017 British Society of Soil Science,
European Journal of Soil Science
,
69
, 39–47
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Page 8
46
B. Minasny and A. B. Mcbratney
large rates, which are not applicable in real farming conditions.
For example, Jordán
et al
. (2010) showed an increase in AWC of
3.7 mm 100 mm
−
1
after application of 10 Mg ha
−
1
of mulch for an
increase in 40 g kg
−
1
of OC. Similarly, Ankenbauer & Loheide
(2017) showed an increase in AWC from 17 g to 37 mm 100 mm
−
1
in a meadow for an increase of 150 g kg
−
1
OC.
Although there is little effect of OM on AWC for plant uptake,
the increase in water content at SAT might be relevant for other
hydrological functions. The increased storage at SAT is useful
for the additional retention of water in soils during periods when
the soil is close to FC. This additional storage resulting from
an increase in OC content can improve the buffering capacity of
soils against flooding events. In addition, increased SAT is usually
accompanied by increased saturated hydraulic conductivity and
infiltration capacity.
Conclusions
From the large number of studies and databases we analysed,
we concluded that the effect of addition of OM on soil-available
water capacity was modest, with average values of between 1.4
and 1.9 mm 100 mm
−
1
per 10 g kg
−
1
increase in OC. Sandy soil
was more responsive to the increase in OC, whereas the effect on
clayey soil was almost negligible. The largest effect of OC was in
large pores, possibly from the formation of macroaggregates, and
its effect decreases with a decrease in size of pores.
The results also suggest that the gradual loss of organic matter
from soil would have a minimal effect on the hydrological cycle.
Global warming might cause a loss in soil carbon, but the effects
on soil water availability to plants and consequent effects on the
hydrological cycle might be less than we thought previously. The
negative feedback could be less serious. In addition, suggestions
that sequestering carbon would increase soil water storage are ques-
tionable. We have shown that even with the best scenario of farm
management practices, available water capacity cannot be increased
meaningfully. Nevertheless, storing carbon in the soil should still be
pursued for improving soil structure, atmospheric CO
2
attenuation
and nutrient cycling. Macropores created by organic matter can still
have important effects in increasing infiltration and gas transport.
Supporting Information
The following supporting information is available in the online
version of this article:
Table S1.
A compilation of studies with estimates of the increase
in grams of water (g H
2
O 100 g
−
1
) with an increase in 1 g OC per
100 g mineral soil.
Table S2.
A compilation of studies with estimates of the increase in
volume of water (mm H
2
O 100 mm
−
1
) with an increase in 1 g OC
per 100 g mineral soil.
Table S3.
The statistical distribution of
Δ
(
𝜃
)/
Δ
(OC) for studies
listed in Table S2 that compare amendment or treatment studies
and PTF studies for coarse, medium and fine-textured soils. The
median test examines whether the equality of the medians from the
two types of studies is different.
Acknowledgements
Budiman Minasny is funded by the Australia Research Council
Future Fellowship.
References
Adams, W. 1973. The effect of organic matter on the bulk and true densities
of some uncultivated podzolic soils.
Journal of Soil Science
,
24
, 10–17.
Ankenbauer, K. & Loheide, S.P. 2017. The effects of soil organic matter on
soil water retention and plant water use in a meadow of the Sierra Nevada,
CA.
Hydrological Processes
,
31
, 891–901.
Assouline, S. & Or, D. 2014. The concept of field capacity revisited: defining
intrinsic static and dynamic criteria for soil internal drainage dynamics.
Water Resources Research
,
50
, 4787–4802.
Basche, A.D., Kaspar, T.C., Archontoulis, S.V., Jaynes, D.B., Sauer, T.J.,
Parkin, T.B.
et al.
2016. Soil water improvements with the long-term use
of a winter rye cover crop.
Agricultural Water Management
,
172
, 40–50.
Bauer, A. 1974. Influence of soil organic matter on bulk density and
available water capacity of soils North Dakota Agricultural Experimental
Station.
Farm Research
,
31
, 44–52.
Bauer, A. & Black, A. 1992. Organic carbon effects on available water
capacity of three soil textural groups.
Soil Science Society of America
Journal
,
56
, 248–254.
Bell, M. & Van Keulen, H. 1995. Soil pedotransfer functions for four
Mexican soils.
Soil Science Society of America Journal
,
59
, 865–871.
Dalgliesh, N., Wockner, G. & Peake, A. 2006. Delivering soil water infor-
mation to growers and consultants. In:
Proceedings of the 13th Australian
Agronomy Conference
, pp. 10–14. Western Australia Australian Society
of Agronomy, Perth.
Daynes, C.N., Field, D.J., Saleeba, J.A., Cole, M.A. & McGee, P.A. 2013.
Development and stabilisation of soil structure via interactions between
organic matter, arbuscular mycorrhizal fungi and plant roots.
Soil Biology
& Biochemistry
,
57
, 683–694.
Efron, B. 1979. Bootstrap methods: another look at the jackknife.
Annals of
Statistics
,
7
, 1–26.
Forrest, J.A., Beatty, J., Hignett, C.T., Pickering, J. & Williams, R.G.P.
1985.
A Survey of the Physical Properties of Wheatland Soils in Eastern
Australia
. CSIRO. Division of Soils Report No 78, Commonwealth
Scientific and Industrial Research Organization, Melbourne.
Franzluebbers, A. 2002. Water infiltration and soil structure related to
organic matter and its stratification with depth.
Soil & Tillage Research
,
66
, 197–205.
Geeves, G.W., Cresswell, H.P., Murphy, B.W., Gessler, P.E., Chartres, C.J.
& Eittle, I.P. 1995.
The Physical, Chemical and Morphological Properties
of Soils in the Wheat-belt of Southern NSW and Northern Victoria
. NSW
Department of Conservation and Land Management/CSIRO. Division
Soils Occasional Report No 178, CSIRO, Glen Osmond.
Giusquiani, P., Pagliai, M., Gigliotti, G., Businelli, D. & Benetti, A. 1995.
Urban waste compost: effects on physical, chemical, and biochemical soil
properties.
Journal of Environmental Quality
,
24
, 175–182.
Gupta, S. & Larson, W. 1979. Estimating soil water retention characteristics
from particle size distribution, organic matter percent, and bulk density.
Water Resources Research
,
15
, 1633–1635.
Haverkamp, R., Zammit, C., Boubkraoui, F., Rajkai, K., Arrúe, J. &
Heckmann, N. 1997.
GRIZZLY, Grenoble Soil Catalogue: Soil Survey
of Field Data and Description of Particle-size, Soil Water Retention and
Hydraulic Conductivity Functions
. Laboratoire d’Etude des Transferts en
Hydrologie et Environnement (LTHE), Grenoble Cedex, 9.
© 2017 British Society of Soil Science,
European Journal of Soil Science
,
69
, 39–47
13652389, 2018, 1, Downloaded from https://bsssjournals.onlinelibrary.wiley.com/doi/10.1111/ejss.12475 by Hayley Peter-Contesse - Oregon State University , Wiley Online Library on [19/10/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Page 9
Limited effect of organic matter on soil water retention
47
Honeysett, J. & Ratkowsky, D. 1989. The use of ignition loss to estimate
bulk density of forest soils.
Journal of Soil Science
,
40
, 299–308.
Hornik, K., Stinchcombe, M. & White, H. 1989. Multilayer feedforward
networks are universal approximators.
Neural Networks
,
2
, 359–366.
Hudson, B.D. 1994. Soil organic matter and available water capacity.
Journal of Soil and Water Conservation
,
49
, 189–194.
Jamison, V. & Kroth, E. 1958. Available moisture storage capacity in
relation to textural composition and organic matter content of sev-
eral Missouri soils.
Soil Science Society of America Journal
,
22
,
189–192.
Jong, R.D. 1983. Soil water desorption curves estimated from limited data.
Canadian Journal of Soil Science
,
63
, 697–703.
Jordán, A., Zavala, L.M. & Gil, J. 2010. Effects of mulching on soil
physical properties and runoff under semi-arid conditions in southern
Spain.
Catena
,
81
, 77–85.
Kay, B. 1998. Soil structure and organic carbon: a review. In:
Soil Processes
and the Carbon Cycle
, (eds R. Lal, J.M. Kimble, R.F. Follett & B.A.
Stewart), pp. 169–197. CRC Press, Boca Raton, FL.
Kladivko, E.J. & Nelson, D.W. 1979. Changes in soil properties from
application of anaerobic sludge.
Journal of the Water Pollution Control
Federation
,
51
, 325–332.
Lal, R. 2004. Soil carbon sequestration impacts on global climate change
and food security.
Science
,
304
, 1623–1627.
Lal, R. 2006. Enhancing crop yields in the developing countries through
restoration of the soil organic carbon pool in agricultural lands.
Land
Degradation & Development
,
17
, 197–209.
MacRae, R. & Mehuys, G. 1985. The effect of green manuring on the
physical properties of temperate-area soils. In:
Advances in Soil Science
(ed. B.A. Stewart), pp. 71–94. Springer, Heidelberg.
Maynard,
A.A.
2000.
Compost:
The
Process
and
Research
.
Bulletin–Connecticut Agricultural Experiment Station No 966. Con-
necticut Agricultural Experiment Station, New Haven, CT.
Minasny, B., Malone, B.P., McBratney, A.B., Angers, D.A., Arrouays,
D., Chambers, A.
et al.
2017. Soil carbon 4 per mille.
Geoderma
,
292
,
59–86.
Nemes, A., Schaap, M., Leij, F. & Wösten, J. 2001. Description of the
unsaturated soil hydraulic database UNSODA version 2.0.
Journal of
Hydrology
,
251
, 151–162.
Prebble, R.E. 1970.
Physical Properties of 17 Soils from Queensland
.
CSIRO Division of Soils Technical Memo 10/70. CSIRO, Townsville.
Ratliff, L., Ritchie, J. & Cassel, D. 1983. Field-measured limits of soil water
availability as related to laboratory-measured properties.
Soil Science
Society of America Journal
,
47
, 770–775.
Rawls, W.J., Pachepsky, Y.A., Ritchie, J.C., Sobecki, T.M. & Bloodworth,
H. 2003. Effect of soil organic carbon on soil water retention.
Geoderma
,
116
, 61–76.
Riley, H. 1981.
Relationships Between Soil Moisture-Holding Properties
and Soil Texture, Organic Matter Content, and Bulk Density
. NASA
Technical Memorandum. NASA-TM-76674. NASA, Washington.
Ritchie, J., Ratliff, L. & Cassel, D. 1987.
Soil Laboratory Data, Field
Descriptions and Field Measured Soil Water Limits for Some Soils of the
United States
. ARS Technical Bulletin. USDA ARS, Temple, TX.
Searle, R. 2014. The Australian site data collation to support the Global-
SoilMap. In:
GlobalSoilMap: Basis of the Global Spatial Soil Information
System
(eds D. Arrouays, N. McKenzie, J. Hempel, A. DeForges & A.
McBratney), pp. 127–132. CRC Press, Boca Raton, FL.
Sharma, P.K. & De Datta, S.K. 1994. Rainwater utilization efficiency in
rain-fed lowland rice.
Advances in Agronomy
,
52
, 85–120.
Soane, B. 1990. The role of organic matter in soil compactibility: a review
of some practical aspects.
Soil & Tillage Research
,
16
, 179–201.
Soil Survey Staff 1995.
Natural Resources Soil Characterization and Profile
Description Data
. United States Department of Agriculture National
Resources Conservation Services (USDA-NRCS), Lincoln, NE.
Tempel, P., Batjes, N. & Van Engelen, V. 1996.
IGBP-DIS Soil Data Set for
Pedotransfer Function Development
. Working Paper No 96. International
Soil Reference and Information Centre (ISRIC), Wageningen.
Williams, A., Hunter, M.C., Kammerer, M., Kane, D.A., Jordan, N.R.,
Mortensen, D.A.
et al.
2016. Soil water holding capacity mitigates
downside risk and volatility in US rainfed maize: time to invest in soil
organic matter?
PLoS One
,
11
, e0160974.
Wösten, J., Lilly, A., Nemes, A. & Le Bas, C. 1999. Development and use
of a database of hydraulic properties of European soils.
Geoderma
,
90
,
169–185.
Yang, F., Zhang, G.-L., Yang, J.-L., Li, D.-C., Zhao, Y.-G., Liu, F.
et al.
2014. Organic matter controls of soil water retention in an alpine
grassland and its significance for hydrological processes.
Journal of
Hydrology Part D
,
519
, 3086–3093.
Zacharias, S. & Wessolek, G. 2007. Excluding organic matter content from
pedotransfer predictors of soil water retention.
Soil Science Society of
America Journal
,
71
, 43–50.
© 2017 British Society of Soil Science,
European Journal of Soil Science
,
69
, 39–47
13652389, 2018, 1, Downloaded from https://bsssjournals.onlinelibrary.wiley.com/doi/10.1111/ejss.12475 by Hayley Peter-Contesse - Oregon State University , Wiley Online Library on [19/10/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License