Interactions between soil texture, soil organic carbon and soil strength:
the specific draught S, measured on Broadbalk wheat experiment
ROTHAMSTED
RESEARCH
Chris Watts, Lawrence Clark, Paul Poulton, David Powlson, Andy Whitmore.
Soil Science Department, Rothamsted Research, Harpenden, Herts., AL5 2JQ
Fig 1 General aerial view of Broadbalk
Fig 2. Measurement of Specific draught, S
Introduction
Strain gauged frame
(to measure draught forces)
Strong soil significantly decreases crop growth and increases the
energy needed for tillage. Using the Broadbalk Wheat experiment
(established in 1843) at Rothamsted we set out to evaluate the
relative influence of soil texture (clay content) and soil organic
carbon (SOC) on soil strength. More specifically, we wished to know
if the small increases in SOC resulting from long-term mineral N
application have any effect on soil strength?
Doppler radar sensor
(forward speed)
Soil strength measurements
We assessed soil strength by measuring the draught force of a
plough. The specific draught, S (draught force per cross section of
area of soil disturbed) is made up of two components; (a) plough
related factors such as mouldboard design, speed and depth of
work and (b) soil strength. By keeping (a) constant we can obtain a
continuous measure of soil strength. Detailed spatial
measurements were also made of clay content and SOC.
Laser proximity sensors
(depth & front furrow width)
Draught force
Soil Strength = Specific Draught, S (kPa)
S (kPa) = Draught Force (kN) / [Depth x width of work (m)]
Fig 3a Different treatments
Fig 3b Soil organic carbon (%) as a result
of different fertilizer and FYM treatments
Fig 3c Contour map showing variation in
clay content, %
Fig 3d Contour map showing Specific
Draught, S (kPa)
N
Broadbalk Wilderness
Strip
20 19 18 17 16 15 14 13 12 11 10 9
Results
Stubbed
Mown
Wooded
8
7
6
5
3 2.2 2.1 1
Straw incorporated
since autumn 1986
Continuous wheat
Continuous wheat
4 1.1.11.2.1 1.4.1 6
The effect of clay content
-
-
P
P
K
K
K
K
Mg Mg
5
(P) (P)
K
K
4
4
P
K
+
1.3.1 4
4
4
3
FYM
-
3
(P) (P) (P) Nil
-
-
K
-
2
1
-
P
(P)
P
-
K
K2
-
-
K
-
Mg Mg Mg Mg Mg
Mg Mg Mg Mg (Mg*) -
Mg2 Mg
(P) (P)
K
K
K
-
-
N
FYM
res
N4
Oats Maize Wheat Wheat Wheat from 1997
S values ranged from 52 to 142 KPa (7 to 21 psi) with higher values
located in areas with high clay content. Contour maps illustrate the
spatial variability of S and show a strong similarity to those
obtained for clay.
Oats Maize Wheat Wheat Wheat from 1996
Oats Maize Wheat Wheat Wheat from 1999
320m
Oats Maize Wheat Wheat Wheat from 2000
The effect of long-term additions of farmyard manure (FYM)
No spring or summer
fungicides
Continuous wheat
Where FYM was added there was on average a 20% reduction in
S compared to the control (no fertilizer). FYM also decreased soil
bulk density and increased SOC above 2%. For these soils this
results in a more stable soil structure which persists through the
growing season and subsequent cultivation.
Strip
19 18 17 16 15 14 13 12 11 10 9
8
7
6
5
3 2.2 2.1 1
Oats Maize Wheat Wheat Wheat from 1998
Wheat
Occassional fallow
No herbicides
Re-drained
Re-drained
autumn
autumn 1993
1993
Continuous
Continuous wheat
Drainage
Drainage
ditchditch
(i). Diminishing returns i.e. limit to draught reduction with
increasing SOC.
(ii). Organic C in soil from larger crops is in forms that have a
disproportionately positive effect on plough draught (and possibly
other soil physical properties) compared to C from FYM.
S, specific draught, kPa
Increases in application rates of mineral N resulted in large
increases in crop yield but relatively little change in SOC. Despite
the modest change in SOC compared to FYM, we measured a
substantial decrease in S from 88 to 73 kPa (13 to 12 psi). We
believe that three factors contribute to this:
90
3.0
88
2.5
86
84
2.0
82
1.5
80
78
1.0
76
0.5
74
72
0.0
0
(iii) Action of roots: biological tillage as a result of roots
exploring the soil. Aggregates adjacent to roots have different
physical characteristics to bulk soil. More plant roots from more
mineral fertilizer.
48
96
144
192
7
6
5
4
3
2
Yield, Mg/ha
Mineral Fertilizer
SOC, Soil Organic Carbon, %
NB Treatments revised for 2001
1
0
288 FYM
Treatment kg N/ha
Conclusions
Organic carbon in soil has a beneficial
effect, (as we already know) but this work
has shown that a small change can have a
disproportionately large effect on soil
strength and possibly other soil physical
properties.
Fig 4 Increased applications of mineral N resulted in a large increase in wheat yield,
but only a small increase in SOC (0.84 to 1.19%). However increased applications
of mineral N also resulted in a large (15%) reduction in S (from 85 to 73 kPa). FYM
represents an annual application of 35 Mg/ha of farm yard manure
Rothamsted receives grant aid from the UK BBSRC