Predicted Log Q
Matric potential -kPa
y = 0.9586x
R² = 0.3996
3.6
3.4
3.2
3.0
2.8
2.6
100
2.4
2.2
Using
50
2.0
log10 Q = 0.35 log 10 Sy + 0.93r + 1.2623
2.5
3.0
3.5
4.0
Measured Log Q
200 kg N/ha (5 cm)
200 kg N/ha (25 cm)
30
Predicted LogQ versus measured
LogQ using Profile Probe data &
DT160 dielectric tensiometer data
100 kg N/ha (5 cm)
25
4.0
100 kg N/ha (25 cm)
3.8
20
15
10
5
05-Jul
Date
21-Jul
07-Jul
24-May
0
y = 0.9646x
R² = 0.4345
3.6
Where:
Q = Resistance of soil
S = Water Content
Ψ = Matric Potential
ρ = Soil Density
Predicted Log Q
Water content cm3/100cm3
35
2.0
05-Jul
21-Jul
07-Jul
0
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
2.0
2.5
3.0
3.5
4.0
Measured Log Q
Data Analysis
By taking penetrometer resistance of soil measurements (Q)
and recording the corresponding matric potential and soil
moisture measurements it is possible to compare measured
log10Q with Predicted log10Q (using a PTF) where: Predicted
log10Q = 0.35 log10Sy + 0.93r + 1.2623. The graphs above
show good 1:1 relationships and confirms the use of a PTF to
predict penetrometer resistance of soil.
Measuring soil moisture
Conclusions
Volumetric Water Content sensors supplied
by Delta-T Devices Ltd UK including the
ML2x ThetaProbe, PR2/6 Profile Probe and
the newly available SM300 volumetric and
soil temperature sensor.
In this work we have demonstrated that the penetrometer
resistance of soil can be predicted with a PTF using matric
potential, soil saturation and soil bulk density data collected from
an open-field test site during a wheat growing period.
Measuring resistance
of soil
The penetrometer used in this work
to measure resistance of soil was
first developed at Penecuik Scottish
Institute of Agricultural Engineering
(now unavailable).
Experimental method
The sensors were installed in rain-fed experimental plots at
Woburn UK and sensor values were logged frequently over the
wheat growing season. Resistance of soil measurements were
taken using a soil penetrometer at frequent intervals throughout
the growing season. The experimental plots had either 100kg of
Nitrogen /ha or 200kg of Nitrogen /ha applied.
150
24-May
In this poster we demonstrate the use of a non-linear
pedotransfer function (PTF) which predicts penetrometer
resistance of soil from the easily measurable soil properties of
matric potential, soil saturation and soil bulk density. Data for this
work was collected over a 2-month wheat growing period from an
open-field test site using a novel prototype dielectric tensiometer
and capacitance measuring soil moisture sensors (ML2x & PR2)
from Delta-T Devices Ltd. By applying this data to the PTF model
proposed by Whalley et al. (2007) it has been possible to predict
penetrometer resistance of soil, over a matric-potential range of
–20kPa to –300kPa. These results are in good agreement with
open-field test site penetrometer measurements. The
instrumentation and data processing techniques presented in
this poster demonstrate a useful method for future root
penetration studies.
200
10-May
Abstract
3.8
26-Apr
Rothamsted Research, West Common, Harpenden, St Albans, AL5 2JQ, UK.
A prototype dielectric tensiometer
sensor from Delta-T Devices Ltd.,
the DT160, measures the dielectric
properties of ceramic blocks that
are in equilibrium with the matric
potential of the surrounding soil. By
using a carefully developed
calibration method for the ceramic
blocks, the sensor measures actual soil matric potential in a
similar manner to a water filled soil tensiometer. Furthermore
dielectric tensiometer sensors have the advantage that they are
able to vastly exceed the “air entry point” value of a water filled
tensiometer (c. -90kPa) and carry on accurately measuring
matric potential beyond this value to in excess of -300kPa.
Unlike the water filled tensiometer, dielectric tensiometers
recover from these high negative potentials upon the soil rewetting (as demonstrated in the following graph). The dielectric
tensiometers have universal calibrations so are field
replaceable. The matric potential measurements obtained by
the DT160 compared well with water filled tensiometers (where
possible).
Predicted LogQ versus measured
LogQ using Theta Probe & DT160
dielectric
4.0
250
12-Apr
W. Richard Whalley, Christopher W. Watts,
Colin P. Webster and Martin A. J. Parry
Matric potential measured
with a dielectric tensiometer
300
10-May
Delta-T Devices Ltd., 130 Low Road, Burwell, Cambridge, CB25 OEJ, UK.
Predicting resistance of soil from soil moisture data
26-Apr
Dick Jenkins and Martin S. Goodchild
Measuring matric potential
12-Apr
Use of a dielectric
tensiometer and other soil
moisture sensors to measure
soil drying by roots of wheat
Data from a prototype dielectric tensiometer and commercially
available soil-moisture sensors from Delta-T Devices Ltd. has
been successfully used to predict the penetrometer resistance of
soil and further verifies the PTF model proposed by Whalley et al.
(2007). Using matric-potential and soil moisture sensors with a
PTF model to predict the strength of field soil may provide a useful
tool for future root penetration studies.
A prototype dielectric tensiometer from Delta-T Devices Ltd. has
enabled reliable real-time prediction of penetrometer resistance
of soil over a much wider range of matric potential than is
possible with a water filled tensiometer. The experimental method
and results have provided verification of the use of dielectric
tensiometer technology as well as its implementation in the
design of the prototype sensor.
Funding Acknowledgement
References
Defra project WU0121: An integrated approach to increasing water
use efficiency and drought tolerance in wheat production in the UK
Whalley, W.R., Cope, R.E., Nicholl, C.J. and Whitmore, A.P. (2004) In-field calibration of a dielectric soil moisture
meter designed for use in an access tube. Soil Use and Management. 20: 203-206.
Whalley, W.R., To J., Kay, B.D. and Whitmore, A.P. (2007) Prediction of the penetrometer resistance of soils with
models with few parameters. Geoderma. 137: 370-377.
Whalley, W.R., Lock, G., Jenkins, M., Peloe, T., Burek, K., Balendonck, J., Take, W.A., Tuzel, IH. and Tuzel, Y.
(2009) Measurement of low matric potentials with porous matrix sensors and water-filled tensiometers. Soil
Science Society of America Journal. 73 :1796-1803.
EPSRC project EP/H040064/1: Non-invasive acoustic-seismic
sensing of soils