Published in Vadose Zone Journal 2:633-641 (2003)
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
SPECIAL SECTION - ADVANCES IN MEASUREMENT AND MONITORING METHODS
Laboratory Calibration, In-Field Validation and Use of a Soil Penetrometer Measuring Cone Resistance and Water Content
G. C. Topp*,a,
D. R. Lapena,
M. J. Edwardsa and
G. D. Youngb
a Eastern Cereal & Oilseed Research Centre, Agriculture & Agri-Food Canada, 960 Carling Ave., Ottawa, ON, K1A 0C6, Canada
b Environmental Sensors Inc. (ESI), Suite 100, 4243 Glanford Ave., Victoria, BC V8Z 4B9, Canada
* Corresponding author (toppc{at}agr.gc.ca).
Received 14 April 2003.
 |
ABSTRACT
|
|---|
Concurrent and coincident measure of penetrometer cone resistance (PR) and water content (WC) were optimized by hourly in-field validation of data from time domain transmissiometry (TDT) for WC and piezoelectric force sensor for PR. A piezoelectric force sensor coupled to a cone is followed by a helical wrapped TDT sensor on a single penetrometer shaft. Detailed laboratory calibrations, coupled with in-field validations, were important to assure the quality of data, which facilitated detailed analyses of PR and WC patterns. The piezoelectric sensor relied on a calibrated spring for the in-field validation. The calibration of the TDT sensor had three stages: a series of fluids of known dielectric constant; soil columns at known, variable water contents; and field soils at a range of ambient conditions. The penetrometer was used to study soil strength and WC behavior in time and space along 300-m plots. The treatments were conventional and no-till, each at two levels of traffic. The crop was corn (Zea mays L.), continuous and in rotation with soybean [Glycine max (L.) Merr.] and wheat (Triticum aestivum L.). The PR vs. WC relationships for two depths (0.21 and 0.27 m), below the level of cultivation, were similar to those at the 0.10-m depth for the nontrafficked no-till plots. These relationships for the 0.21- and 0.27-m depths were not influenced by tillage, traffic, and corn cropping system treatments. The variable depth of plowing in tilled plots was found to influence the data consistency for the 0.21-m depth, indicating the penetrometer's high sensitivity to the soil conditions.
Abbreviations: PR, penetrometer cone resistance • SEE, standard error of estimate • TDR, time domain reflectometry • TDT, time domain transmissiometry • WC, water content
|
INTRODUCTION
|
|---|
SOIL PENETROMETERS are largely used to assess soil compaction for off-road agricultural, civil, and geotechnical purposes and to estimate the resistance experienced by growing roots. Water content has a major influence on soil strength, including PR, and yet no definitive theory defines the PR vs. WC relationship. Field variation in soil water contents could mask soil treatment effects on soil strength (Busscher et al., 1997; Lapen et al., 2003). In addition, Ley et al. (1993) found that soil strength vs. soil WC relationships can be different for different management treatments. These and other findings demonstrate the importance of simultaneous and conjoint measurement of PR and WC. The flexibility of design of time domain reflectometry (TDR) transmission lines or probes has led to combinations of TDR or TDT with cone penetrometers for such simultaneous and conjoint measurements (Morrison et al., 2000; Young et al., 2000; Vaz and Hopmans, 2001).
The evaluation of data from combination probes is more complex, since interactions between parameter measurements must be removed or taken into account. One interaction is the effect of friction imposed by the WC sensor on cone resistance values. Vaz and Hopmans (2001) used a WC sensor of equal diameter as the cone on a cone penetrometer while Young et al. (2000) used a WC sensor of slightly larger diameter than the cone to help ensure good WC sensor–soil contact as the probe moved vertically through the soil. Morrison et al. (2000) incorporated a short transmission line in the cone. In cone penetrometers with electrical force sensors, it has been customary and easier to have the force sensor located at the top of the shaft, eliminating the need for the force sensor to enter the soil (Bengough and Mullins, 1990; Lowery and Morrison, 2002). Recognizing the need to separate cone resistance forces from forces required to insert the WC sensor, Adams et al. (2000) reported successful penetrometer operation with a load cell mounted at the base of the cone, between the cone and WC sensor.
Irrespective of the approach used to estimate cone resistance, intrinsic soil dielectric properties may be altered by the soil disturbance resulting from the passage of the cone through the soil. Rothe et al. (1997) evaluated the impact on TDR measurements of the disturbance caused by insertion of varying diameter rods. They concluded that soil should be removed before insertion of rods >10 mm in diameter to alleviate the effect of soil disturbance around the rods on the WC measurement. With a diameter >12.7 mm, the combination WC–penetrometer probe (Young et al., 2000; Vaz and Hopmans, 2001) was expected to show effects of disturbance that have not been detected and quantified. Topp et al. (2001a)( 2001b) showed possible disturbance effects that implied greater concentration of water and soil adjacent to the probe. In contrast, Vaz and Hopmans (2001) detected effects of disturbance for only one soil type, a sand, and found that the disturbance appeared to cause less dense soil and/or less water adjacent to the probe. Vaz and Hopmans (2001) indicated how difficult it was to confirm and quantify independently the soil disturbance because of the limited extent of the disturbed soil around the probe. Topp et al. (2001a) showed X-ray CAT scan images for relatively dry soil, indicating higher densities around probes occurred 1 to 3 mm from the probe surface.
We address three main issues: (i) the basic calibration of both the WC and PR sensors, (ii) the methods used for validation of both the WC and PR sensor outputs during operation in the field, and (iii) the results of the PR–WC instrument used to characterize PR vs. WC relationships for depths of 0.21 and 0.27 m. The objectives for this paper are first to show that simple and periodic in-field validations provided data of high quality and second that reliable PR vs. WC relationships at the 0.21- and 0.27-m depths were not influenced by cultural practices.
|
MATERIALS AND METHODS
|
|---|
The Combination Penetrometer
The general operation of the combination penetrometer with both WC and PR measurement capability was described by Young et al. (2000) and Topp et al. (2001a) (Fig. 1). The penetrometer system uses a lightweight, 12-V electric motor driven, screw jack assembly to provide a constant velocity for probe insertion of approximately 28 mm s-1. The length of travel is 400 mm. The drive mechanism is mounted on a supporting frame made of lightweight materials, which also houses the sensor signal conditioning electronics and a datalogger–controller. The frame is supported vertically by three horizontal legs on which the operator stands to provide reaction force to hold the mechanism down during operation. The power for the whole system, including the drive motor, is supplied by a 12-V, gel-cell battery carried in a backpack.
View larger version (112K):
[in this window]
[in a new window]
|
Fig. 1. The combination penetrometer showing also the details of the time domain transmissiometry helical parallel pair transmission line, force sensor, and cone. The length of the water content sensing region was 60 mm.
|
|
Topp et al. (2001a)(2001b) described briefly the helical parallel pair WC sensor, which is similar to that of Vaz and Hopmans (2001), which they called coiled. Our sensor used an additional 50-
coaxial cable connected to the lower end of the parallel pair helix to convey the transmitted signal after it passed along the helix to the TDT circuit for analysis. The time delay in the return of the transmitted signal caused by soil or other media surrounding the helical probe was converted to a relative voltage. This voltage relates directly to propagation velocity and thus to WC. The WC sensor consists of a stainless-steel tubular core in which cables pass from the WC sensor and cone force sensor (Fig 1). The tubular core is surrounded by epoxy resin in which the helical pair is embedded.
Field and Laboratory Calibration and Validation of the TDT Water Content Sensor
The calibration of the TDT WC sensor was achieved in three stages: (i) sensor immersed in fluids as ideal (i.e., fully deformable) media for evaluation of the TDT sensor; (ii) sensor inserted into laboratory soil columns to compare TDT with TDR and account for soil disturbance by the probe, if any; and (iii) comparison of TDT sensor performance with TDR in field soils.
Reference Calibration in Fluids of Known Relative Permittivity
A series of organic liquids of known relative permittivity, which spanned the range of relative permittivity values encountered in soils (Table 1), was used to calibrate the sensor. The gain and zero of the TDT circuit were set so that voltage output was well within its operating range (0–5 V) when in air (minimum relative permittivity) or immersed in water (maximum relative permittivity). Voltage readings were recorded when the probe was in air and when fully immersed in each liquid (Table 1). The voltage difference between air and each medium was a direct result of the decrease in propagation velocity along the helical probe caused by the interaction of the electromagnetic wave with the liquid. For normal field operations, reference determination checks on the performance of the TDT probe were made in three readily available fluids (air, tap water, and 2-propanol) approximately every 30 field measurements.
Laboratory, Field Calibration, and Validation of the TDT Sensor in Soils
A series of soil columns was prepared at a sequence of water contents starting from air dry. Three soils of differing texture, North Gower SiCL, Matilda SL, and Rideau C, were used. Soil samples, both air dried or wetted, were sieved (2 mm) and packed by hand into a cylindrical plastic container 0.25 m long and 0.15 m in diameter. At a particular WC the sieved soil was added to the container in preweighed segments to 0.05 m depth. Each segment was hand packed by tapping as needed to force a 0.15-m-diam. plunger to the top of the 0.05-m incremental height. The intended bulk density for the soils was 1.20 Mg m-3 for the SiCL, 1.35 Mg m-3 for the SL, and 1.20 Mg m-3 for the C. The wetting sequence used was from initial air dry to fully wetted through a series of additions of tap water. The soil column was repacked and TDR and TDT measurements were made after each incremental wetting and before adding the water for the next wetting step. Each wetting increment was performed by adding approximately 150 mL of tap water to the soil required to fill the column to the desired density. About one-third of the soil was spread evenly on a tray to about 20 mm deep. A portion of the water was sprinkled over the surface of the spread soil, and another one-third of the soil was added and water sprinkled on it and so on until all soil and 150 mL of water had been added. Soil and liquid were mixed thoroughly in the tray and passed through a series of sieves to break into about 2-mm particles and packed uniformly as described above. Measurements were made. The soil was removed from the column, and another wetting increment was commenced. The upper limit on volumetric WC was determined by the limit on manipulation and mixing of the wetted soil to allow uniform packing.
A 1502C cable tester (Tektronix Inc., Beaverton, OR), and 0.20-m three-pronged wave guides (3-mm-diam. prongs, 18-mm spacing) were used to estimate the bulk relative permittivity of the soil in the column at three randomly selected locations not influenced by insertion of the penetrometer probe. To assess qualitatively the vertical uniformity of WC in the columns the 0.20-m probe was partially inserted vertically to a depth of 0.06 m to estimate WC of that depth increment. Then the probe was inserted to 0.12 m and measured and then to 0.18 and 0.20 m, with TDR readings taken at each depth. From these incremental estimates a WC profile was estimated in 0.06-m increments by TDR. The penetrometer was positioned above the soil column so that the probe was inserted directly into the soil as in field operation. The data were collected for the upper 0.16 m of the soil column, the limit being set by the cone tip reaching the bottom of the cylinder. All measurements were performed in a laboratory at 20°C.
An in-field calibration of the WC sensor was performed by relating TDR-measured water contents (TRASE model 6050, Soilmoisture Equipment Corporation, Santa Barbara, CA) of the top 0.15 m to the average of the outputs of the TDT sensor for the same depth interval. Within 0.10 m of the location of insertion of the TDT probe, TDR determinations were made using a three-pronged probe with 6-mm-diam. prongs and 35-mm spacing. The TDT outputs were used to develop a calibration from soil as found under agricultural field conditions. Empirical relationships between TDT outputs vs. TDR-estimated relative permittivity from field soil and laboratory columns were compared with each other, and each was evaluated against TDT output vs. reference liquid permittivity values.
Field and Laboratory Validation of the Force Sensor Calibration
The piezoelectric force sensor was calibrated using static loading in the laboratory as reported previously (Adams et al., 2000). For the in-field validation, a 150-mm-long, 38-mm-diam. coil spring was mounted in a cavity attached below a small platform. Axial compression of the spring by the penetrometer cone occurred by the cone engaging the cap on the spring through a hole in the platform (Fig. 2). A 100-mm compression of the chosen spring was achieved by approximately 0.4 kN force. An empirical relationship between depth of spring compression and static loading on the spring was developed from laboratory measurements, which was used as a reference against which the in-field performance was checked.
Typically, the force sensor calibration was checked in the field using the spring system after approximately 30 in-field readings. At the same time readings were made of the TDT sensor output from immersion in air, water, and 2-propanol. The sensor signals from these tests were compared with expected outputs to check for zero and calibration drifts and to provide data to correct for drifts, if any.
Field Site and Measurements
Field measurements were performed on a tile-drained field (300 m long by approximately 120 m wide) located near Winchester, ON, Canada (45° 03' N, 75° 21' W) during 2001. The soil is classified as a North Gower clay loam (Orthic Humic Gleysol). Soil textures at depths between 0 to 0.3 m and 0.3 to 0.5 m are approximately 0.20 sand, 0.28 clay, and 0.52 silt Mg Mg-1 and 0.10 sand, 0.42 clay, and 0.48 silt Mg Mg-1, respectively. The field had been under timothy (Phleum pratense L.) and brome grass (Bromus inermis Leyss.) for 9 yr before 1996, after which, the field was subdivided into crop plots under various tillage, cropping system, and traffic treatment ("worst" and "best" managed) combinations (Table 2). Treatment descriptions and plot names are given in Table 2. The field was never tilled while under hay production. The plots were approximately 7 m wide by 300 m long. Moldboard plowing at a nominal depth of 0.17 m was done each fall after harvest. A field traffic treatment (i.e., worst management) was conducted by making single wheel-beside-wheel passes with a 7710 Ford tractor (5400-kg total mass, front and rear tires 13.6R24 at 137, and 18.4R34 at 124 kPa, respectively) over the length of selected plots when the soil was at or slightly above the upper plastic limit (
0.35 Mg Mg-1) as seeding time approached each year. Preferential trafficking was done before spring secondary tillage operations (cultivator) and seeding activities. This treatment was employed to simulate soil structural degradation, such as increased strength, densities, and surface sealing, imposed by wheel traffic during wet soil conditions. Best management was normal field operations (no preferential trafficking) when soil conditions were at water contents <0.32 Mg Mg-1. Fertilizer for all plots was broadcast in spring at rates of 150 kg ha-1 actual N NH4NO3; there was no side dressing. The same corn planter was used on all plots.
Several times between planting and fall plowing in 2001, combination PR–WC measurements were made at seven geo-referenced sample sites spaced systematically over the length of each plot. The multiple intrarow measurements in corn at a particular site were made within approximately 1 m of one another. The PR and WC information of interest in this study were for the 0.18- to 0.24-m (0.21-m) and 0.24- to 0.30-m (0.27-m) soil depth increments, that is, just below cultivation zone at the site. The PR readings recorded within this 0.06-m depth range (approximately 20 measurements) were averaged, and this average was used in subsequent analyses. For WC, the singular TDT-based WC reading representing the respective 0.21- and 0.27-m depths was considered collocated with the average PR of the corresponding 0.06-m increment of soil. The seasonal changes in WC over the 2001 growing season was sufficient to provide PR–WC data for a wide range of soil water conditions. During this period the soil became dry enough that two persons were insufficient weight as a reaction force to achieve penetrometer insertion to full depth.
|
RESULTS AND DISCUSSION
|
|---|
Reference Calibration in Fluids of Known Relative Permittivity
The TDT output vs. relative permittivity of different fluids provided a reference relationship against which to evaluate soil-based TDT readings. The fluids provided a complete and uniform contact around the probe, and therefore, the fluids were considered effectively "undisturbed" or "ideal-deformable" media. The reference liquids (Table 1) showed a highly linear response between the square root of relative permittivity (
r) and TDT voltage output (V), Fig. 3, with an r = 0.99. A linear regression r = 1.26 + 2.39V provides a very convenient calibration relationship by which to convert V from TDT to r. These voltage readings in Fig. 3 are all relative to readings taken in air; that is, the voltage difference from that measured in air. The intercept (r = 1.26) gives a relative permittivity higher than that of air because the epoxy material in the helical transmission line forms part of the measured dielectric, resulting in an overestimate of the surrounding air for which corrections could be made. Ideally, these corrections would be based on dielectric mixing law approaches. However, the high correlation suggests that such a correction would have had a minor impact on the form of the relationship. Therefore, no attempt was made to remove the influence of the epoxy in the helical probe from the TDT readings.
TDT Sensor Measurements in Soil Columns
The uniformity of soil packing in the vertical direction in the columns was assessed only qualitatively using TDR. Two representative profiles (Fig. 4) of the TDT voltage readings for two WC levels in the Rideau C column, for example, indicate that vertical uniformity was fully adequate, considering the difficulty of achieving uniform packing of prewetted soil. The limited assessment using the incremental insertions of the TDR confirmed the level of uniformity observed in Fig. 4. Lateral uniformity was ascertained from randomly located replicate TDR measurements between the central TDT location and the column wall. Therefore, the apparent relative permittivity, ra, by TDR using the 0.20-m wave guides fully inserted was used for the bulk column for comparison to TDT (V) measures at three depth positions for each soil column. The TDR values resulted from the average of three readings around the TDT probe insertion locale. The chosen TDT depth increments were based on the sensor length of 0.06 m, starting immediately after complete immersion of the sensor at the soil surface.
View larger version (12K):
[in this window]
[in a new window]
|
Fig. 4. Two representative time domain transmissiometry voltage profiles from the Rideau C soil column at mid-range water contents. The depth axis is the cone depth below the penetrometer base.
|
|
For all three depth increments, the North Gower SiCL (Fig. 5 and Table 3) ra by TDR vs. V by TDT relationships showed global linear form. In addition, regression slopes were generally consistent with the fluid reference regression slope (2.39 V-1). Slopes were 2.23, 2.31, and 2.42 V-1 for the depth increments of 0 to 0.06, 0.06 to 0.12 and 0.10 to 0.16 m, respectively (Table 3). For all three depth increments together, the slope and intercept were very similar to that for the fluid regression best fit line. Relative to the 0- to 0.06-m depth observations, the TDT voltages for the other increments were, on average, slightly larger for given reference ra. These findings suggest that with progressive movement of the probe through the column, there was greater potential for the TDT to overestimate WC within the probe's sensing region. In terms of WC, the maximum deviation from the fluid reference line, as derived from the bottom depth increment was 
0.02 m3 m-3, an overestimate we considered minimal. Overestimation was not unexpected since insertion of the probe displaces water and soil toward the probe perimeter. The pressure gradients imposed by the probe would likely retain more water in the soil next to the probe, rather than vice versa.
View this table:
[in this window]
[in a new window]
|
Table 3a. Regression parameters for linear regression relationships of form y = y0 + (ax) for time domain transmissiometry (TDT) measures.
|
|
View this table:
[in this window]
[in a new window]
|
Table 3b. Regression parameters for linear regression relationships of form y = y0 + (ax) for spring and piezoelectric force sensor.
|
|
For the Rideau C soil the disturbance, probably compression, resulted in higher TDT readings at all depths, that is, a tendency for data to be to the right of the fluid calibration line in Fig. 6, particularly at high and low water contents. The different depths did not respond differently and the regression had an intercept of 1.23, a slope of 2.22 V-1, and r = 0.987 (Table 3). The 7% reduction in slope from that of the fluid calibration represents an equivalent increase in WC of 0.036 m3 m-3 at 0.4 m3 m-3. Conceivably this could have arisen from compaction of the soil immediately adjacent to the sensor leading to a local increase in WC. The observation of increased deviation with increasing WC supports this hypothesis.
The Matilda SL column experiments showed somewhat different patterns in TDT vs. ra relationships, relative to the C and SiCL soils (Fig. 7). The 0- to 0.06-m depth increment relationship was linear and fit well with the reference fluid best fit line having a slope of 2.52 V-1 and an intercept of 1.19 (Table 3). There was a tendency for the 0- to 0.06-m increment data to display lower relative TDT voltage in the range of ra = 3.5 to 4.5, indicating that TDT slightly underestimated the true WC in the mid WC range for this soil type. The relationship between ra and TDT V for the 0.06- to 0.12- and 0.10- to 0.16-m depth increments shows a similar nonlinear form. Time domain transmissiometry for these increments systematically overestimated ra in the range of ra = 2 to 4. However, the degree of over-estimation, expressed in terms of soil WC, did not exceed 0.05 m3 m-3. The 0.06- to 0.12- and 0.10- to 0.16-m depth increment observations ultimately converge with the reference fluid best fit line at around ra = 4, which suggests that the factors that augmented water next the probe were effectively minimized at soil water contents approaching saturation. From this exploratory study, it is not evident why an apparent soil disturbance effect from the probe manifests itself at mid WC levels for this sandy loam soil.
Field Validation of the TDT Sensor Calibration
The field validation of the TDT probe calibration in both North Gower SiCl and Matilda SL soils showed effectively that the relationship between TDT V and TDR ra compared favorably with the reference fluid best fit line (Fig. 8 and Table 3). Overall, there was slight overestimation in the TDT field results, as regression slope and intercept of the TDT V vs. TDR ra field observations were 2.32 V-1 and 1.18, respectively. In addition, the field regression results were on average similar to soil column regression parameters (Table 3). However, in terms of WC, such an overestimate, as determined via differences between the field and reference fluid regression slopes was quite small, <0.01 m3 m-3. The soil WC range from about 0.14 to about 0.36 m3 m-3 covers the most dynamic and usual range encountered in most agricultural soils. The disturbance effect imposed by the 6-mm-diam. TDR rods in the field soils was not considered to be significant (Rothe et al., 1997), and considering spatial variability in WC associated with a field environment, the TDR vs. TDT relationships were considered excellent. The TDR probe was inserted near but not coincident with the TDT probe. Even with the variability suggested in Fig. 8 a standard error of estimate (SEE) of 0.18 was obtained, which corresponds to a WC resolution of ±0.02 m3 m-3 for the TDT-based estimates. This resolution is similar in magnitude as that for TDR, indicating that the TDT approach is quite comparable in both accuracy and precision to TDR.
Adopting the linear equation given by Topp and Reynolds (1998) between 
v and ra:
 | [1] |
we can derive a calibration relationship by substitution of ra from the regression relationship (dashed line in Fig. 8, Table 3a) for the field-based validation data. The resulting calibration equation is
 | [2] |
where v is the volumetric WC (m3 m-3) and V is the TDT voltage (V) output relative to that obtained with the probe suspended in air. Since disturbance effects appear to vary with soil type, and given that the slight overestimation in field TDT relative to the field TDR estimates was considered to approximate the TDR measurement resolution, no attempt was made to account for an estimated offset in the calibration relationship with respect to the reference fluid best fit line.
As with many environmental instruments, the TDT has shown two kinds of drift with time, sensitivity and zero offset changes. A small zero offset change (e.g., the reading in air) can occur with no change in sensitivity. In addition, some very small changes in "gain" or sensitivity were also observed but not related to any specific causal factor. Although we have not characterized or quantified these drifts, we have followed a field validation procedure where periodic measurements were made in air, water, and 2-propanol at ambient field temperatures. Regression slopes of the field verification TDT outputs vs. the reference liquid relationship were considered excellent, as the average slope for 51 observations was 0.998 with a standard error of mean of 0.003 (Table 3).
Field Validation of the Piezoelectric Cell Force Sensor Calibration
The laboratory force (static weight) vs. depth of spring compression relationship had strong linearity (Table 3b), indicating that the spring approach was an easy, rapid in-field validation method for checking the piezoelectric force sensor response. The average regression slope of the force (piezoelectric cell) vs. depth of spring compression was 3.99 N mm-1, with a SEE of 4.51 N, somewhat higher than that for the lab of 3.72 N mm-1 with SEE of 2.17 N. As the penetrometer position had to be realigned for each field validation measurement, any slight nonalignment may have resulted in friction effects between the spring and the side wall of its encasement during spring compression. This friction may explain the 7% increase in slope in the field tests relative to the laboratory static weight tests where friction would have been minimized by better controlled alignment.
Penetrometer Cone Resistance vs. Water Content Relationships Below the Depth of Cultivation
Since WC can confound interpretation of treatment effects on soil cone resistance parameters, it is often critical when comparing PR results taken at different water contents for a given soil type, to detrend the "effects" of WC from the PR measure. Therefore, we tested running hypotheses that linear PR vs. WC relationships with negative slopes existed at depths below the dynamic plow layer for the plot treatments and that PR vs. WC regression slopes for each plot treatment were statistically similar.
The observed water contents covered a range representing adequately common field conditions at the 0.21-m depth, approximately 0.10 to 0.45 m3 m-3, and at the 0.27-m depth, approximately 0.13 to 0.49 m3 m-3 (Table 4). The PR data ranged from 0.98 to 5.59 MPa at the 0.21-m depth and from 0.85 to 6.38 MPa at the 0.27-m depth (Table 4). These data show slightly lower water contents in the tilled soils at the shallower depth, probably the result of greater plant water uptake. Representative scatterplots of PR vs. WC (Fig. 9) show apparent linear, negative slope relationships as expected. Linear regression coefficients for each plot and depth (Table 5), show the general similarities among plots and depths. In general, except in the Best Conventional Till Rotation Corn (BCTRC) and Worst Conventional Till Continuous Corn (WCTCC) plots, relationships were similar (Table 5). The data scatter is quite high, as indicated by R2 values ranging from 0.34 to 0.76. The rigor of the in-field validation procedures as described above minimizes considerably the possibility that the data scatter arose from instrumental malfunction.
View this table:
[in this window]
[in a new window]
|
Table 4. Summary statistics for penetrometer cone resistance (PR, MPa) and water content (WC, m3 m-3) used to assess PR vs. WC relationships.
|
|
View larger version (11K):
[in this window]
[in a new window]
|
Fig. 9. Representative penetrometer cone resistance (PR) vs. water content relationships for (a) 0.21-m and (b) 0.27-m depths from the Best No-Till Continuous Corn (BNTCC) plot.
|
|
View this table:
[in this window]
[in a new window]
|
Table 5. Regression coefficients penetrometer cone resistance (PR) on water content (WC) (PR = slopeWC + intercept).
|
|
Paired t tests of regression slopes for each depth confirmed that there were no significant differences (0.05 level) across the eight plots at the 0.21-m depth. At 0.27 m, however, pair differences were found at the 0.05 significance level. WCTCC had a slope different from WNTCC, WNTRC, and BNTRC. Additionally, BNTRC had a slope different from BCTRC. We cannot provide a possible causal hypothesis for these findings, but we doubt that they arose from differences in tillage and trafficking at the surface, as the shallower 0.21 depth slopes were quite similar. The differences may have basis in the original soil and landscape properties; for example, the WCTCC plot has often remained wet longer during the spring cultivation and planting period than most other plots.
Other studies (Lapen et al., 2003; Ley et al., 1993) have shown dynamic tillage and traffic effects on the PR vs. WC relationship for soils in the cultivation layer. The soils examined here were beneath nominal cultivation depth and dynamic tillage or traffic effects did not appear strongly manifested, spatially or temporally. Lapen et al. (2003) found for BNTRC and BNTCC plots that PR vs. WC relationships in the top 0.15 m of the soil had intercepts similar to those shown in Table 5, but the slopes tended to be more negative (-13.5 and -15.5 MPa m3m-3). For the more trafficked WNTRC and WNTCC plots, they found the regression was piece-wise linear, with more significant negative slope at WC below approximately 0.2 m3m-3. For tilled plots, robust negative linear PR vs. WC relationships were not evident consistently from planting to after harvest (Lapen et al., 2003).
During analysis of the data from tilled plots we were able to detect instances where plowing had most likely been deeper than the nominal 0.17 m, as the penetrometer detected anomalously low concurrent PR and WC values, possibly from the presence of crop residue from the previous year. From careful preanalysis of the profile data we were able to remove the influence of the variable plow depth on the PR vs. WC relationship. This was also recognized as another possible application for the PR–WC combination penetrometer: mapping depth of plowing in tilled fields.
|
CONCLUSIONS
|
|---|
The three phases of calibration of the TDT WC sensor were useful for (i) confirming instrument function using fluids, (ii) validating operation of the WC sensor in relation to the soil behavior behind the advancing penetrometer cone, and (iii) showing that from field tests the disturbance effects from the cone were <0.01 m3 m-3. The accuracy and precision of the TDT sensor were equal to those of TDR.
The periodic in-field validation of both the WC and cone resistance sensor outputs maintained high precision operation during a wide range of field conditions. The series of repeated PR and WC measurements at georeferenced locations in the plots during the 2001 growing season were used to develop PR vs. WC relationships for two depths just below cultivation depth. The resulting PR vs. WC relationships were negative, significant, and predominantly linear. Linearity in PR vs. WC relationships was not often observed in a companion study of PR vs. WC relationships in the plow layer in tilled plots. Thus, this study demonstrates the potential of detrending the effects of WC on cone penetration measures for variably managed soils at depths immediately below nominal cultivation depth. The combination PR–WC penetrometer was effective for characterizing the cone resistance as a function of WC to the depth of 0.3 m.
|
REFERENCES
|
|---|
- Adams, B.A., G. St-Amour, and G.C. Topp. 2000. Evaluation of a piezoelectric load cell for use on cone penetrometers. J. Agric. Eng. Res. 76:205–210.
- Bengough, A.G., and C.E. Mullins. 1990. Mechanical impedance to root growth: A review of experimental techniques and root growth responses. J. Soil Sci. 41:341–358.
- Busscher, W.J., P.J. Bauer, C.R. Camp, and R.E. Sojka. 1997. Correction of cone index for soil water content differences in a coastal plain soil. Soil Tillage Res. 43:205–217.
- Lapen, D.R., G.C. Topp, M.E. Edwards, E.G. Gregorich, and W.E. Curnoe. 2003. Combination cone penetration resistance/water content instrumentation to evaluate cone penetration-water content relationships in tillage research. Soil Tillage Res. In press.
- Ley, G.J., C.E. Mullins, and R. Lal. 1993. Effects of soil properties on the strength of weak structured tropical soils. Soil Tillage Res. 28:1–13.
- Lide, D.R. 2003. CRC handbook of chemistry and physics. 84th ed. CRC Press, Boca Raton, FL.
- Lowery, B., and J.E. Morrison, Jr. 2002. Soil penetrometers and penetrometry. p. 363–388. In J. Dane and C. Topp (ed.) Methods of soil analysis. Part 4. SSSA Book Ser. 5. SSSA, Madison, WI.
- Morrison, J.E., Jr., B. Lowery, and J.L. Hart. 2000. Soil penetrometer for sensing soil water content as in soil characterization for site-specific farming. p. 121–129. In Proc. Agrophysical and Ecological Problems of Agriculture in the 21st Century. 6–9 Sept. 1999. International Soil and Tillage Research Organization, St. Petersburg, Russia.
- Rothe, A., W. Weis, K. Kreutzer, D. Matthies, U. Hess, and B. Ansorge. 1997. Changes in soil structure caused by the installation of time domain reflectometry probes and their influence on the measurement of soil moisture. Water Resour. Res. 33:1585–1593.
- Topp, G.C., B.A. Adams, G.D. Young, and D.R. Lapen. 2001a. Practical considerations for the use of a combination penetrometer and water content sensor. p. 403–414. In W.D. Reynolds et al. (ed.) Proc. Soil Structure/Carbon Workshop, Leamington, ON. 23–25 Aug. 1999. Eastern Canada Soil Structure Workshop and Agriculture and Agri-Food Canada, Harrow, ON, Canada.
- Topp, G.C., D.R. Lapen, G.D. Young, and M. Edwards. 2001b. Evaluation of shaft-mounted TDT readings in disturbed and undisturbed media. In C.H. Dowding (ed.) Proceedings of TDR2001—Second Symposium and Workshop on Time Domain Reflectometry for Innovative Geotechnical Applications. Available at http://www.iti.northwestern.edu/tdr/tdr2001/proceedings/ (verified 9 Sept. 2003.) Infrastructure Technology Institute, Northwestern Univ., Evanston, IL.
- Topp, G.C., and W.D. Reynolds. 1998. Time domain reflectometry: A seminal technique for measuring mass and energy in soil. Soil Tillage Res. 47:125–132.
- Vaz, C.M.P., and J.W. Hopmans. 2001. Simultaneous measurement of soil penetration resistance and water content with a combined penetrometer–TDR moisture probe. Soil Sci. Soc. Am. J. 65:4–12.[Abstract/Free Full Text]
- Young, G.D., B.A. Adams, and G.C. Topp. 2000. A portable data collection system for simultaneous cone penetrometer force and volumetric soil water content measurements. Can. J. Soil Sci. 80:23–31.
TOP
ABSTRACT
INTRODUCTION
