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Structural heterogeneity of the soil tilled layer as characterized by 2D electrical resistivity surveying
Soil & Tillage Research 79 (2004) 239–249 www.elsevier.com/locate/still
Structural heterogeneity of the soil tilled layer as characterized by 2D electrical resistivity surveying A. Bessona,b,c, I. Cousina, A. Samoue¨liana, H. Boizardb, G. Richardc,* a
INRA, Unite´ de Science du Sol, Avenue de la Pomme de Pin, 45166 Ardon Cedex, France b INRA, Unite´ d’Agronomie Laon-Reims-Mons, 80200 Estre´es-Mons, France c INRA, Unite´ d’Agronomie Laon-Reims-Mons, rue Fernand Christ, 02007 Laon, France
Abstract Our research was aimed at analysing the possibilities of using a geophysical method, the electrical resistivity method, to describe the structure of a cultivated loamy soil. Soil electrical resistivity was measured in laboratory conditions on two soil blocks (0.30 m 0.30 m 0.20 m) with 2D Wenner configuration using an inter-electrode spacing of 0.015 m. The two soil blocs exhibited different structure: one with a compacted structure (bulk density equal to 1.59 Mg m3 with a standard deviation of 0.05 Mg m3) and the second with a porous structure (bulk density equal to 1.39 Mg m3 with a standard deviation of 0.04 Mg m3). The electrical resistivity results showed a significant 10 V m difference between the compacted block (30 V m) and the porous block (40 V m) due to the difference in their bulk density. This structural distinction by electrical resistivity needs temperature correction using the Campbell equation. The soil electrical resistivity was also measured in the field with a 2D Wenner configuration using an inter-electrode spacing of 0.10 m along a 3.20 m transect. After the electrical measurements, a pit was dug and the contours of porous and compacted zones in the ploughed layer were identified, the boundaries between the ploughed layer, the plough pan and the pedological horizons were defined. Comparisons between inverted electrical resistivity maps and visual morphological descriptions showed the ability of electrical resistivity to detect wheels tracks. However, electrical surveying in a heterogeneous field after ploughing did not correspond to the visual morphological description, the latter being 2D whereas the electrical resistivity map is a 2D projection of a 3D sensing. As a non-destructive method, the electrical resistivity method could improve the quantitative description of the tilled layer and permit a temporal survey. # 2004 Elsevier B.V. All rights reserved. Keywords: Structure; Tilled layers; Electrical resistivity; Visual morphological description; Bulk density
1. Introduction Soil structure, i.e. the arrangement of soil particles at various scales, is a key component of the physical * Corresponding author. Fax: +33 3 23 79 36 15. E-mail address: [email protected] (G. Richard).
quality of agricultural fields: it governs the mechanical and transport properties that are involved in numerous aspects of soil physical degradation (poor rootability, aeration and water infiltration). Soil structure on agricultural fields is difficult to characterize because it is variable in space and time. The mechanical stresses applied to the soil during tillage and traffic do not
0167-1987/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2004.07.012
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concern the whole soil volume and the soil is affected by long-term stresses due to climate and biological activity. Soil structure characterization in the field is based on measurements of soil properties such as bulk density, porosity, penetration resistance, water infiltration or on the visual description of a soil profile (Roger-Estrade et al., 2004). X-ray tomography, mercury porosimetry on undisturbed soil cores, image analysis of soil sections can also be used in the laboratory. One of the main problems with these methods for the study of soil structure is that they are destructive and cannot easily be used for spatial and temporal monitoring of the soil structural heterogeneity. Geophysical non-destructive methods such as electrical resistivity have already been demonstrated as useful quantitative methods for sub-surface soil survey, soil mapping at the farm scale (Bourennane et al., 1998), monitoring of water content or solute transfer (Binley et al., 1996; Goyal et al., 1996; Michot et al., 2003), characterization of soil properties (Banton et al., 1997) or description of porosity evolution (Dannowski and Yaramanci, 1999; Samoue¨ lian et al., 2003). Indirect agronomic results have been deduced from electrical surveys; Bottraud et al. (1984) followed the water distribution for 2.5 months in a sandy soil and related the hydraulic properties of soil to the growth of vines. Dabas et al. (1989) related soil thickness, calculated from electrical resistivity measurements, to crop yield at the farm scale. The electrical resistivity of a soil would depend on its volumetric clay content because the electric conduction occurs within the water-filled pores and at the surface of the clay particles. Consequently, electrical resistivity would depend on soil bulk density and more generally on soil structure. However, no attempt has been made to characterise the structure of the tilled soil layers. The aim of our study was to examine the feasibility of using electrical resistivity to describe the structure of tilled layers in agricultural fields. We first compare electrical resistivity measurements in a laboratory experiment using two homogeneous soil cores with contrasting bulk density. We then apply the electrical resistivity method in the field to describe the heterogeneity of the soil structure in the ploughed layer. Results are discussed in comparison with a direct visual description of the soil structure at the profile scale.
2. Material and methods 2.1. Theoretical principles of electrical resistivity measurements The electrical resistivity of a soil layer is its capacity to limit the transfer of electric current. It is calculated from the potential difference due to the continuous and low frequency artificial current injected by electrodes into the soil. There are different electrode configurations, called electrodes arrays (Rhoades et al., 1976) where the electrical current (I) is injected by two electrodes conventionally named A and B and the potential difference (DV) is measured by two other electrodes M and N. The measured electrical resistivity of the prospected medium is called apparent resistivity (ra) and is calculated by Eq. (1): ra ¼
DV 2p I 1=MA 1=MB þ 1=NB 1=NA
(1)
where MA (resp. MB, NB, NA) represents the distance between electrodes M and A (resp. M and B, N and B, N and A). The electrode configuration used in our experiments is the Wenner array. Four electrodes are arranged in-line, with A and B electrodes at the external positions and M and N electrodes in between. The distances AM, MN, NB are equal. For the characterization of the soil cultivated layer, a 2D electrical profile was constructed (Fig. 1a). For that, the Wenner array is moved on a line from one point to another to measure the electrical resistivity of adjacent locations. By increasing the distance between all the four electrodes, the depth of investigation increases and deeper zones in the soil profile can be characterised. The measured resistivities can thus be quickly obtained. They are called ‘‘apparent resistivities’’ because each value corresponds to an integrated volume. To be correctly interpreted, the measured integrated values must be converted so that we can rely, at one location, a resistivity to another soil parameter. This conversion can be made by inversion software and the results are then called ‘‘inverted resistivities’’. The Res2Dinv software (Loke and Barker, 1996), widely used in electrical resistivity surveys, calculates a distribution of ‘‘inverted resistivity’’ from measured
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Fig. 1. Principles of 2D electrical resistivity measurement. (a) Electrode positions for a Wenner array configuration. A and B are the injection electrodes. Electrical potential is measured by electrodes M and N. The distance AM = MN = NB represents the inter-electrode spacing. (b) Grid used by Res2Dinv software to calculate the inverted electrical resistivity values for a 32 electrode Wenner array.
apparent resistivity. The soil profile is discretized into elementary blocks with an increasing thickness with depth as shown in Fig. 1b. It then calculates a set of apparent resistivity which is compared to the measured one. An estimation of the difference between measured and calculated apparent resistivity is given by the root mean square error (RMS). Four to six iterations are generally necessary to obtain the most reliable and realistic inverted resistivity distribution with a RMS lower than 10% (Delapierre, 1998). The calculated data consist then in a vertical map of inverted resistivity, whose depth and width depend on the number and spacing of the electrodes. At each point of the map, the inverted resistivity value corresponds to the value of resistivity at that location, without any integration. Final 2D diagrams are obtained after linear interpolation. Several parameters related to the soil characteristics (porosity, water content, clay content, salinity) or to the climatic conditions (temperature) influence
electrical resistivity values. At one site and for a given water content, different resistivity values can be compared after correction of the temperature effect. The latter is calculated from the Campbell equation (Campbell et al., 1948): rT ¼ r25 C ½1 þ aðT 25 CÞ
(2)
where T represents the temperature, rT the electrical resistivity measured at temperature, r25 C the electrical resistivity at the 25 8C reference temperature, and a a correction factor equal to 0.02. 2.2. Experimental site Experiments were conducted at the experimental site of INRA Estre´ es-Mons (508N, 38E, elevation 85 m), in northern France (Boizard et al., 2002). The studied soil is a Haplic Luvisol (FAO Classification, 1998) developed on loess (Table 1). We focused our
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Table 1 Physical characteristics of the soils in plots P1 and P2 Clay (0–2 mm g kg1)
Fine silt (2–20 mm g kg1)
Coarse silt (20–50 mm g kg1)
Fine sand (50–200 mm g kg1)
Coarse sand (200–2000 mm g kg1)
Organic carbon content (g kg1)
Plot P1
Horiz. L, 0–30 cm Horiz. B, 30–60 cm
192 320
306 312
439 344
28 23
3 1
9 7
Plot P2
Horiz. L, 0–30 cm Horiz. B, 30–60 cm
197 248
264 280
490 433
36 32
13 7
9 5
study on the layers down to 0.50 m depth. Two plots were chosen:
Plot P1: a fine soil structure was first obtained by digging a bare soil during a dry period in autumn. Localised compacted bands were then created under wet conditions in spring using a heavy tractor (81.4 kN with rear tyres 0.65 m width inflated at 200 kPa). We aimed at producing homogeneous porous zones and more compacted ones in an even soil profile with known positions.
Plot P2: the second plot was from a long-term field experiment (Boizard et al., 2002). The crop rotation was sugar beet (Beta vulgaris L.)/winter wheat (Triticum aestivum L.)/maize (Zea mays L.)/winter wheat with an annual winter ploughing. Because of the soil compaction produced during crop harvesting in autumn in wet conditions, the soil structure is heterogeneous, with a percentage of compacted clods generally between 10 and 40%. Measurements were performed in spring before the sowing of maize. 2.3. Laboratory experiments Two soil blocks (0.30 m 0.30 m 0.20 m in size) were sampled from plot P1 described above: one from the compacted zone and one from the porous zone. Their respective bulk density values were determined on 98 cm3 soil cores sampled during the field experiment. Ten cylinders were sampled in the compacted zone and 7 in the porous zone. The soil blocks were stored in a cold room at T = 5 8C. Electrical resistivity measurements were performed immediately after removing the samples from the cold room (temperature T1). Two days later, after rewarming at room temperature, a second set of electrical resistivity measurements were performed (tempera-
ture T2). In both experiments, the soil block temperature was recorded by the use of a resistivimeter coupled to a thermoprobe PT100. The mean temperatures T1 and T2 resulted from measurements done before and after electrical resistivity prospecting at 0.20 m depth in the middle of blocks. The electrical measurements (apparent resistivity data) were obtained from a set-up composed of 16 separate steel electrodes (1 mm diameter) spaced 0.015 m apart in Wenner arrays (Samoue¨ lian et al., 2003). The potential differences were recorded on a Syscal R1 resistivimeter (Iris Instrument, Orleans, France) connected to a 12 V battery that results in a dc current amplified at 100 V. A multinode allowed the electrical current to be distributed to the 16 electrodes. The inverted resistivities were calculated with the Res2Dinv software (Loke and Barker, 1996). At the end of the experiment, we determined the gravimetric soil water content on 12 clods (several cubic cm3) sampled in each soil block. 2.4. Field experiments Field experiments were carried out on the two plots at the same period of the year on profiles 3.20 m wide. On plot P1, two experiments (P11 and P12) perpendicular to the traffic direction were conducted 1.50 m apart. They covered two wheel tracks located, respectively, between 0.40–1.00 and 2.25–2.85 m from the left corner of the profile. On plot P2, only one experiment (P21) was conducted. The electrical measurements were made using 32 separate brass electrodes (3 mm diameter) spaced 0.10 m apart. Except for the number and characteristics of the electrodes and the distance between two consecutive electrodes, the experimental protocol was identical to that used in the laboratory. Temperature measurements were made by a resistivimeter coupled to a
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thermometric probe PT100 at 0.05, 0.15 and 0.30 m depth at four locations along the studied profile: 0.20, 1.00, 1.80 and 2.60 m from the left corner of the profile. These measurements enabled to obtain a representative mean value of temperature in the whole profile. The measured electrical resistivity values were then corrected by the Campbell equation (2) (reference temperature: 25 8C). The inverted resistivities were calculated with the Res2Dinv software. Calculations of inverted resistivity were performed for seven layers with the following thickness: from 0.05 m for the first layer (half of the inter electrode spacing) to 0.168 m (Fig. 1b). At the end of the electrical prospecting, a soil pit was dug to characterise the soil structure with different methods:
2.5. Data presentation and analyses
A visual morphological profile was made in plots P11 and P21 by the method of Manichon (1987). Two main structural zones were visually distinguished as a function of the visible porosity and the roughness of the rupture face when soil lumps are manually broken: compacted zones without any visible porosity and a smooth rupture face, porous zones with visible porosity and a rough rupture face. A sequence of digital photographs was taken and joined together to represent the whole 3.20 m wide profile.
Thirty eight undisturbed soil cores were sampled in 98 cm3 metal cylinders on plot P11 under or outside the wheel tracks. Bulk density, gravimetric and volumetric water content were measured on these soil cores.
On plots P12 and P21, gravimetric soil water content measurements were also determined. On four vertical profiles located at 0.35, 1.35, 1.75 and 2.60 m from the left corner of the profile, soil samples were taken at 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30 m depth.
3.1. Laboratory analyses
All the measurements (laboratory and field data) were described by classical statistical variables. Both apparent and inverted resistivity values are presented in the following part. As the populations of apparent and inverted resistivities were asymmetrical and dispersed, the median values were compared with nonparametric tests of Kolmogorov–Smirnov and Mann–Whitney (P < 0.05). The distributions of the bulk densities and soil water contents were normal and mean values were compared with the parametric test of Student at the 5% significant level.
3. Results
The mean values of bulk density and volumetric water content were significantly larger in the compacted block than in the porous block (Table 2). Gravimetric water content was the same in the two blocks. For each soil block, the median value of apparent resistivities was higher for measurements at 4 8C than at 20 8C and the difference in apparent resistivity between the two temperatures was about 30 V m (Fig. 2). After temperature correction by the Campbell equation (2), the difference between the two measurements for each soil block was not significantly different, which validated the use of this equation for electrical resistivity measurements in soils. The apparent resistivity values were significantly larger for the porous block than for the compacted block (Fig. 2). The interquartile interval was larger for the porous block than for the compacted block which showed the greater heterogeneity of the porous block. The apparent resistivity data, after temperature correction, were inverted with the Res2DInv software
Table 2 Bulk density, volumetric water content and inverted electrical resistivity for the porous and the compacted blocks in laboratory experiments
Porous block Compacted block
Bulk density (Mg m3)
Volumetric water content (m3 m3)
Inverted electrical resistivity (V m)
1.39 (0.04) (n = 7) 1.59 (0.05) (n = 10)
0.320 (0.007) (n = 12) 0.301 (0.012) (n = 12)
38 (22) (n = 68) 27 (13) (n = 68)
For bulk density and volumetric water contents, the main numbers are mean and the numbers between brackets are standard deviations. For inverted electrical resistivity, the main numbers are the median values and the numbers between brackets are interquartile intervals.
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Fig. 2. Apparent resistivity values for the porous and the compacted blocks. T1, T2: experimental measurements at temperatures T1 and T2; T125, T2-25: experimental measurements corrected by the Campbell equation for estimation at 25 8C.
for each experiment in each soil block (Fig. 2). The inverted resistivities were not equal to the apparent resistivities, the discrepancy being higher in the porous block. This was due to structure heterogeneities within each block: with a perfectly homogeneous block from an electrical point of view, the apparent and inverted resistivities would theoretically have been equal. By mixing all the inverted values of the experiments (at T1 and T2) for each block, we calculated a unique median value representative of the porous and compacted structure (Table 2). The difference in inverted resistivity between the two blocks was 9 V m, with a lower resistivity value in the compacted block, that was consistent with the apparent resistivity values. As for the apparent data, the interquartile interval was larger for the porous block than for the compacted block certainly due to higher internal structural heterogeneity of the porous block.
For each depth, differences in median values for plots P11 and P12 were not statistically different. For plots P11 and P12, the median value decreased with depth from the second level. For the plot P21, the median value decreased from the surface. The interquartile intervals decreased with depth for each soil profile, indicating a lower heterogeneity of resistivity values in depth. The interquartile interval for the first level was particularly high showing the strong heterogeneity of the near-surface electrical values (<0.15 m). A more precise analysis of the lateral distributions of electrical resistivity values on plots P11 and P12 showed that the highest electrical resistivity values were located at two places along the soil profile: between 0.40 and 0.95, and 2.30–2.75 m for P11; between 0.40 and 1.05, and 1.75–2.85 m for plot P12. These places corresponded to wheels tracks. On the plot P21, the heterogeneity was high as well in the first level but no particular structure in electrical resistivity was seen.
3.2. Field experiments 3.2.1. Electrical resistivity measurements Whatever the soil profile, the inverted resistivity map showed both vertical and horizontal variabilities (Fig. 3). On each soil map, the median value and interquartile intervals have been calculated along horizontal lines for all the seven depth levels (Fig. 4).
3.2.2. Visual morphological description Within P11 and P12 soil profiles, a visual morphological description was made of different zones with regard to soil bulk density (Fig. 5). We identified compacted zones, where no porosity could be seen with the naked eye. In the remaining zones, more or less porous soil volumes were identified. In
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Fig. 3. Inverted electrical resistivity panels for the prospecting for the three plots. Black lines represent the contours of the compacted zones as interpreted from Fig. 5. The arrows point on small compacted clods not detected by the electrical resistivity prospection.
plot P11, large compacted zones (about 0.60 m 0.10 m) were identified at locations 0.40–1.00 and 2.30–2.90 m. They corresponded to wheel track ways. Several other small clods, from 5 to 10 cm in size are located between or under the wheel tracks. Around 0.20–0.25 m deep, a continuous compacted layer, several centimeters wide, corresponded to the depth of the last ploughing. In plot P21, voids due to tillage or macro fauna activity (1–5 cm diameter) were identified and straw residues from the previous wheat crop were buried in the 0–0.30 m layer. Compacted clods of different sizes were located all along the soil profile. Other cracked clods were numerous, usually located in the first 20 cm. Limits of the plough pan could be identified in several places.
3.2.3. Bulk density and water content measurements In plot P11, 38 undisturbed soil cores were taken to determine both the bulk density and the water content (Fig. 5). The mean bulk density measured in the compacted zones was similar than that of the compacted block (Tables 2 and 3). The mean bulk density measured in the porous zones and its standard deviation were higher than those of the porous block (Tables 2 and 3). A more precise analysis of the porous zone showed a significant difference of bulk density according to the position of the cylinders. Just under the compacted zones, the mean bulk density value was 1.53 Mg m3. In the middle of the profile, outside any wheel tracks, the mean bulk density value was 1.39 Mg m3. There was a density gradient
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Fig. 4. Median values and interquartile intervals of inverted electrical resistivity measurements, calculated at the seven prospected levels on plot P11.
created by wheel tracks. Gravimetric water contents (0.21 g g1) were not significantly different between compacted and porous zones. Nevertheless, the volumetric water content was higher in the compacted zones than in the porous zones. In plot P12, no large soil pit was dug and a complete description of the
water content was not done. Nevertheless, gravimetric water content measured in compacted zones (resp. porous zones) in plot P12 (0.20 kg kg1) was not different from that measured in compacted zones (resp. porous zones) in plot P11. In plot P21, the gravimetric water content was significantly different
Fig. 5. Visual morphological description of the P11 and P21 soil profiles. Yellow crosses on the profile P11 represent the locations of soil cores sampled for bulk density and water content measurements. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.
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Table 3 Mean bulk density, mean volumetric water content and median inverted electrical resistivity in porous zones and compacted zones for plot P11 during field experiments Bulk density (Mg m3) a
Porous zones Compacted zones Pedological horizon
b
1.48 (0.09) , n = 22 1.59 (0.05), n = 12 1.53 (0.01), n = 4
c
Volumetric water content (m3 m3)
Inverted electrical resistivity (V m)
0.307b (0.039), n = 22 0.339 (0.008), n = 12 0.349 (0.011), n = 4
40 (15)d, n = 60e 30 (6), n = 36 23 (4), n = 41
a
Porous zones were located outside the wheel tracks or in the wheel tracks under the compacted area. Numbers between brackets are standard deviations. c Number of replicates. d Interquartile interval. e Due to the size of the cylinder used to measure bulk density and of the grid used to calculate inverted electrical resistivity, three or four inverted electrical resistivity values were considered for one bulk density measurement. b
(0.23 g g1) from those measured in plots P11 and P12. From our measurements, no spatial variation of water content was found.
4. Discussion Laboratory experimental results show that the resistivity value is equal to 38 V m in the porous block (bulk density of 1.39 Mg m3) and 27 V m in the compacted block (bulk density of 1.59 Mg m3) for a loamy soil at 0.20 kg kg1 water content and 25 8C temperature. Insofar as the porous and compacted blocks were sampled just after compaction, at the
same time and in the same soil and location, the difference between the two samples only results from the difference in bulk density. No variation in other soil parameters like texture and salinity, could be suspected. As a consequence, the significant difference in electrical resistivity between the two blocks (11 V m) was due to the difference in soil structure. Similar values of electrical resistivity were obtained, in field conditions, for the same structures and water content, when the effect of temperature variation on electrical resistivity was corrected using the Campbell equation. Therefore our results show that soil electrical resistivity can be used to discriminate, at least, two extreme structural states of the same soil. The
Fig. 6. Inverted electrical resistivity according to the measured bulk densities in the porous and the compacted zones on plot P11.
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small difference (11 V m) requires us to control the temperature measurement conditions. Moreover, the soil cores that were sampled for bulk density determination on the plot P11 allow us to establish the variation of electrical resistivity with bulk density (Fig. 6). Electrical resistivity clearly decreased with increasing bulk density, with a non-linear relationship. Any other relationship cannot be found in the literature. The shape of the relationship, which probably depends on water content and soil type, has to be established for various soils conditions. The effect of soil bulk density on soil electrical resistivity allows us to distinguish three main structural features: 1. The plough pan: around 0.25–0.30 m on plot P11, low values of resistivity (around 29 V m) corresponds to the plough pan and are equal to the compacted zone value. This is consistent with the hypothesis that the compacted zones and the plough pan have the same origin. Under that limit, the lower resistivity value (23 V m) is measured in the B horizon (Table 1). The clay content of the B horizon is higher than that of the surface horizon which can explain a lower resistivity (Fukue et al., 1999; Robain et al., 2001). 2. The wheel tracks: on plot P11, the distribution of inverted electrical resistivity values in the first 0.15 m shows a high heterogeneity. It is lower in two bands located at 0.40–0.95 and 2.30–2.75 m. The comparison with the visual description of the soil profile suggests that these lower values are located in compacted bands corresponding to former wheel tracks. The other higher values are located in zones identified as more being porous. 3. The clods: on plot P11, some compacted clods (size < 0.10 m) were included in the ploughed layer outside the wheel tracks (Fig. 5). They were not detected by the electrical experiment, probably because of the 0.10 m inter-electrode spacing which did not allow detection of objects smaller than 0.10 m. On plot P21, the 40–50 V m values were preferentially located in the first 15 cm layer near the surface where the soil was effectively the most porous without any compacted clods. Compacted clods were mainly located in the 0.15–0.30 m layer of the plot P21. Their diameter varied from 5 to 25 cm. The electrical resistivity
map did not allow us to localise the compacted clods, even the largest ones. It was the same thing for voids, cavities and straw residues that are supposed to induce a high electrical resistivity. This discrepancy would be due to a difference in the soil volume that is characterized by the morphological description or by the electrical method. For the visual morphological description, a strict 2D vertical plan is analysed. With the electrical resistivity method, a 3D space is investigated but its exact volume cannot be determined. The 2D inverted resistivity gives a characterization of the soil structure that corresponds to the projection of a 3D integrated volume. As one possible consequence, the simulation of water transfers in cultivated soils may be more accurate if an electrical description of the cultivated soil layer is used.
5. Conclusions Soil structure, which is a key component of the soil physical quality, is still difficult to characterise because of its heterogeneity in agricultural fields. Our study, aimed at evaluating the use of a geophysical method, uses 2D electrical resistivity measurements to describe the structural heterogeneity of cultivated soils. We showed that soil electrical resistivity is sensitive to bulk density: an increase of the bulk density from 1.39 to 1.59 Mg m3 in a loamy soil corresponds to a decrease of the electrical resistivity of 11 V m. This variation allows us to distinguish structural features such as plough pan or wheel tracks in field conditions using a 2D investigation. The use of this method to characterise the structure of tilled horizons, such as clod size, needs the testing of a 3D investigation procedure. This work is a first step to obtain a non-destructive method to characterise the variability of the structure of cultivated soils, in both in time and space.
Acknowledgements The authors are grateful to Bertrand Chauchard, Pierre Courtemanche and Paul Re´ gnier for their assistance with the laboratory and field experiments.
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The electrical resistivity device, especially multinodes, were rented by courtesy of the Geofcan French group.
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(Site gallo-romain d’Orbe-Bosce´ az, Vaud, Suisse). Ph.D. Thesis. University of Lausanne, Switzerland, 200 pp. FAO, 1998. World reference base for soil resources. In: World Soil Resources Report No. 84. FAO, ISRIC, ISSS, Rome, 91 pp. Fukue, M., Minatoa, T., Horibe, H., Taya, N., 1999. The microstructure of clay given by resistivity measurements. Eng. Geol. 54, 43–53. Goyal, V.C., Gupta, P.K., Seth, P.K., Singh, V.N., 1996. Estimation of temporal changes in soil moisture using resistivity method. Hydrocarb. Process. 10, 1147–1154. Loke, M.H., Barker, R.D., 1996. Rapid least-squares inversion of apparent resistivity pseudo-sections using a quasi-Newton method. Geophys. Prospect. 44, 131–152. Manichon, H., 1987. Observation morphologique de l’e´ tat structural et mise en e´vidence d’effets de compactage des horizons travaille´ s. In: Monnier, G., Goss, M.J. (Eds.), Soil Compaction and Regeneration, A.A. Balkema, Rotterdam, The Netherlands, pp. 39–52. Michot, D., Benderitter, Y., Dorigny, A., Nicoullaud, B., King, D., Tabbagh, A., 2003. Spatial and temporal monitoring of soil water content with an irrigated corn crop cover using electrical resistivity tomography. Water Res. Res. 39, 1138–1160. Rhoades, J.D., Kaddah, M.T., Halvorson, A.D., Prather, R.J., 1976. A technique for establishing accurate four-electrode soil electrical conductivity–salinity calibration. Soil Sci. 123, 137–141. Robain, H., Bellier, G., Camerlynck, C., Vergnaut, D., 2001. Relation entre re´ sistivite´ et teneur en eau. Importance des caracte´ ristiques granulome´ triques, mine´ ralogiques et rhe´ ologiques des sols. In: Proceedings of the Third Conference of GEOFCAN, Paris, France, September 25–26, pp. 101–105. Roger-Estrade, J., Richard, G., Caneill, J., Boizard, H., Coquet, Y., Defossez, P., Manichon, H., 2004. Morphological characterisation of soil structure in tilled fields: from a diagnosis method to the modelling of structural changes with time. Soil. Till. Res. 79, 33–49. Samoue¨ lian, A., Cousin, I., Richard, G., Tabbagh, A., Bruand, A., 2003. Electrical resistivity imaging for detecting soil cracking at the centimetric scale. Soil Sci. Soc. Am. J. 67, 1319–1326.
Structural heterogeneity of the soil tilled layer as characterized by 2D electrical resistivity surveying A. Bessona,b,c, I. Cousina, A. Samoue¨liana, H. Boizardb, G. Richardc,* a
INRA, Unite´ de Science du Sol, Avenue de la Pomme de Pin, 45166 Ardon Cedex, France b INRA, Unite´ d’Agronomie Laon-Reims-Mons, 80200 Estre´es-Mons, France c INRA, Unite´ d’Agronomie Laon-Reims-Mons, rue Fernand Christ, 02007 Laon, France
Abstract Our research was aimed at analysing the possibilities of using a geophysical method, the electrical resistivity method, to describe the structure of a cultivated loamy soil. Soil electrical resistivity was measured in laboratory conditions on two soil blocks (0.30 m 0.30 m 0.20 m) with 2D Wenner configuration using an inter-electrode spacing of 0.015 m. The two soil blocs exhibited different structure: one with a compacted structure (bulk density equal to 1.59 Mg m3 with a standard deviation of 0.05 Mg m3) and the second with a porous structure (bulk density equal to 1.39 Mg m3 with a standard deviation of 0.04 Mg m3). The electrical resistivity results showed a significant 10 V m difference between the compacted block (30 V m) and the porous block (40 V m) due to the difference in their bulk density. This structural distinction by electrical resistivity needs temperature correction using the Campbell equation. The soil electrical resistivity was also measured in the field with a 2D Wenner configuration using an inter-electrode spacing of 0.10 m along a 3.20 m transect. After the electrical measurements, a pit was dug and the contours of porous and compacted zones in the ploughed layer were identified, the boundaries between the ploughed layer, the plough pan and the pedological horizons were defined. Comparisons between inverted electrical resistivity maps and visual morphological descriptions showed the ability of electrical resistivity to detect wheels tracks. However, electrical surveying in a heterogeneous field after ploughing did not correspond to the visual morphological description, the latter being 2D whereas the electrical resistivity map is a 2D projection of a 3D sensing. As a non-destructive method, the electrical resistivity method could improve the quantitative description of the tilled layer and permit a temporal survey. # 2004 Elsevier B.V. All rights reserved. Keywords: Structure; Tilled layers; Electrical resistivity; Visual morphological description; Bulk density
1. Introduction Soil structure, i.e. the arrangement of soil particles at various scales, is a key component of the physical * Corresponding author. Fax: +33 3 23 79 36 15. E-mail address: [email protected] (G. Richard).
quality of agricultural fields: it governs the mechanical and transport properties that are involved in numerous aspects of soil physical degradation (poor rootability, aeration and water infiltration). Soil structure on agricultural fields is difficult to characterize because it is variable in space and time. The mechanical stresses applied to the soil during tillage and traffic do not
0167-1987/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2004.07.012
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concern the whole soil volume and the soil is affected by long-term stresses due to climate and biological activity. Soil structure characterization in the field is based on measurements of soil properties such as bulk density, porosity, penetration resistance, water infiltration or on the visual description of a soil profile (Roger-Estrade et al., 2004). X-ray tomography, mercury porosimetry on undisturbed soil cores, image analysis of soil sections can also be used in the laboratory. One of the main problems with these methods for the study of soil structure is that they are destructive and cannot easily be used for spatial and temporal monitoring of the soil structural heterogeneity. Geophysical non-destructive methods such as electrical resistivity have already been demonstrated as useful quantitative methods for sub-surface soil survey, soil mapping at the farm scale (Bourennane et al., 1998), monitoring of water content or solute transfer (Binley et al., 1996; Goyal et al., 1996; Michot et al., 2003), characterization of soil properties (Banton et al., 1997) or description of porosity evolution (Dannowski and Yaramanci, 1999; Samoue¨ lian et al., 2003). Indirect agronomic results have been deduced from electrical surveys; Bottraud et al. (1984) followed the water distribution for 2.5 months in a sandy soil and related the hydraulic properties of soil to the growth of vines. Dabas et al. (1989) related soil thickness, calculated from electrical resistivity measurements, to crop yield at the farm scale. The electrical resistivity of a soil would depend on its volumetric clay content because the electric conduction occurs within the water-filled pores and at the surface of the clay particles. Consequently, electrical resistivity would depend on soil bulk density and more generally on soil structure. However, no attempt has been made to characterise the structure of the tilled soil layers. The aim of our study was to examine the feasibility of using electrical resistivity to describe the structure of tilled layers in agricultural fields. We first compare electrical resistivity measurements in a laboratory experiment using two homogeneous soil cores with contrasting bulk density. We then apply the electrical resistivity method in the field to describe the heterogeneity of the soil structure in the ploughed layer. Results are discussed in comparison with a direct visual description of the soil structure at the profile scale.
2. Material and methods 2.1. Theoretical principles of electrical resistivity measurements The electrical resistivity of a soil layer is its capacity to limit the transfer of electric current. It is calculated from the potential difference due to the continuous and low frequency artificial current injected by electrodes into the soil. There are different electrode configurations, called electrodes arrays (Rhoades et al., 1976) where the electrical current (I) is injected by two electrodes conventionally named A and B and the potential difference (DV) is measured by two other electrodes M and N. The measured electrical resistivity of the prospected medium is called apparent resistivity (ra) and is calculated by Eq. (1): ra ¼
DV 2p I 1=MA 1=MB þ 1=NB 1=NA
(1)
where MA (resp. MB, NB, NA) represents the distance between electrodes M and A (resp. M and B, N and B, N and A). The electrode configuration used in our experiments is the Wenner array. Four electrodes are arranged in-line, with A and B electrodes at the external positions and M and N electrodes in between. The distances AM, MN, NB are equal. For the characterization of the soil cultivated layer, a 2D electrical profile was constructed (Fig. 1a). For that, the Wenner array is moved on a line from one point to another to measure the electrical resistivity of adjacent locations. By increasing the distance between all the four electrodes, the depth of investigation increases and deeper zones in the soil profile can be characterised. The measured resistivities can thus be quickly obtained. They are called ‘‘apparent resistivities’’ because each value corresponds to an integrated volume. To be correctly interpreted, the measured integrated values must be converted so that we can rely, at one location, a resistivity to another soil parameter. This conversion can be made by inversion software and the results are then called ‘‘inverted resistivities’’. The Res2Dinv software (Loke and Barker, 1996), widely used in electrical resistivity surveys, calculates a distribution of ‘‘inverted resistivity’’ from measured
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Fig. 1. Principles of 2D electrical resistivity measurement. (a) Electrode positions for a Wenner array configuration. A and B are the injection electrodes. Electrical potential is measured by electrodes M and N. The distance AM = MN = NB represents the inter-electrode spacing. (b) Grid used by Res2Dinv software to calculate the inverted electrical resistivity values for a 32 electrode Wenner array.
apparent resistivity. The soil profile is discretized into elementary blocks with an increasing thickness with depth as shown in Fig. 1b. It then calculates a set of apparent resistivity which is compared to the measured one. An estimation of the difference between measured and calculated apparent resistivity is given by the root mean square error (RMS). Four to six iterations are generally necessary to obtain the most reliable and realistic inverted resistivity distribution with a RMS lower than 10% (Delapierre, 1998). The calculated data consist then in a vertical map of inverted resistivity, whose depth and width depend on the number and spacing of the electrodes. At each point of the map, the inverted resistivity value corresponds to the value of resistivity at that location, without any integration. Final 2D diagrams are obtained after linear interpolation. Several parameters related to the soil characteristics (porosity, water content, clay content, salinity) or to the climatic conditions (temperature) influence
electrical resistivity values. At one site and for a given water content, different resistivity values can be compared after correction of the temperature effect. The latter is calculated from the Campbell equation (Campbell et al., 1948): rT ¼ r25 C ½1 þ aðT 25 CÞ
(2)
where T represents the temperature, rT the electrical resistivity measured at temperature, r25 C the electrical resistivity at the 25 8C reference temperature, and a a correction factor equal to 0.02. 2.2. Experimental site Experiments were conducted at the experimental site of INRA Estre´ es-Mons (508N, 38E, elevation 85 m), in northern France (Boizard et al., 2002). The studied soil is a Haplic Luvisol (FAO Classification, 1998) developed on loess (Table 1). We focused our
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Table 1 Physical characteristics of the soils in plots P1 and P2 Clay (0–2 mm g kg1)
Fine silt (2–20 mm g kg1)
Coarse silt (20–50 mm g kg1)
Fine sand (50–200 mm g kg1)
Coarse sand (200–2000 mm g kg1)
Organic carbon content (g kg1)
Plot P1
Horiz. L, 0–30 cm Horiz. B, 30–60 cm
192 320
306 312
439 344
28 23
3 1
9 7
Plot P2
Horiz. L, 0–30 cm Horiz. B, 30–60 cm
197 248
264 280
490 433
36 32
13 7
9 5
study on the layers down to 0.50 m depth. Two plots were chosen:
Plot P1: a fine soil structure was first obtained by digging a bare soil during a dry period in autumn. Localised compacted bands were then created under wet conditions in spring using a heavy tractor (81.4 kN with rear tyres 0.65 m width inflated at 200 kPa). We aimed at producing homogeneous porous zones and more compacted ones in an even soil profile with known positions.
Plot P2: the second plot was from a long-term field experiment (Boizard et al., 2002). The crop rotation was sugar beet (Beta vulgaris L.)/winter wheat (Triticum aestivum L.)/maize (Zea mays L.)/winter wheat with an annual winter ploughing. Because of the soil compaction produced during crop harvesting in autumn in wet conditions, the soil structure is heterogeneous, with a percentage of compacted clods generally between 10 and 40%. Measurements were performed in spring before the sowing of maize. 2.3. Laboratory experiments Two soil blocks (0.30 m 0.30 m 0.20 m in size) were sampled from plot P1 described above: one from the compacted zone and one from the porous zone. Their respective bulk density values were determined on 98 cm3 soil cores sampled during the field experiment. Ten cylinders were sampled in the compacted zone and 7 in the porous zone. The soil blocks were stored in a cold room at T = 5 8C. Electrical resistivity measurements were performed immediately after removing the samples from the cold room (temperature T1). Two days later, after rewarming at room temperature, a second set of electrical resistivity measurements were performed (tempera-
ture T2). In both experiments, the soil block temperature was recorded by the use of a resistivimeter coupled to a thermoprobe PT100. The mean temperatures T1 and T2 resulted from measurements done before and after electrical resistivity prospecting at 0.20 m depth in the middle of blocks. The electrical measurements (apparent resistivity data) were obtained from a set-up composed of 16 separate steel electrodes (1 mm diameter) spaced 0.015 m apart in Wenner arrays (Samoue¨ lian et al., 2003). The potential differences were recorded on a Syscal R1 resistivimeter (Iris Instrument, Orleans, France) connected to a 12 V battery that results in a dc current amplified at 100 V. A multinode allowed the electrical current to be distributed to the 16 electrodes. The inverted resistivities were calculated with the Res2Dinv software (Loke and Barker, 1996). At the end of the experiment, we determined the gravimetric soil water content on 12 clods (several cubic cm3) sampled in each soil block. 2.4. Field experiments Field experiments were carried out on the two plots at the same period of the year on profiles 3.20 m wide. On plot P1, two experiments (P11 and P12) perpendicular to the traffic direction were conducted 1.50 m apart. They covered two wheel tracks located, respectively, between 0.40–1.00 and 2.25–2.85 m from the left corner of the profile. On plot P2, only one experiment (P21) was conducted. The electrical measurements were made using 32 separate brass electrodes (3 mm diameter) spaced 0.10 m apart. Except for the number and characteristics of the electrodes and the distance between two consecutive electrodes, the experimental protocol was identical to that used in the laboratory. Temperature measurements were made by a resistivimeter coupled to a
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thermometric probe PT100 at 0.05, 0.15 and 0.30 m depth at four locations along the studied profile: 0.20, 1.00, 1.80 and 2.60 m from the left corner of the profile. These measurements enabled to obtain a representative mean value of temperature in the whole profile. The measured electrical resistivity values were then corrected by the Campbell equation (2) (reference temperature: 25 8C). The inverted resistivities were calculated with the Res2Dinv software. Calculations of inverted resistivity were performed for seven layers with the following thickness: from 0.05 m for the first layer (half of the inter electrode spacing) to 0.168 m (Fig. 1b). At the end of the electrical prospecting, a soil pit was dug to characterise the soil structure with different methods:
2.5. Data presentation and analyses
A visual morphological profile was made in plots P11 and P21 by the method of Manichon (1987). Two main structural zones were visually distinguished as a function of the visible porosity and the roughness of the rupture face when soil lumps are manually broken: compacted zones without any visible porosity and a smooth rupture face, porous zones with visible porosity and a rough rupture face. A sequence of digital photographs was taken and joined together to represent the whole 3.20 m wide profile.
Thirty eight undisturbed soil cores were sampled in 98 cm3 metal cylinders on plot P11 under or outside the wheel tracks. Bulk density, gravimetric and volumetric water content were measured on these soil cores.
On plots P12 and P21, gravimetric soil water content measurements were also determined. On four vertical profiles located at 0.35, 1.35, 1.75 and 2.60 m from the left corner of the profile, soil samples were taken at 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30 m depth.
3.1. Laboratory analyses
All the measurements (laboratory and field data) were described by classical statistical variables. Both apparent and inverted resistivity values are presented in the following part. As the populations of apparent and inverted resistivities were asymmetrical and dispersed, the median values were compared with nonparametric tests of Kolmogorov–Smirnov and Mann–Whitney (P < 0.05). The distributions of the bulk densities and soil water contents were normal and mean values were compared with the parametric test of Student at the 5% significant level.
3. Results
The mean values of bulk density and volumetric water content were significantly larger in the compacted block than in the porous block (Table 2). Gravimetric water content was the same in the two blocks. For each soil block, the median value of apparent resistivities was higher for measurements at 4 8C than at 20 8C and the difference in apparent resistivity between the two temperatures was about 30 V m (Fig. 2). After temperature correction by the Campbell equation (2), the difference between the two measurements for each soil block was not significantly different, which validated the use of this equation for electrical resistivity measurements in soils. The apparent resistivity values were significantly larger for the porous block than for the compacted block (Fig. 2). The interquartile interval was larger for the porous block than for the compacted block which showed the greater heterogeneity of the porous block. The apparent resistivity data, after temperature correction, were inverted with the Res2DInv software
Table 2 Bulk density, volumetric water content and inverted electrical resistivity for the porous and the compacted blocks in laboratory experiments
Porous block Compacted block
Bulk density (Mg m3)
Volumetric water content (m3 m3)
Inverted electrical resistivity (V m)
1.39 (0.04) (n = 7) 1.59 (0.05) (n = 10)
0.320 (0.007) (n = 12) 0.301 (0.012) (n = 12)
38 (22) (n = 68) 27 (13) (n = 68)
For bulk density and volumetric water contents, the main numbers are mean and the numbers between brackets are standard deviations. For inverted electrical resistivity, the main numbers are the median values and the numbers between brackets are interquartile intervals.
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Fig. 2. Apparent resistivity values for the porous and the compacted blocks. T1, T2: experimental measurements at temperatures T1 and T2; T125, T2-25: experimental measurements corrected by the Campbell equation for estimation at 25 8C.
for each experiment in each soil block (Fig. 2). The inverted resistivities were not equal to the apparent resistivities, the discrepancy being higher in the porous block. This was due to structure heterogeneities within each block: with a perfectly homogeneous block from an electrical point of view, the apparent and inverted resistivities would theoretically have been equal. By mixing all the inverted values of the experiments (at T1 and T2) for each block, we calculated a unique median value representative of the porous and compacted structure (Table 2). The difference in inverted resistivity between the two blocks was 9 V m, with a lower resistivity value in the compacted block, that was consistent with the apparent resistivity values. As for the apparent data, the interquartile interval was larger for the porous block than for the compacted block certainly due to higher internal structural heterogeneity of the porous block.
For each depth, differences in median values for plots P11 and P12 were not statistically different. For plots P11 and P12, the median value decreased with depth from the second level. For the plot P21, the median value decreased from the surface. The interquartile intervals decreased with depth for each soil profile, indicating a lower heterogeneity of resistivity values in depth. The interquartile interval for the first level was particularly high showing the strong heterogeneity of the near-surface electrical values (<0.15 m). A more precise analysis of the lateral distributions of electrical resistivity values on plots P11 and P12 showed that the highest electrical resistivity values were located at two places along the soil profile: between 0.40 and 0.95, and 2.30–2.75 m for P11; between 0.40 and 1.05, and 1.75–2.85 m for plot P12. These places corresponded to wheels tracks. On the plot P21, the heterogeneity was high as well in the first level but no particular structure in electrical resistivity was seen.
3.2. Field experiments 3.2.1. Electrical resistivity measurements Whatever the soil profile, the inverted resistivity map showed both vertical and horizontal variabilities (Fig. 3). On each soil map, the median value and interquartile intervals have been calculated along horizontal lines for all the seven depth levels (Fig. 4).
3.2.2. Visual morphological description Within P11 and P12 soil profiles, a visual morphological description was made of different zones with regard to soil bulk density (Fig. 5). We identified compacted zones, where no porosity could be seen with the naked eye. In the remaining zones, more or less porous soil volumes were identified. In
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Fig. 3. Inverted electrical resistivity panels for the prospecting for the three plots. Black lines represent the contours of the compacted zones as interpreted from Fig. 5. The arrows point on small compacted clods not detected by the electrical resistivity prospection.
plot P11, large compacted zones (about 0.60 m 0.10 m) were identified at locations 0.40–1.00 and 2.30–2.90 m. They corresponded to wheel track ways. Several other small clods, from 5 to 10 cm in size are located between or under the wheel tracks. Around 0.20–0.25 m deep, a continuous compacted layer, several centimeters wide, corresponded to the depth of the last ploughing. In plot P21, voids due to tillage or macro fauna activity (1–5 cm diameter) were identified and straw residues from the previous wheat crop were buried in the 0–0.30 m layer. Compacted clods of different sizes were located all along the soil profile. Other cracked clods were numerous, usually located in the first 20 cm. Limits of the plough pan could be identified in several places.
3.2.3. Bulk density and water content measurements In plot P11, 38 undisturbed soil cores were taken to determine both the bulk density and the water content (Fig. 5). The mean bulk density measured in the compacted zones was similar than that of the compacted block (Tables 2 and 3). The mean bulk density measured in the porous zones and its standard deviation were higher than those of the porous block (Tables 2 and 3). A more precise analysis of the porous zone showed a significant difference of bulk density according to the position of the cylinders. Just under the compacted zones, the mean bulk density value was 1.53 Mg m3. In the middle of the profile, outside any wheel tracks, the mean bulk density value was 1.39 Mg m3. There was a density gradient
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Fig. 4. Median values and interquartile intervals of inverted electrical resistivity measurements, calculated at the seven prospected levels on plot P11.
created by wheel tracks. Gravimetric water contents (0.21 g g1) were not significantly different between compacted and porous zones. Nevertheless, the volumetric water content was higher in the compacted zones than in the porous zones. In plot P12, no large soil pit was dug and a complete description of the
water content was not done. Nevertheless, gravimetric water content measured in compacted zones (resp. porous zones) in plot P12 (0.20 kg kg1) was not different from that measured in compacted zones (resp. porous zones) in plot P11. In plot P21, the gravimetric water content was significantly different
Fig. 5. Visual morphological description of the P11 and P21 soil profiles. Yellow crosses on the profile P11 represent the locations of soil cores sampled for bulk density and water content measurements. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.
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Table 3 Mean bulk density, mean volumetric water content and median inverted electrical resistivity in porous zones and compacted zones for plot P11 during field experiments Bulk density (Mg m3) a
Porous zones Compacted zones Pedological horizon
b
1.48 (0.09) , n = 22 1.59 (0.05), n = 12 1.53 (0.01), n = 4
c
Volumetric water content (m3 m3)
Inverted electrical resistivity (V m)
0.307b (0.039), n = 22 0.339 (0.008), n = 12 0.349 (0.011), n = 4
40 (15)d, n = 60e 30 (6), n = 36 23 (4), n = 41
a
Porous zones were located outside the wheel tracks or in the wheel tracks under the compacted area. Numbers between brackets are standard deviations. c Number of replicates. d Interquartile interval. e Due to the size of the cylinder used to measure bulk density and of the grid used to calculate inverted electrical resistivity, three or four inverted electrical resistivity values were considered for one bulk density measurement. b
(0.23 g g1) from those measured in plots P11 and P12. From our measurements, no spatial variation of water content was found.
4. Discussion Laboratory experimental results show that the resistivity value is equal to 38 V m in the porous block (bulk density of 1.39 Mg m3) and 27 V m in the compacted block (bulk density of 1.59 Mg m3) for a loamy soil at 0.20 kg kg1 water content and 25 8C temperature. Insofar as the porous and compacted blocks were sampled just after compaction, at the
same time and in the same soil and location, the difference between the two samples only results from the difference in bulk density. No variation in other soil parameters like texture and salinity, could be suspected. As a consequence, the significant difference in electrical resistivity between the two blocks (11 V m) was due to the difference in soil structure. Similar values of electrical resistivity were obtained, in field conditions, for the same structures and water content, when the effect of temperature variation on electrical resistivity was corrected using the Campbell equation. Therefore our results show that soil electrical resistivity can be used to discriminate, at least, two extreme structural states of the same soil. The
Fig. 6. Inverted electrical resistivity according to the measured bulk densities in the porous and the compacted zones on plot P11.
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small difference (11 V m) requires us to control the temperature measurement conditions. Moreover, the soil cores that were sampled for bulk density determination on the plot P11 allow us to establish the variation of electrical resistivity with bulk density (Fig. 6). Electrical resistivity clearly decreased with increasing bulk density, with a non-linear relationship. Any other relationship cannot be found in the literature. The shape of the relationship, which probably depends on water content and soil type, has to be established for various soils conditions. The effect of soil bulk density on soil electrical resistivity allows us to distinguish three main structural features: 1. The plough pan: around 0.25–0.30 m on plot P11, low values of resistivity (around 29 V m) corresponds to the plough pan and are equal to the compacted zone value. This is consistent with the hypothesis that the compacted zones and the plough pan have the same origin. Under that limit, the lower resistivity value (23 V m) is measured in the B horizon (Table 1). The clay content of the B horizon is higher than that of the surface horizon which can explain a lower resistivity (Fukue et al., 1999; Robain et al., 2001). 2. The wheel tracks: on plot P11, the distribution of inverted electrical resistivity values in the first 0.15 m shows a high heterogeneity. It is lower in two bands located at 0.40–0.95 and 2.30–2.75 m. The comparison with the visual description of the soil profile suggests that these lower values are located in compacted bands corresponding to former wheel tracks. The other higher values are located in zones identified as more being porous. 3. The clods: on plot P11, some compacted clods (size < 0.10 m) were included in the ploughed layer outside the wheel tracks (Fig. 5). They were not detected by the electrical experiment, probably because of the 0.10 m inter-electrode spacing which did not allow detection of objects smaller than 0.10 m. On plot P21, the 40–50 V m values were preferentially located in the first 15 cm layer near the surface where the soil was effectively the most porous without any compacted clods. Compacted clods were mainly located in the 0.15–0.30 m layer of the plot P21. Their diameter varied from 5 to 25 cm. The electrical resistivity
map did not allow us to localise the compacted clods, even the largest ones. It was the same thing for voids, cavities and straw residues that are supposed to induce a high electrical resistivity. This discrepancy would be due to a difference in the soil volume that is characterized by the morphological description or by the electrical method. For the visual morphological description, a strict 2D vertical plan is analysed. With the electrical resistivity method, a 3D space is investigated but its exact volume cannot be determined. The 2D inverted resistivity gives a characterization of the soil structure that corresponds to the projection of a 3D integrated volume. As one possible consequence, the simulation of water transfers in cultivated soils may be more accurate if an electrical description of the cultivated soil layer is used.
5. Conclusions Soil structure, which is a key component of the soil physical quality, is still difficult to characterise because of its heterogeneity in agricultural fields. Our study, aimed at evaluating the use of a geophysical method, uses 2D electrical resistivity measurements to describe the structural heterogeneity of cultivated soils. We showed that soil electrical resistivity is sensitive to bulk density: an increase of the bulk density from 1.39 to 1.59 Mg m3 in a loamy soil corresponds to a decrease of the electrical resistivity of 11 V m. This variation allows us to distinguish structural features such as plough pan or wheel tracks in field conditions using a 2D investigation. The use of this method to characterise the structure of tilled horizons, such as clod size, needs the testing of a 3D investigation procedure. This work is a first step to obtain a non-destructive method to characterise the variability of the structure of cultivated soils, in both in time and space.
Acknowledgements The authors are grateful to Bertrand Chauchard, Pierre Courtemanche and Paul Re´ gnier for their assistance with the laboratory and field experiments.
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The electrical resistivity device, especially multinodes, were rented by courtesy of the Geofcan French group.
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