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Viremouneix, T. et al. Proc. Int. Soc. Sugar Cane Technol., Vol. 26, 2007 ____________________________________________________________________________________________

ELECTRICAL RESISTIVITY MEASUREMENTS FOR FAST AND PRECISE LARGE SCALE CHARACTERISATION OF THE AGRICULTURAL

LAND OF CAMEROON SUGAR SOCIETY (SOSUCAM)

By

T. VIREMOUNEIX1, L. GUIARD2, M. DABAS3 and B. TSOGO ZAMBA1

1SOSUCAM, BP 857, Yaoundé, Cameroon 2SOMDIAA, 39 rue Jean-Jacques Rousseau, 75001 Paris, France

3GEOCARTA, 16 rue du Sentier, 75002 Paris, France [email protected]

KEYWORDS : Sugarcane, Resistivity, Soil Map, Crop Management, Sustainable Agriculture.

Abstract

A GOOD knowledge of soil properties and their agricultural suitability is a major prequisite for managing agriculture in a planned, sustainable, and environmentally friendly manner. To optimise and extend its agricultural area, Cameroon Sugar Society (SOSUCAM) has thus decided to use an innovative technology based on a survey of the electrical resistivity of soils. In the first step of the project, the feasibility and interpretation of the measurements were tested on the Oxisols of SOSUCAM by an electrical survey conducted at two depths (0.5 and 1 m) on a 40 ha sugarcane field. Specialised software was then used to interpolate the measurements. Soil samples were taken and analysed simultaneously, and sugarcane was monitored in specific areas through measurements of growth, tillering and yield. Areas with homogenous resistivity and common properties have been identified using the map produced. The electrical values were correlated with the fields physical properties such as its stoniness, depth and clay %. We have also observed different sugarcane growth patterns depending on resistivity values. In view of these good results, SOSUCAM has decided to extend this experiment to 26 000 ha of sugarcane fields and fallow land, without any particular reference to soil type, through the adaptation of a specific and patented technology, named ARP06© (for Automatic Resistivity Profiling). This technology, which has an adequate system of electrodes configuration, makes it possible to prospect continuously at different depths and get data to elaborate, through interpolation, resistivity maps. Resistivity surveying should thus be a quick and reliable technique to map the spatial variability of soil properties on large sugarcane areas and to define the latters agricultural potential. Furthermore, it should provide other important elements in field planning, beyond logistic constraints: delineating homogenous zones should improve the use of results of physical and chemical analyses and to identify zones where better practices and operations can be adopted.

Introduction The Cameroon Sugar Society (SOSUCAM) is a sugar estate established in 1966 which has

gradually expanded to over a 20 000 ha area in central Cameroon. The company started characterising its soils in 2004, using surveys and auger sampling, since soil data are essential for optimum agricultural management. This venture is restricted by the length of time required, cost, and level of precision achieved. SOSUCAM has therefore considered the use of resistivity mapping,

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initially used in archaeology and geophysics but now adopted in agriculture, since it is faster and non destructive (Grant and West, 1965; Rhoades and Ingvalson, 1971; Boigontier, 2000; Kearer et al., 2002). SOSUCAM initiated a large scale field mapping project in November 2006 following a testing stage to determine the usefulness and feasibility of this technique conducted on its land. We will present the different techniques used in this project and the first results obtained, which demonstrate that links exist between soil properties, plant growth data and resistivity when the latter are compared to concurrent observations and analyses. Thus, when used together with other local observations and parameters, resistivity measurements could be an appropriate indicator of agronomic parameters. This technique shows interesting prospects for the management of sugarcane fields and the selection of new agricultural land. Material and methods

Geographical situation and agro-pedological background SOSUCAM is established in central Cameroon, approximatively 150 km northeast of the

capital Yaoundé, in an area previously under grass savannah and forest edges. Most of the soils are classified as Oxisols, consisting of ferralitic soils with varying degrees

of oxidation, hydromorphic soils and poorly evolved soils (Vallerie, 1973). The fields are situated in interfluvial zones bordered by forested areas and consist typically of three main units: 1) a plateau and a convex higher hillside zone with well-oxidised, reddish, deep ferralitic soils, 2) a straight mid-hillside zone with frequent patches of lateritic and gravelly soil profiles overlying red ochre soils, and 3) a straight to concave lower hillside zone, with hydromorphic soils.

It was considered appropriate to proceed with the soil mapping and characterisation project because of the high pedological heterogeneity.

Introduction to soil measurements The soils resistivity, expressed in terms of ohm.m (Ω.m) is an expression of its ability to

restrict the passage of an electrical current. This parameter is often used by geophysicists and pedologists. Indeed, under the various soil conditions where it has been studied, resistivity has been shown to be closely related to intrinsic and perennial soil characteristics such as clay fraction, water holding capacity, rockiness, depth and geological substratum (Dabas et al., 1989a). Mapping the apparent resistivity of soils could be a means of highlighting the spatial variability of their intrinsic perennial properties.

Testing phase: manual resistivity measurements A manual method to measure electrical resistivity was used in the feasibility study phase of

the project. This system allowed us to explore down to 50 cm and 1 m depth and was made up of a set of horizontal in-line electrodes, with a space of 1 and 2 metres (Wenner configuration), a resistivity meter and a GPS (Figure 1). Measurements were taken in 2005 on 10 m by 25 m grids (i.e. 40 measurements per ha) over an area of 40 ha, in field (A100) which was relatively heterogeneous.

Pre-industrial phase: continuous measurements on tractor mounted system (ARP©) A tractor mounted by the ARP06© system (Automatic Resistivity Profiling) was used in the

2nd phase of the project at the end of November 2006 (Dabas et al., 1989b). This specific instrumentation, manufactured by the French Jammet Company1, was custom

designed for SOSUCAM by the Géocarta Company to allow continuous electrical measurements on sugarcane planted fields.

It consisted of a frame on which four pairs of isolated electrodes (coulter disks) were placed (one dipolar pair emitting the current (injection) followed by three receiving dipoles), a 1 Jammet Company : 12 Grande Rue 45390 Echilleuses, France

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differentially corrected global positioning system (dGPS), a radar (for position control and movement) and an on-board PC for real-time data logging and control (Figure 2).

The system was configured to take measurements at depths of 0.5, 1.1 and 2.1 m (Figure 3). The electrodes were positioned to take measurements in the inter-row of the standing crop (non-destructive measurements).

Fig. 1Manual resistivity measurements on field A100 using Wenner array (july 2005).

Fig. 2Tractor mounted model ARP06© for continuous

electrical measurements.

Voie 3 Voie 3Voie 2 Voie 2Voie 1 Voie 1

+ -

0.5 m

1.1 m

2.1 m

gén.

Fig. 3Equipment operation with

electrodes, generator and resistivimeter (schematic)

The measurements using the ARP06© were initiated at the end of November 2006 on

SOSUCAM fields where cane height allowed tractor traffic (i.e. 23 months old). Determinations were made at a rate of almost 50 ha/day. Uneven terrain restricted the average speed of the device to 10 km/h. Measurements were taken in passes through the field at 10.5 m (7 cane rows) intervals.

Measurements were also made in fallow areas planned for extension following an initial path clearance using a Bulldozer blade to uproot the vegetation and allow tractor movement. Each clearance pass was made at 50 m intervals.

Field data were analysed by the Géocarta Company, interpolated by specific developed software. Interpretive georeferenced maps, at a resolution of 6 m by 6 m, were also created for each of the three soil depths.

Complementary studies and observations The study on the relationship between field resistivity and soil physical properties was based

on soil auger samples, from an isotropic 50 by 50 m sampling grid. Different parameters were observed such as soil depth, proportion of coarse elements (> 2 mm), texture (evaluated by touch)

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and colour (using the soil colour Munsel chart), and specific maps were elaborated. Furthermore, following the first measurements done in 2005, we recognised five zones, characterised by their resistivity values, namely: low (lower field area zone B and plateau zone A), medium (zone C), high (zone D) and very high resistivity (zone E).

Many plots were localised on these areas (10 repetitions for each area), where specific soil sampling was done, in order to study soil chemical properties2, but also agronomic observations, such as stalk elongation measurements and tiller counts, in order to evaluate the link between resistivity and cultural characteristics.

Stalk length measurements were made every 15 days from the age of 4.5 months to 7.5 months after harvest on 5 sugarcane stalks in the different plots of each area. To estimate tillering per hectare, we counted the number of stalks in a 100 m² area for each plot.

Finally, we created a yield map, by the monitoring of the harvest of a little field (7.2 ha): the load of each trucks trailer was followed by GPS, in order to determine the real distance of load.

Using the weigh data obtained at the entry of the factory, it was possible to link tonnage to the areas where sugarcane was loaded, and thus to realise this map. This map was then compared to the resistivity one. Results and discussion

Measurement feasibility and validation of ARP06 technology The first step of the project has cleared all the doubts regarding the feasibility of resistivity

measurements on our soils, whose specific properties could have distorted the electrical measurements (high iron concentration, presence of gravelly and laterite layers).

The electrical signals produced were quantifiable, even though their values were much higher than those normally recorded in developed countries (values varying between 100 to more than 2000 Ω.m).

The in-field resistivity variation was recognisable from the resistivity map produced from these measurements (Figure 4).

The manual measurements yielded results similar to the continuous survey with the ARP06©, with a useful correlation (Figures 5 and 6).

In addition ARP06 made it possible to collect more data in less time at higher resolution. In comparison to manual measurements, the measurements made in the second sampling

conducted with ARP06© tend to be lower. Differences in the methodology could partially contribute to this difference. Differing

hydrological and cultural conditions that occurred during the two observation periods are likely to be the largest contributor to the differences observed (Benderitter and Shott, 1997; Goutouly, 2006).

On the one hand, November coincides with the end of the main rainfall season (from August to early November) while July is in a drier intermediate season.

On the other hand, the test field (A100) was replanted in August 2006 and land preparation led to topsoil decompaction and higher soil porosity, which improved the ability for electrical current to pass through the soil.

It is important to note that the absolute value of the soils apparent resistivity is not important as long as their relative variations remain the same.

With respect to data collection, the ARP technology appears to be six times faster at twelve times higher spatial resolution than the classical manual surveying technique for map production (Table 1). 2 soil samples were analysed by the laboratory of the CIRAD (Montpellier, France)

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Fig. 4Resistivity map at 1 m depth from manual measurements made in July 2005 on field A100.

Fig. 5Compared resistivity map at 1 m depth produced by ARP06© in November 2006 on the

same field

Fig. 6Comparison between resistivity measures done by both methods at 1 m

depth on field A100 Manual (July 2005) and ARP06 (November 2006)

Table 1Comparison of grid intensity, measurement frequency and resolution

among 3 kinds of mapping techniques used in this evaluation (ARP06 measures, manual resistivity measures, and auger sampling).

Resistivity ARP06© Resistivity manual Auger survey

Sampling grid 20 cm x 10.5 m 25 m x 10 m 50 m x 50 m Measurements per ha 14 087 80 4 Area (ha/day) 50 ha/day 14 ha/day 8 ha/day

Resistivity measures with both methods1m depth

0

500

1 000

1 500

2 000

2 500

0 500 1 000 1 500 2 000 2 500ARP (ohm.m)

man

ual (o

hm.m

)

r = 0.75r² = 0.56

Significant range : 95%

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Resolution (after Interpol.) 1/1200 1/4050 1/15 000 Furthermore, a larger spacing of continuous measurements was simulated by considering

only one line on two in the data treatment (21 m instead of 10 m), without a visual loss in quality of the obtained map. If it is confirmed by further observations and studies (such as geostatistics semi-variograms and kriging for example-), it would be possible to improve productivity.

Relationships between resistivity and agro-pedological characteristics Soil physico-chemical properties In the first instance, we have been able to compare resistivity values and maps to values

inferred from observations and auger samples obtained from an isotropic 50 by 50 m sampling grid. There is a correlation between apparent resistivity, soil depth and percentage of coarse particles (>2 mm) (Figures 7 and 9), which is statistically significant at a 95% range (Figure 8 and Figure 10). In addition, the texture, which was evaluated by the touch method (so relatively subjective and without important variability), shows nevertheless some links between resistivity map and clay content (Figure 11).

It appears that the highly gravelly and shallow zones (first signs of laterite layer) have high resistivity values. Resistivity in the shallow zones decreased with increasing amounts of clay. The presence of the laterite layer is obvious where there is a change of incline while the deep soils of the higher plateau (central part of the field) have lower resistivity levels. Finally, the hydromorphic zones situated at the bottom of the field (upper part of the map) have remarkably low resistivity levels caused probably by their higher water content and percentage of fine particles, coming from soil erosion. This shows also the importance of geomorphologic knowledge in order to interpret resistivity map.

Fig. 7Soil depth (auger sampling) and ARP06 resistivity map (1 m field A100).

Fig. 8Relationship between soil depth and resistivity at 1 m depth (field A100)

significant range: 95%.

Fig. 9% coarse particles (>2 mm) (auger sampling) and ARP06 resistivity map (1 m field A100). Fig. 10Relationship between % coarse

particles (>2 mm) and resistivity at 1 m depth

Soil depth vs Resistivity 1m

0

200

400

600

800

1 000

1 200

1 400

1 600

1 800

0 20 40 60 80 100Soil depth (cm)

ohm

.m -

1m

r = 0.72r² = 0.51

% coarse particles vs Resist. 1m

0200400600800

1 0001 2001 4001 6001 800

0% 20% 40% 60% 80% 100%% Coarse particles 0-1m

ohm

.m -

1m

r = 0.71r² = 0.50

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significant range : 95%.

Fig. 11Percent clay content (evaluated by touch on soil sampling) and ARP06

resistivity measures (depth: 1 m field A100).

Chemical analysis of soil samples from the five resistivity zones highlights some correspondence between resistivity and chemical values (Table 2).

In general, sum (S) of the 4 exchangeable cations (Ca2+, Mg2+, K+ and Na+), CEC values (S, H+, Fe2+ and Al3+) and resistivity are inversely related.

Contrary to the areas A, B and C, characterised by low values of resistivity and a relatively high exchange capacity and high base saturation, the higher resistivity zones, D and E, had low CEC, S values and a very high Al/CEC ratio, approaching or exceeding the critical value for aluminium toxicity of 40.

This trend is confirmed by the value of the saturation level S/T, lower in the areas D and E: this indicates a potential difference in soil fertility (Baizen, 2000).

In addition, the high values of P and K in area A could probably be due to lixiviation (case of K) and superficial erosion (case of P), causing the move of the nutrients from the top to the bottom of the field.

Table 2Results of soil chemical analyses (CIRAD laboratory) on the 5 different zones in field A100,

determined by resistivity values.

Resis1 m

(Ω.m) pH

H2O O.M. (%)

N (%)

P (mg/kg)

K (me%)

CEC(T)(me%)

S (me%)

S/T (%)

Al/T (%)

A-low-lower part 252.9 4.8 1.5 0.040 26.3 0.30 3.0 1.27 42.2 27.3 B-low-plateau 337.2 5.0 1.5 0.052 14.3 0.08 2.5 1.04 40.7 30.2 C-average 398.3 5.0 1.5 0.050 12.8 0.08 2.5 1.14 44.9 29.2 D-high 469.4 4.9 1.5 0.035 15.7 0.11 2.4 0.81 33.6 32.9 E-very high 756.7 4.8 1.5 0.028 14.3 0.13 2.4 0.70 29.1 44.0 Mean 429.8 4.9 1.5 0.043 16.3 0.13 2.6 1.00 38.8 32.3

Relationships with agronomic characteristics Elongation measurements and tiller counts Stalk elongation has been monitored for each resistivity zone at regular 15-day intervals.

The results of this agronomic monitoring show that there was better cane growth (Figure 12) in low resistivity zones (A, B and C) compared to high resistivity zones (D and E).

Furthermore, we can notice that the good growth period after rainfall is longer in low resistivity areas, which indicates better water holding properties and water use there. This is

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presumably related to soil depth and texture.

Fig. 12Stalk elongation rate and length in relation to resistivity zones over eight

months and total rainfall (field A100).

Furthermore, tiller counts were taken after three, four and a half and five months following

the harvest in May 2005 (Table 3). Except for the specific hydromorphic soils, where the tillering is lower during the whole

cultural period (explained by the waterlogged condition occurring in that part of the field), it seems to be a general trend to lower tillering at the first cultural stage (3 month) in the more resistive zones (C, D and E). However, the high tillering at 3 months on the low resistivity area, B, is compensated by a higher mortality rate, certainly due to more competition.

Table 3Tiller count data for each resistivity zone in field A100 taken three, four and an half and six months after harvest in May 2005, and evaluation of mortality rate.

Tillering Diff. from mean Mortality rate

Zone Resis.1 m 3 months 4.5 months 6 months 3 months 6 months 36 monthsA-low-lower part 252.9 94 955 63 760 64 926 11.9% 11.8% 31.6% B-low-plateau 337.2 121 384 85 124 78 378 12.7% 6.5% 35.4% C-average 398.3 108 022 90 386 75 108 0.3% 2.1% 30.5% D-high 469.4 103 943 81 362 71 838 3.5% 2.4% 30.9% E-very high 756.7 109 348 83 166 77 625 1.5% 5.5% 29.6% Mean 429.8 107 720 80 976 73 575 30.5%

Cane yield (t/ha) We prepared a map of gross sugarcane yield in December 2006 based on close monitoring

of the harvest and loading of a 7.2 ha field. We made subsequent resistivity measurements on this

0.0

10.3 12.06.5

1.06.7

0.9 0.27.1 4.1

21.4

7.10.0 0.0 0.4 0.0 2.5 1.3

Rainfall21.5

Growth-A

Growth-B

Growth-C

Growth-D

Growth-E

Length-A

Length-BLength-C

Length-D

Length-E

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 90

20

40

60

80

100

120

140

160

180

Rainfall Growth-AGrowth-B Growth-CGrowth-D Growth-ELength-A Length-BLength-C Length-DLength-E

Grow

th (c

m/da

y)

Leng

th (cm

)

Field A100 - Growth and length stalkRepartition by resistivity zones

Duration (month)

Date of last harvest :Date of first controle : Age first controle :Date of last controle : Age last controle :

08/05/05

14/12/057.3 month

24/09/054.6 month

Rainf

all (

mm)

Variety : Co997Ratoon : R7

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same field after harvest. Apart from border effects such as proximity to forested zones and traffic areas, the maps of resistivity, soil depth and yield are in relatively close agreement. Gross sugarcane yield per ha appears to be inversely related to soil depth and resistivity (Figures 13, 14 and 15). This is confirmed by the statistical analysis of the data (Figure 16), which shows a significant correlation (with 95% confidence range), in spite of a relatively weak r² value, due essentially to the method of yield mapping, less precise than the resistivity measurements, but also to other agricultural non-controlled factors (such as fertilisation, weed control).

Fig. 13Resistivity map

prospected by ARP06 2 m.

Fig. 14Soil depth map done by auger sampling (1 m depth).

Fig. 15Cane yield map t/ha done by load monitoring (December

2006).

0

20

40

60

80

100

120

0 500 1000 1500 2000 2500Resistivity - 2.1m - ohm.m -1

TC/ha

r = 0.60r² = 0.36

95% confidence range

Data not considered in the regression

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Fig. 16Relationship between resistivity and gross sugarcane yield (t/ha) collected in field C2 in December 2006.

Limitations and discussion The results obtained with this new technology were encouraging and promising. This

methodology is also being tested on a larger scale. Resistivity values were linked with most of the parameters observed (soil physico-chemical properties, cane growth and yield). The most unfavourable production occurred in the high resistivity zones and the waterlogged low-lying hydromorphic soil area. The hydromorphic soils area is characterised by the lowest resistivity, poorest growth and yield. This indicates that other considerations (namely geomorphic knowledge) are necessary in the site characterisation process apart from resistivity alone. So resistivity surveying must always be associated with specific and local observations to determine adequate correlation.

In addition, the practical aspects related to optimal data collection in this are not complete. Further studies to maximise the distance between measurements (currently at 10 m) to increase the working rate without losing resolution need to be conducted.

Applications and future management of resistivity maps The maps should allow us to rapidly delineate fields and identify homogeneous resistivity

units. These areas would be characterised by their soil properties and also their agronomic potential. On this basis, and in association with adequate experimentation, it would also be possible to determine and select the most appropriate series of tillage operations and the best cane varieties. Furthermore, it could be helpful to realise targeted chemical analyses on the delineating homogeneous zones, in order to optimise and manage better the fertilisation of sugarcane on these areas.

Finally, this method could be helpful to identify the most suitable zones and design appropriate field plans in potential expansion areas. In contrast to the past, where the choice and organisation of the fields were realised only on logistic constraints, this technology would permit us to consider other important parameters, such as culture potential and field homogeneity. So, the first measurements for additional expansion in November 2006 allowed us to select a 90 ha portion that is characterised by low resistivity, without hydromorphic soils, and therefore greater agronomic potential from the 175 ha area surveyed (Figure 17).

Fig. 17Zone surveyed by ARP06 for expansion (175 ha) and selected zone (90 ha).

Conclusion Electrical resistivity measurements may be feasible under the soil and climatic conditions of

SOSUCAM. These measurements have allowed SOSUCAM to embark on a large scale field mapping project using an innovative continuous soil resistivity surveying technology (ARP). This

260


technology is relatively fast, reasonably reliable and non-destructive to soils. The concurrent analyses and observations confirmed that there could be relationships between resistivity and soil and plant growth parameters. This needs nevertheless to be further studied and confirmed. Resistivity could be used as an efficient indicator of agro-pedological properties of sugarcane fields, linked to field characteristics and observations (topography, morphology).

Resistivity maps supported by local observations have a good prospect for use in agricultural development and management. Electrical resistivity measurements might be used for field planning by locating homogeneous agricultural management units for which optimised cultural operations can be chosen.

The electrical resistivity mapping of expansion zones prior to agricultural development might also make it possible to choose the most suitable zones for sugarcane cultivation.

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Paris, 2ème édition. Benderitter, Y. and Shott, J.-J. (1997). Chute de pluies et résistivité du sol. Colloque GEOFCAN

Abstracts et résumés étendus, 127131. Boigontier, D. (2000). La mise en oeuvre de lagriculture de précision, 365374, Actes du colloque

des 29 et 30 Mai 2000, ENESAD, Educagri eds., Dijon, 431 p. Dabas, M., Hesse, A., Jolivet, A., Tabbagh, A. and Ducomet, G. (1989a). Intérêt de la

cartographie de la résistivité électrique pour la connaissance du sol à grande échelle, Science du Sol, 27 (1): 6568.

Dabas, M., Hesse, A. and Jolivet, A. (1989b). Prospection électrique de sub-surface automatisée, éditions CIRA (Centre Interdisciplinaire de Recherches Archéologiques), Bruxelles, 7381.

Goutouly, J.-P., Rousset, D., Perroud, H. and Gaudillère, J.P. (2006). Caractérisation de la variabilité spatiale et temporelle de l'état hydrique d'un sol de vigne par résistivité électrique, Vie Congrès International des terroirs viticoles 2006, 292297.

Grant, F.S. and West, G.F (1965). Interpretation Theory in Applied Geophysics. McGraw Hill Book Co., New York.

Kearer, P., Brooks, M. and Hill, I. (2002). An Introduction to Geophysical Exploration. Blackwell Science (3), 183207.

McCarter, W.J. (1984). The electrical resistivity characteristics of compacted clays. Géotechnique, 36: 263267.

Rhoades, J.D. and Ingvalson, R.D. (1971). Determining salinity in field soils with soil resistance measurements. Soil Sci. Soc. Amer. Proc., 35: 5460.

Vallerie, M. (1973). Contribution à létude des sols du Centre-Sud Cameroun. Type de différenciation morphologique et pédogénétique sous climat subéquatorial. Trav. et doc. ORSTOM, N029, Paris Ed. 111 p.

Carte pédologique et daptitude pour la canne à sucre du site sucrier de Camsuco à Bandjock-Nkoteng, Rapport technique N°5, Projet PNUD-FAO, ONAREST, 51.

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APPLICATION DES MESURES DE RESISTIVITE POUR UNE CARACTÉRISATION RAPIDE ET PRÉCISE EN GRANDES SURFACES DES TERRES DE LA SOCIÉTÉ

SUCRIÈRE DU CAMEROUN Par

T. VIREMOUNEIX1, L. GUIARD2, M. DABAS3 et B. TSOGO ZAMBA1 1SOSUCAM, BP 857, Yaoundé, Cameroon

2SOMDIAA, 39 rue Jean-Jacques Rousseau, 75001 Paris, France 3GEOCARTA, 16 rue du Sentier, 75002 Paris, France

[email protected] MOTS CLEFS: Canne a Sucre, Résistivité, Carte des Sols, Unités de Gestion Culturale, Agriculture Raisonnée et Durable.

Résumé LA CONNAISSANCE des propriétés des sols et de leurs aptitudes culturales est un enjeu majeur pour la conduite d'une agriculture raisonnée, pérenne et respectueuse de l'environnement. Dans une perspective d'optimisation et d'extension de ses surfaces, la SOSUCAM a ainsi décidé d'avoir recours à une technique innovante de caractérisation basée sur des mesures électriques de la résistivité des sols. La première étape du projet a consisté à tester la faisabilité et l'interprétation des mesures sur les sols ferralitiques de la SOSUCAM, par des profilages électriques effectués à 2 profondeurs (0.5 et 1 m). Parallèlement, des échantillons de sols ont été prélevés et analysés et la culture suivie en des zones spécifiques, par des contrôles de croissance, de tallage et de rendement. La carte obtenue a permis de définir des zones de résistivité homogène ayant des propriétés particulières. Il a été alors mis en évidence des corrélations entre les valeurs électriques et certaines caractéristiques physico-chimiques de la parcelle comme la pierrosité, la profondeur, ou la texture Nous avons également mis en relation des différences de développement de la canne à sucre selon la valeur de résistivité. Considérant ces premiers résultats encourageants, la SOSUCAM a décidé de poursuivre cette expérience à grande échelle sur 26 000 hectares en culture ou en friche, sans référence précise de pédologie, en y adaptant une technologie spécifique capable de prospecter et de recueillir des données en continu (système breveté ARP©-Géocarta), permettant après interpolation l'élaboration de cartes de résistivité. La prospection par mesures de résistivité devrait ainsi être une technique fiable et rapide pour approcher la variabilité spatiale des propriétés des sols des grands périmètres canniers et définir leurs potentialités. Elle apportera en outre d'autres éléments déterminants dans le choix de l'aménagement parcellaire au-delà des contraintes logistiques : la définition de zones homogènes permettra de valoriser les analyses physico-chimiques et de définir des secteurs aux pratiques et itinéraires techniques mieux adaptés.

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MEDICIONES DE RESISTIVIDAD ELÉCTRICA PARA UNA CARACTERIZACIÓN RÁPIDA Y PRECISA A GRAN ESCALA DE LA TIERRA AGRÍCOLA DE LA SOCIEDAD

AZUCARERA DE CAMERÚN (SOSUCAM) Por

T. VIREMOUNEIX1, L. GUIARD2, M. DABAS3 y B. TSOGO ZAMBA1 1SOSUCAM, BP 857, Yaoundé - Cameroon

2SOMDIAA, 39 rue Jean-Jacques Rousseau, 75001 Paris - France 3GEOCARTA, 16 rue du Sentier, 75002 Paris - France

[email protected] PALABRAS CLAVE: Caña de Azúcar, Resistividad, Mapa de Suelos, Manejo de Cultivo, Agricultura Sostenible.

Resumen UN BUEN conocimiento de las propiedades del suelo y su aptitud agrícola es un pre-requisito mayor para el manejo del suelo de una manera planificada, sostenible y ambientalmente amigable. Para optimizar y ampliar su área agrícola, la Sociedad Azucarera de Camerún (por sus siglas en francés: SOSUCAM) ha por lo tanto decidido usar una tecnología innovadora basada en un muestreo de la resistividad eléctrica de los suelos. En el primer paso del proyecto, la viabilidad e interpretación de las mediciones fueron probadas en suelos oxisoles de SOSUCAM por medio de un muestreo eléctrico llevado a cabo a dos profundidades (0.5 y 1 m) en un campo de caña de azúcar de 40 ha. Posteriormente se usó un software especializado para interpolar las mediciones. Las muestras de suelos fueron tomadas y analizadas simultáneamente, y la caña de azúcar fue monitoreada en áreas específicas por medio de mediciones de crecimiento, macollamiento y rendimiento. Las áreas con resistividad homogénea y propiedades comunes fueron identificadas usando el mapa producido. Los valores eléctricos fueron correlacionados con las propiedades físicas de campo, tales como pedregosidad, profundidad y porcentaje de arcilla. También observamos diferentes patrones de crecimiento de caña de azúcar dependiendo de los valores de resistividad. En vista de los buenos resultados, SOSUCAM decidió ampliar este experimento a 26 000 ha de campos de caña de azúcar y tierras de barbecho, sin ninguna referencia particular a tipo de suelo, por medio de la adaptación de una tecnología específica y patentada, llamada ARP06© (Perfil Automático de Resistividad, en inglés: Automatic Resistivity Profiling). Esta tecnología, la cual tiene un sistema adecuado de configuración de electrodos, hace posible las continuas prospecciones a diferentes profundidades y obtener información para elaborar mapas de resistividad por medio de la interpolación. El muestreo de la resistividad debiera por tanto ser una técnica rápida y confiable para el mapeo de la variabilidad especial de las propiedades de los suelos en grandes áreas cañeras y para definir el potencial agrícola de éstas. Más aún, debería proveer otros elementos importantes en la planeación de campo, más allá de las restricciones logísticas: al delinear zonas homogéneas se debería mejorar el uso de los resultados de los análisis físicos y químicos e identificar zonas en las que se pueden adoptar mejores prácticas y operaciones.

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electrical resistivity measurements for fast … viremouneix... · electrical resistivity...

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