DEMATTE José Alexandre Melo (1), COOPER Miguel (2), MAULE Rodrigo Fernando (2), FIORIO Peterson Ricardo (2)

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1 Scientific registration n : 2237 Symposium n : 17 Presentation: poster Soil characterization and discrimination on a toposequence over diabase through their reflected energy Caractérisation des sols d une toposéquence sur diabase au moyen de leur réflectance spectrale DEMATTE José Alexandre Melo (1), COOPER Miguel (2), MAULE Rodrigo Fernando (2), FIORIO Peterson Ricardo (2) (1) Escola Supeiror de Agricultura Luiz de Queiroz /USP, Dep. Ciência do Solo, Avenida Pádua Dias, nº 11, C.P. 9, CEP Piracicaba, São Paulo, Brazil carpa.ciagri.usp.br; prfiorio@ carpa.ciagri.usp.br 1 - Introduction The characterization and the study of soils is the basis to soil mapping and land use. With this, soil use can be rationalized and agriculture management impact reduced, getting higher yields. For soil evaluation, physical, chemical and mineralogical analyses are done, that are normally expensive and slow. In this aspect, new techniques have to be developed with the object of, in addition to conventional analyses, help in the qualitative and quantitative soil diagnosis. Brazil presents a great territorial extent and poorly detailed soil surveys. These surveys represent an important tool for adequate agricultural practices. Remote sensing has been used to study soils through their spectral reflectance curves. The importance of spectral characterization of soil properties has been shown by Henderson et al. (1992). Many authors (Mathews et al., 1973; Montgomery, 1976; Sherman and Waite, 1985) determined that the spectral behaviour of soils varies according to various factors: organic matter content, color, texture, cation exchange capacity, clay minerals, iron oxide content, and humidity. Thus, descriptive analysis of spectral curves preconized by Condit (197) and Stoner and Baumgardner (1981), can be used to evaluate soils differences, as shown by Demattê et al. (1996) and Formaggio et al. (1996). 2 - Objectives The soils in this study are in a weathering sequence disposition. Thus, the objectives of this work were to relate the properties of soils developed along a toposequence in Brazil with their respective spectral reflectance curves, and to compare the limits of soil change on this toposequence obtained from the analytical data and spectral curves. We also intend to verify the alterations on soil properties by weathering along this sequence and its relationship to spectral reflectance. 1

2 3 - Material and Methods The study area is located in the city of Piracicaba, São Paulo state, Brazil, at the coordinates 22 o 44' S and 47 o 33 W. Geologically it is situated on the top of an interfluve formed by the Piracicaba River over predominantly sandy Neocenozoic deposits correlated with the Rio Claro formation, that are underlain by the lower Permian sediments of the Irati formation. Such stratigraphy is interrupted, at the study site, by an intrusion of hipabissal basic rocks (diabase) in the form of a great sill during the Jurassic/Cretaceous periods, related to the Serra Geral formation, that after past erosive phases, ended up being exposed by the deepening of the valley, being the parent material of the studied soils (Cooper et al.,!996). The soils were sampled along a transect, that evolves from medium textured soils to very clayey soils, at the depths of -2, 2-4 and 6-8 cm. The distance between samples were pre-determined with 1 m in the top, 5 m on the shoulder of the toposequence and 3 m in the rest of the area. Soil samples were dried, ground and sieved, for posterior spectral reflectance obtention. The Infra Red Intelligent Spectroradiometer sensor was used in the laboratory with a 2 nm spectral resolution between 4 to 1 nm and 4 nm between 1 to 25 nm, wich includes visible and near infrared wavelength as well. The sensor was positioned vertically at a distance of 27 cm over the soil sample. The samples were placed in a 9 cm-diameter petri dish, forming a 1.5 cm layer of earth. The light source, a 6 W halogen lamp with non-collimated rays for the sample plane, was positioned 61 cm from the sample container at a 15 o zenital angle. A white plate with 1 % reflectance was used as a primary standard. The ratio between the spectral reflected radiant flux of a soil sample and the spectral reflected radiant flux of a vised and illuminated reference soil material with the same surface conditions, was used to calculate "the spectral bi-directional reflectance factor". Chemical, physical and mineralogical analysis were done on soil samples. 4 - Results and discussion The following soils were characterized by analytical data in the studied area: Oxisol (LVA, Red Yellow Latosol), Kandiudox (TRL, Terra Roxa Estruturada Latossólica), Kandiudalf (TR, Terra Roxa Estruturada), Mollisol (BV, Brunizem), Vertisol (VE), and an association between a Plintosol and a Vertisol (P+VE). Respectively, these soils occur from the top to the bottom of the toposequence. In this sense, alterations on analytical soil properties occurred due to weathering, and were detected by spectral reflectance curves. Evaluation of the spectral curves permit to separate 9 groups according to their reflectance features and intensity, as the following (Figure 1). Group 1 corresponds to the LVA, the most weathered soil of the sequence. It presented low iron content, kaolinitic mineralogy and absence of gibbsite, confirmed by the spectral curves with weak absorption bands at 5 nm an 85 nm, moderate at 22 nm and absent at 2265 nm. Comparing this group of soils with the area s soil map, a good correlation was observed between the features shown in the curve and the soil class in this segment. The LVA had a curve type 3. An interesting feature observed was the absence of gibbsite in these curves, showing that these soils are predominantly kaolinitic. 2

3 Group 2, a transition between the LVA and the TRL, was differentiated from group 1 by the presence of an absorption feature due to iron oxides (85 nm) at the depth of 6-8 cm. As the inclination of the slope sequence increased, the influence of the diabase also increased. Consequently did the iron oxide content, higher for TE. Thus, in group 3 of the TRL and 4 TR, the spectral feature of the iron appeared in all the profile. In group 5, a shallow TR is present, and an increase in 2:1 mineral is observed, evidenced by a strong absorption at 19 nm. These group of curves represented the soils of the backslope. An increase of 2:1 minerals occurred in these curves, evidenced by the higher absorption at 19nm. On the other hand, the presence of kaolinite (band at 22nm), is still important. This can be due to the proximity of the parent material to the soil surface and to the development of a poorly weathered soil situated on a surface that suffers erosion. Group 6 is an association between a BV and a vertic BV, showing a strong iron feature at 85 nm. Group 7, VE, presented a clear band at wave length of 19nm, showing the dominance of 2:1 minerals in the clay fraction of these soils. A kaolinite band was also observed, but it was less developed than the previous groups, meaning a lower participation in the composition of the clay fraction. Other studies of these soils have shown that montmorillonite is the predominant clay mineral, with the presence of kaolinite in less quantities, evidencing poorly weathered soils. In the lower layers, the influence of iron was observed, characterized by the indenting of the band between 7 and 1nm. This influence is extended to the 2-4cm layer, although it is less evident. The iron is originated from the diabase and is transported by the seasonal fluctuations of the water table provoking its precipitation in the form of concretions mixed in the middle of the vertic material. This does not occur in the superficial horizons probably due to the impedance of the passage of water by the vertic horizons, that are denser and, consequently, with a finer porosity, that would favor the iron precipitation. Another factor that could influence the iron feature is the high quantity of organic matter in the surface horizon that hides its influence. On the other hand, although the total iron is high, being the VE a young soil, most of the iron is in an amorphous state with very little contribution of cristalline iron. Therefore, the differences in the 85nm bands can be also related to the iron forms, as shown by Demattê et al. (1996). Groups 8 and 9, were discriminated as a BV and an association P+VE. The soils of group 8 are shallow and present a dominance of 2:1 clay minerals evidenced by the well developed band at 19nm and a smaller proportion of kaolinite evidenced by a smaller band at 22nm. The superficial layer curve presented a lower intensity and masking in the bands between 5 and 13nm, probably due to the higher content of organic matter of the mollic horizon. A lower presence of iron oxides was observed, probably masked by the organic matter. The soils of group 9 are characterized by presenting 2:1 clay minerals in sub-surface (layer 6-8cm), probably montmorillonite, evidenced by the well developed band at 19nm, and, in less quantity, kaolinite (band at 22nm). The feature that most characterizes these sub-superficial curves was the highly pronounced indenting between the bands of 7 and 1nm, and the narrowing of the curve between 4 and 55nm. 3

4 5 - Conclusions It was possible to discriminate soils using spectral data, as well as to relate their attributes occurring on the toposequence. The weathering promoted alterations on soils properties that were detected by reflectance data. The overlapping of the radiometric data of the spectral curves, with the information collected from the field, showed a high descriptive correlation between the soil attributes and reflectance. Therefore, the use of spectral curves in the soil s characterization, discrimination and classification is a useful tool to aid soil surveys. On the other hand, the state of art of this technique is still limited, consequently the support of soil analysis and field studies are necessary for the interpretation and decision making on soils. 6 - References Cooper, M; P Vidal-Torrado.; I.F. Lepsch Stratigraphy and morphogenesis of na Oxisol- Alfisol toposequence in Piracicaba (SP) Brazil. CD-ROM. In: R.R. Aloisi (ed.) Res. 13 Congresso Latino Americano de Ciência do Solo, Águas de Lindóia. Agosto Demattê, J.A.M., G. J. Garcia, and L.I. Prochnow. 1996a. Relações entre a reflectância espectral e atributos físicos, químicos e mineralógicos de solos desenvolvidos de rochas eruptivas. CD-ROM. In: R.R. Aloisi (ed.) Res. 13 Congresso Latino Americano de Ciência do Solo, Águas de Lindóia. Agosto Formággio, A.R., J.C.E. Epiphânio, M.M. Valeriano, and J.B. Oliveira Comportamento espectral ( nm) de solos tropicais de São Paulo. R. bras. Ciência do Solo, 2: Henderson, T.L., M.F. Baumgardner, D.P. Franzmeier, D.E. Stott, and D.C. Coster High dimensional reflectance analysis of soil organic matter. Soil Sci. Soc. Am. J., 56: Stoner, E.R., and M.F. Baumgardner Characteristic variations in reflectance of surface soils. Soil Sci. Soc. Am. J., 45: Sherman, D.M., and T.D. Waite Eletronic spectra of Fe +3 oxides and oxide hydroxides in the near IR to near U.V. Am. Miner., 7: Montgomery, O.L An investigation of the relationship between spectral reflectance and chemical, physical and genetic characteristics of soils. PhD Thesis, Purdue University, West Lafayette. IN (Diss. Abstr. 77: 1747). Mathews, H.L., R.L. Cunningham, and G.W. Petersen Spectral reflectance of selected Pennsylvania soils. Soil Sci. Soc. Am. Proc., 37: Condit, H.R. The spectral reflectance of American soils. Photogram Eng; v. 36, p , 197. Keywords: spectral reflectance, tropical soils, Brazil, toposequence, soil discrimination Mots clés: réflectance spectrale,sols tropicaux, Brésil, toposéquence, discimination de sols 4

5 g1t2a g1t2b g1t2c.5 g1t6a g1t6b g1t6c.5 g1t9a g1t9b g1t9c Wavelegth (nm LV 2. TRL cm 2-4cm 6-8cm dam Figura 1: Textural sequence and spectral curve distribution on the summit 1 5

6 6,5,1,15,2,25, Wavelength) (nm) g1t11a g1t1b g1t1c,5,1,15,2,25, ) g1t15a g1t15b g1t15c,5,1,15,2,25,3, g1t18a g1t18b g1t18c,5,1,15,2,25, ) g1t19a g1t19b g1t19c,5,1,15,2, g1t24a g1t24b g1t24c,5,1,15,2, Comprimento de onda (nm) Reflectância g1t25a g1t25b,5,1,15,2, g1t29a g1t29b g1t29c Figura 2: Soil sequence, horizons and spectral curve groups. 2