PEDOGENESIS AND SOIL CLASSIFICATION OF AN EXPERIMENTAL FARM IN MOSSORÓ , STATE OF RIO GRANDE DO NORTE , BRAZIL 1 LUNARA

The Rafael Fernandes experimental farm is located in the Alagoinha district, northwest of Mossoró, State of Rio Grande do Norte, Brazil, over two pedogenic formations of different edaphic characteristics, thus needing a more detailed description of its soil types, since this area hosts several experimental fields for researches on agriculture. The objective of this work was to characterize the morphological, physical, chemical and mineralogical features of the most representative soils of this farm, in order to understand its pedogenesis and define its classifications. The whole area was surveyed in order to assess its more representative pedogenic formations. Three soil profiles were classified and described, from which samples were collected in all horizons for physical, chemical and mineralogical analysis. The parent materials and the relief were the determining factors for pedogenic processes of desilication, podzolization and laterization, generating the three main soil types of the area, which were classified up to the 4 th categorical level and described as: Typic Rhodustults (P1), Rhodic Haplustox (P2) and Typic Plinthustalfs (P3).


INTRODUCTION
The soil is part of the landscape and its distribution is subject to topographical and lithological variations.Different soil types in different reliefs can be related by analyzing the landscape.The soil distribution in a landscape is dependent, in part, by the rocks, climatic conditions and relief, which define the main soil characteristics and morphological properties.
The natural variability of soil types in a landscape is the result of complex interactions between various formation factors: parent material, relief, climate, organisms and time (RESENDE et al., 2014).Soil formation factors and processes define their physical, chemical and mineralogical properties, as well as the agricultural potential of each soil type.Thus, studies involving soil characterization are fundamental for solving problems related to its physical, chemical and morphological processes.
Most of the soils of Mossoró, State of Rio Grande do Norte, Brazil, were formed by associations between the Plio-pleistocene (Barreiras Formation) and Cretaceous (Jandaira Formation) reliefs, with geological composition consisting mainly of limestone of the Jandaira Formation, which forms part the Mesozoic deposits in the Cretaceous period, associated with sedimentary sandstone rocks of the Barreiras Group (CPRM, 2010).The main soil types found in Mossoro are: Ultisols, Aridisols, Alfisols, Vertisols, Molisols, Inceptisols and Oxisols.
The Mossoro northwest region has two distinct pedogenic formations: a large area with Oxisols of medium texture and plain relief in the coast area, and a small area with Ultisols of clayey texture and plain relief in the Chapada do Apodi (SNCLS, 1971).The Rafael Fernandes experimental farm is located between these two pedogenic formations of different edaphic characteristics, thus needing a more detailed description of its soil types.This description is very important for future researches in this area, which hosts several experimental fields for researches on agriculture.
Thus, the objective of this work was to characterize the morphological, physical, chemical and mineralogical features of the most representative soils of this farm, in order to understand its pedogenesis and define their classification.The whole area was surveyed in order to assess its more representative pedogenic formations.

MATERIAL AND METHODS
The study area was located at the Rafael Fernandes experimental farm, in the district of Alagoinha (5º03'37"S, 37º23'50"W), northwest of Mossoró, State of Rio Grande do Norte, Brazil, which has approximately 400 hectares.According to Carmo Filho and Oliveira (1995) and the classification of Köppen, the local climate is BSwh', dry and very hot, with a dry season, often from June to January, and a wet season from February to May, annual average precipitation of 673.9 mm and average relative humidity of 68.9%.The region vegetation type is (Eas) open arborous Caatinga (RADAMBRASIL, 1979).
The whole area of the experimental farm was surveyed in order to assess the more representative pedogenic formations.Three locations were selected and trenches were opened to define their soil profiles.The soils, which were under native vegetation in good preservation conditions, were described and disturbed samples were collected in all horizons according to Santos et al. (2013a), and classified up to the 4 th categorical level, according to the Brazilian System of Soil Classification (SANTOS et al., 2013b).The main morphological evaluations considered the horizons height and transition, color, structure, consistency through the profiles, lithology, climate, vegetation, relief and altitude, as described by Santos et al. (2013a).The samples were air dried, ground and sieved in a 2-mm mesh sieve, in order to obtain the air-dried ground soil (ADGS).Physical analyzes of the ADGS were performed with three replications.The granulometry was evaluated by the pipette method, according to Donagema et al. (2011).
Chemical analyzes of the ADGS were carried out with three replications, evaluating the pH in water and KCl (1:2.5),P-available extracted with Mehlich-1 and determined by colorimetry, Ca 2+ and Mg 2+ extracted with KCl 1 mol L -1 and determined by atomic absorption spectrophotometry, K + extracted with Mehlich-1 and determined by flame spectrophotometry, exchangeable Al 3+ extracted with KCl 1 mol L -1 and determined by titration, and H + Al extracted with calcium acetate 1 mol L -1 and determined by titration with NaOH 0.025 mol L -1 .The total organic carbon (TOC) was determined by the method proposed by Yeomans and Bremner (1988).These analyzes provided the indices of cation exchange capacity at pH 7.0 (T), base saturation (V%) and exchangeable aluminum saturation (m%) (DONAGEMA et al., 2011).Fe was extracted by sulfuric digestion according to the methodology proposed by Vettori (1969), and determined by atomic absorption spectrophotometry.
The mineralogical characterization was carried out by separating the granulometric fractions (Donagema et al., 2011), using only the clay fraction.The identification of the minerals in this granulometric fraction was carried out by X-ray diffractometry (XRD) using a diffractometer (SHIMADZU XRD-6100) and applying the kα1 emission of cooper.The source potential used was 40 kV and the electric current was 30 mA.The scan rate applied was 0.02º per second.Scanning range (2θ) was 5 to 70°.The identification of peaks was performed with the software X-ray 1.0.0.37 and the minerals were identified according to Chen (1977).

RESULTS AND DISCUSSION
The morphological, physical, chemical and mineralogical characteristics of the soil profiles evaluated showed the influence of the formation factors parent material and relief in the pedogenic processes, which generated more developed soils, even under their semiarid climatic conditions; expressed by chemical weathering processes, consequently forming more resistant clay minerals.Three main soil types were found in the experimental farm, which were classified up to the 4 th categorical level: Typic Rhodustults (P1), Rhodic Haplustox (P2) and Typic Plinthustalfs (P3), which are formations from the pedogenic processes of podzolization, desilication and laterization, respectively.
The morphological attributes had distinct pedogenic characteristics in the studied profiles.P1 showed structures that remains granular in the surface horizons, and changes to subangular blocks in its subsurface horizons due to clay translocation, which changes its structure type (Table 2).The main pedogenic process of the P2 and P3 was not podzolization, however, they also had an increased clay in the subsurface, which classified the P2 in the argisolic sub-group (4 th categorical level), and the P3 in the Argiluvic sub-order (2 nd categorical level).
The dry and wet soil consistency was soft and friable, unchanged by the clay concentration in the subsurface in P1, however, due to the clay accumulation, its plasticity and stickiness were changed, especially in the subsurface horizons.The wet consistency in P2 indicated plasticity and tackiness from the surface to the deepest horizons, showing that the desilication significantly alters inorganic compounds, forming more stable clay mineral (clay 1:1 and Fe oxides and Al oxides) from the first centimeters of the soil, making the Oxisol characterized by a homogeneity of horizons.
A marked change in consistency was found in the P3, especially in the plinthic horizon, with hard, very hard and extremely hard consistencies, both in dry and wet soils, due to the presence of plinthite, which are formed from the weathering of rocks rich in Fe.The accumulation of Fe oxides, which forms the plinthite, can be identified by a red, yellow-red or dark red mottled material, (SANTOS et al., 2013 b).These mottled material indicate the occurrence of alternating cycles of wetting and drying, that significantly express the geochemical conditions to which the P3 profile is subjected, which is also confirmed by the clear transition between the BA and the Btf horizon.This profile was the only one that had no diffuse transition.
Regarding the parent material, the experimental farm is located over sandstones of the Cenozoic age, known as sediments of the Barreiras Group.The reddish color of the studied soils, with hue of 2.5 YR in most horizons of the P1 and P2 profiles (Table 2) can be explained by the presence of Fe oxides.The color of the P3 differed from the other profiles, with yellowish color mottled material in the surface and intense reddish in the Btf horizon, probably due to the influence of the slightly relief difference of this area, whose altitude is lower compared to the other profiles (Table 2).This lower area may have cyclically accumulated more water, making the environment periodically hydromorphic, segregating the Fe in the subsurface, which is confirmed by the intense presence of plinthite from the depth of 90 cm.
Table 2. Morphological attributes of soil profiles of an Experimental Farm "Rafael fernandes" in Mossoró.
The granulometric composition of the P1 showed sandy and loamy sand textures in the surface horizons, and sandy clay loam texture in the subsurface; P2 showed sandy loam texture in the surface horizons, and sandy clay loam and sandy clay in the subsurface; and P3 showed textures varying from sandy to sandy loam in the surface, and sandy clay loam texture in the subsurface (Table 3).These compositions contributed to identify the pedogenic processes that occurred in the profiles by understanding the vertical movement of the thinner material that generated more clayey textures in the The profiles showed acid reaction in all horizons evaluated (Table 4).The pH in KCl were lower than the pH in H 2 O, resulting in a negative ΔpH in all profiles, indicating a predominance of negative charges on the colloid surfaces.The total organic carbon (TOC) contents were relatively low in all profiles and horizons (Table 4), due to the semiarid climate, little primary biomass and low and irregular precipitation, which make the plant material decomposition a fast process.
P1 and P2 showed low base saturation (V<50%) in the subsurface horizons, denoting its dystrophic character, while P3 showed high base saturation (V>50%), denoting its eutrophic character (Table 4).Soil from quartz sandstones tend to have little bases, with low nutrient reserves, especially under humid climates, in which the material permeability favors the leaching (KAMPF; CURI, 2009).All soil profiles showed aluminum content below the amount established by the Brazilian System of Soil Classification to characterize them as aluminc or alitic, however, they presented high acidity potential, with low soil pH, especially in the P2.Kampf and Curi (2009) reported that negative charges of clay mineral surfaces are compensated by H + and Al 3+ , in a base leaching condition caused by weathering.
The Fe 2 O 3 contents found by sulfuric digestion (Table 4) confirmed the richness of Fe oxides in the soils, especially in horizons Btf (46.6 g kg -1 ) and Crf (62.1 g kg -1 ) of the P3, which showed the highest content of Fe 2 O 3 , due to its wetting and drying cycles that caused Fe segregation and consequently, Fe accumulation in subsurface horizons.
The mineralogical composition of the clay fraction of the studied profiles showed that the soils are, basically, kaolinitic-oxidic.In general, the minerals found were similar in all profiles, even in soils that underwent different pedogenic processes.The soil mineralogy is directly connected to the parent material composition, therefore, the fact that all soil profiles described had the same parent material (Table 2) explains this mineral similarity.
The three soil profiles showed well-defined peaks of kaolinite (Kt) (Figure 2), showing a more advanced level of crystallization, which is normal for sediments of soils of the Barreiras Group, due to the intense weathering that the parent material underwent and climatic conditions (MELO et al., 2001).Illite (Il) was found in all profiles, indicating that these soils also have less-weathered minerals, which can be explained by the climatic factor and because illite is a partially expansive, 2:1 mineral.
Goethite (Gt) and hematite (Hm) were found in P1 and P2, with different peaks (Figure 2), explaining the reddish color of these soils.Fe oxides are the main pigment agents in many tropical soils, compared to other mineral groups, regardless of their concentration (MELLO; ALLEONI, 2009).However, less-defined peak formations were found in P3, denoting the more amorphous nature of Fe oxides, which is due to the influence of the predominant pedogenic process, in which wetting and drying cycles slow the crystallization rate of Fe oxides, once these alternate cycles indicate new Fe inputs that adhere to nodes, promoting the crystallization of oxides (KAMPF; CURI, 2009).

CONCLUSIONS
The morphological, physical, chemical and mineralogical characteristics of the soil profiles evaluated showed the influence of the formation factors paternal material and relief in the pedogenic processes, which generated more developed soils, even under their semiarid climatic conditions.The three soil types found in the studied experimental farm were Typic Rhodustults (P1), Rhodic Haplustox (P2) and Typic Plinthustalfs (P3).The profiles showed, in general, a structure that remains granular in the surface horizons and changes to sub-angular blocks in the subsurface horizons.The textures found in the soil profiles showed surface horizons with sandier texture and subsurface horizons with more clayey texture.The chemical attributes showed total organic carbon (TOC) contents relatively low, dystrophic character in P1 and P2 and eutrophic character in P3.The clay fraction of the soil profiles showed mineralogical composition with predominance of kaolinite, illite and presence of the Fe oxides goethite and hematite.

Figure 1 .
Figure 1.Map of location of soil profiles of experimental farm "Rafael fernandes".

Table 1 .
Location of the described soil profiles

Table 3 .
Physical attributes of soil profiles of an Experimental Farm "Rafael fernandes" in Mossoró.

Table 4 .
Chemical attributes of soil profiles of an Experimental Farm "Rafael fernandes" in Mossoró.