Gênese, caracterização e comparação de métodos analíticos de solos de natureza calcária do Grupo Bambuí, no estado do Tocantins

Abstract

Calcareous soils represent a significant portion of agricultural areas of the planet; commonly have high contents of calcium, magnesium, carbonates, associated with high pH values. In general, although there is little information about their attributes, they are located in areas with great potential for agricultural use. This study aimed to evaluate aspects involved in the genesis of calcareous soils of the Bambuí Group in the municipalities of Lavandeira and Aurora, southeast of Tocantins state, using morphological, physical, chemical and mineralogical attributes, and to compare methods for analytical evaluation ofthis soil attributes. Mostly, these soils havea low degree of pedogenetic development, highlighting the pedogenic processes of melanization and calcification, as well as high levels of silt fraction, which are related to the occurrence of siltstones layers associated with the limestone. From the evaluations, it is suggested that the carbonate character or calcic horizon must be included in the large group level in the class of Luvissolos Háplicos in the Brazilian System of Soil Classification. In this study, the profil P4 of toposequence T2 showed high calcium carbonate content, but were classified as "Órtico" respectively; thus omitting a feature of great importance in the management aspect. This attribute should be emphasized in the classification, since it influences the soil pH, available phosphorus content and micronutrients. The use of the suffix "r" should be reviewed in the Manual for Soil Description and Sampling in the Field, as this suffix has been used exclusively to the horizon and / or layer C, omitting this feature to other horizons, as it is featured in profile P1 of toposequence T2. As for the analytical methods, the phosphorus (P) determination more efficiently to calcareous soilsuses a buffered alkaline solution extraction (Olsen et al., 1954), and the use of acidic nature extractors (Mehlich 1) may overestimate the values of available phosphorus. Moreover, the method for determining calcium carbonateequivalent, with the changes proposed by Donagemma et al. (2011), quantified higher concentrations of calcium carbonate (CaCO3), suggesting that other methods have underestimating the CaCO3 content of the samples. As for the particle size analysis of fine earth, the usage of sodium hydroxide as chemical dispersant was more efficient in determining the clay fraction, with better dispersion of this fraction compared to the sodium hexametaphosphate solution. In contrast, the method for organic carbon measurement proposed by Yeomans and Bremner (1988) was more efficient, when compared to the method of Donagemma et al. (2011), as shown by the values similar to the ones obtained from elemental analysis (CHNS-O) in samples pretreated with an acid solution.