EDS/SEM study on microaggregates of Brazilian Latosols, in relation to P adsorption and clay fraction attributes

Abstract

Little has been published on the diversity and importance of microaggregates in tropical latosols and their chemistry and fertility. This study presents a detailed analytical and submicroscopical characterization of microaggregates in selected Brazilian Latosols, representing all latosols orders, by means of electron microprobe and scanning electron microscopy (SEM) at high magnification. Chemical data for microaggregates were related to selected soil attributes, including P sorption. In these latosols, no chemical/mineralogical gradients exist between the different plasma zones of the aggregates, indicating an absence of physical–chemical microaggregate nucleation. The microgranular structure occurs in all latosol Bw horizons, with varying degrees of organization, from ‘strongly developed’ to ‘moderately developed’, varying from discrete micropeds to weakly defined /coalesced micropeds, the latter forming a massive kaolinitic groundmass. The P adsorption capacity (PAC) of the soil was significantly correlated with Al2O3 in both external and internal aggregate zones, whereas no correlation was found between PAC and SiO2, and Fe2O3 amounts. The PAC was attributed to both well-crystalline and poorly crystalline Al forms. The EDS/SEM data for the clay plasma are consistent with the results of H2SO4 extraction. Regarding EDS analysis of microaggregates, it is shown that soil thin sections of 18 cm2 sizes adequately represent the soil composition in deeply weathered, mineralogically monotonous Brazilian latosols. Due to its differential distribution between coarse and fine fractions, Ti amounts obtained by EDS and H2SO4 are poorly correlated. Ti-EDS data of soil plasma only determines microcrystalline anatase whereas Ti–H2SO4 also includes Ti in sand size resistant minerals in the coarse fractions, not selected for EDS analysis in our procedure. The highly weathered latosols studied possess Bw horizons with a pedobiological microgranular structure, whose degree of development and aggregate roundness are influenced by soil mineralogy. At the microaggregate scale, quartz grains, Ti-minerals, charcoal particles, Fe and gibbsite nodules or concretions occur randomly distributed within the clay plasma, as skeletal grains or inclusions.