Resumo Os latossolos argilosos são dominantes ao Norte e Este de Manaus, desenvolvendo-se sobre os baixos platôs sedimentares (Formação Barreiras). A sua evolução depende do clima úmido megatérmico e da cobertura florestal equatorial densa. Visando o estudo da dinâmica nestes solos, foram efetuadas análises microscópicas, químicas e mineralógicas em cerca de 50 amostras coletadas em toposseqüências selecionadas nas bacias hidrográficas estudadas pelo INPA. Os resultados mostraram que a circulação da água nestes latossolos se efetua por uma rede de vazios largos, contida nos setores "bioturbados". No seu Interior observa-se dessilicificação intensa e neoformação de gibbsita. O complexo sortivo é globalmente muito desaturado, mas constata-se também uma retenção muito forte da cátions sob forma não trocável. Estas características estão de acordo com a idéia de circulação da água pelo "by pass" sugerida pelos hidrólogos. Esta dinâmica seria dependente das ações da fauna e do flora.
Summary In the region of Manaus, the low sedimentary plateaux (Terciary formations of the Barreiras Group) are dominated by clayey yellow latosols with a very low base saturation (Acrorthox and Haplorthox according to the Soil Survey Staff). These soils present in general a fairly homogeneous profile particularly considering the colour, the texture (very clayey) and the structure (very weakly developed). But in the edge of the plateau, near the axes of drainage, the soils change lateraly and gradualy into a soil more differenciated with a B22Cn containing white and violescent red nodules. Microscopic and mineralogical analysis show at the first, the large dominance of a kaolinitic plasma with argillasepic fabric, showing no recognizable agregates norvoids, enclosing nothing but isolated quartz grains and white and violescent red micronodules. The abondance of the micronodules increases strongly in the neighbourhood and in the Interior of the microagregated filling of "aggrotubules" of biologic origin, which contains most of the interconnected voids. It is exactly in theese "bioturbed" volumes that appear and developp lateraly in the edge of the plateaux, the nodules of a centimetric size and whitisch colour (containing gibbsite, kaolinite and quartz) and/or violescent red colour (centaining hematite, gibbsite, kaolinite and quartz). The observation under S.E.M. together with the microprobe analyse show that the plasma of the S. matrix consists essencially of packing of very fine particles of kaolinite (<0,2μ) forming the cryptovoids. The S. matrix of the white nodules consists of finely crystallised gibbsite and kaolinite, while in the red ones, besides these constituents a high concentration of iron oxydes (55% in weight) is present. The walls of the intranodular voids are coated with gibbsite crystal ("crystal chambers") or with iron cutans ("ferrans") The results of dispersion test and chemical and physico-chemical analysis show particularly the weak colloidal characteristic of the soil clay as well as their aptitude to retain strongly the cations under a non exchangeable form. Finally, as a conclusion, it can be said that the results shown out a group of characteristics, some of them being common to the latosols in general, and some other more peculiar to the studied soils: — Bimodal distribution of the voids radius: cryptovoids (<0,1μ) and meso or macrovoids (>30μ). The cryptovoids are situated within the plasmic structure of the S. matrix, while most of the others are enclosed In the "aggrotubules". — Retention of high water content in the field capacity in the cryptovoids (30-40% in volume). Rapid drainage of excedent of water by the packing porosity of aggrutubules. — Preservation of kaolinite and small content of quartz in the S. matrix, submited to slow movement of the soil solution. Strong desilicification and formation (or conservation) of gibbsite in the aggrotubules where strong leaching condition is present. — Very weak colloidal activity of the clay fraction and strong fixation of cations under a nearly non exchangeable form, as considering the state of the absorption complex and the presence of a microporosity limiting the exchange possibilities.
