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Land-use changes with natural vegetation removal may impact the quantity and quality of humic substances (HS), including their molecular nature. Nuclear magnetic resonance (NMR) experiments were used in the present study to evaluate the alterations in the molecular composition of fulvic (FA) and humic (HA) acids from soils under eucalypt plantations in three major biomes in Brazil: Atlantic Forest, Cerrado, and Grassland. The major NMR-identifiable components in these HS were aromatics including aromatic C-O, COO/NC = O groups, peptides, carbohydrates, lignin-derived moieties and nonpolar alkyls. In all biomes the dipolar dephasing technique indicated the presence of significant amounts of condensed aromatic C, possibly inherited from charred materials derived from natural and anthropogenic fires in the region. The nonpolar alkyl C to O-alkyl C ratio averaged 1.4 for HA and 1.1 for FA. Humic substances from eucalypt soils showed greater contribution of nonpolar alkyl groups and a smaller abundance of O-alkyl groups in comparison to the native vegetation soil. Sugarcane (Saccharum officinarum L.) cultivation increased HA aromatics in comparison to those from the native Atlantic Forest soil, but when sugarcane was substituted by eucalypt the aromatics decreased and O-alkyl C recovered in HA and FA. There was evidence of greater contribution of lignin-derived C for HA and FA in sites planted with Brachiaria spp. pastures. Except for the HA from one Cerrado soil (Itacambira), aromaticity of HA decreased following planting to eucalypt. These changes in HS molecular composition across biomes may have impact on soil organic matter processes and they should be taken into account in future studies.

Highly weathered kaolinitic soils in Brazil often have adequate levels of K and Mg to support plant growth. The source of K and Mg in these soils and their relationship with the dominant mineral, kaolinite, is addressed in this article. Crystallographic, chemical, and morphological properties of kaolinite, from selected highly weathered Brazilian soils, were investigated by x-ray diffraction (XRD), analytical electron microscopy (AEM), chemical, and thermal methods. Kaolinite properties showed significant variations such as d(001) spacing ranging between 0.713 to 0.728 nm, width at half height (WHH) between 0.30 to 0.97 °20, and dehydroxylation temperature between 489 to 518°C. Kaolinite in the clay fraction has relatively poor crystal order with a mean crystallinity index value(CI) of 12.7. The dominant forms of the clay-fraction kaolinite were elongated and rounded, with relatively lower proportions of hexagonal particles. The silt-fraction kaolinite showed a tendency to form subspherical large aggregates with high stability. The average Fe 2 O 3 level in the kaolinite of the clay fraction (19.1 g kg -1 ) was higher than that obtained for the silt fraction (6.6 g kg -1 ). The smaller kaolinite particles of the clay fraction showed a lower degree of crystal order, higher K and Mg levels, and lower dehydroxylation temperatures. From the strong relationship between the asymmetry index (AI) of the (001) diffraction line and the level of K in kaolinite from the younger soils, we believe that both K and Mg in kaolinite are part of residual micaceous layers interleaved in kaolinite crystals.

Four Coastal Plain soils and one Piedmont soil heavily fertilized in the past were intensively cropped in greenhouse pots to determine changes in extractable soil P as measured by North Carolina, Bray I and Olsen extractants. Slopes relating changes in extractable P with P removed by cropping were very similar for the three sandy soils. Changes in extractable P with the North Carolina and Bray I extractants were correlated with P buffer capacity, % clay, and soil surface area. The principal source of P removed by cropping was NH₄F-P in sandy Coastal Plain soils; NH₄F-P and NaOH-P supplied equal amounts in the clayey Piedmont soil. Sandy Coastal Plain soils which originally contained more than 74 ppm North Carolina extractable P and 71 ppm Bray I extractable P supplied 82 to 108 kg P/ha to nine crops of pearl millet (Pennisetum americanum, cv. Gahi-1).