ABSTRACT Soil structure plays an important role in water retention, infiltration capacity, porosity, and penetration resistance. The aim of this study was to evaluate the unsaturated hydraulic conductivity and physical properties of Oxisol soils (bulk density and macro-, micro-, and total porosity) in the structures identified using the profil cultural method in areas under two different management practices (perennial pasture and sugarcane). Three pits were dug in each plot to find out how homogeneous morphological structural units (HMSU) were organized in the soil. Next, unsaturated hydraulic conductivity (Ki) was measured in situ inside each HMSU using a triple-ring infiltrometer at multiple suction. Six samples were collected (two samples for each kind of structural organization in each pit) to determine soil bulk density, total porosity, and macro- and microporosity. Each management practice resulted in a distribution of structures in the soil profile. The behavior of morphologically similar clods was the same, regardless of the physical manipulation of the soil. The distribution of structures based on the management practice determines specific hydraulic conductivities in the soil profile.

ABSTRACT: Knowledge of soils in the Amazon rainforest is becoming increasingly essential due to dynamics adopted by farmers that substitute forest for pastures, together with growing pressure from diverse segments of society towards adoption of sustainable production systems. The objective of this study was to characterize the soils along two toposequences, one under forest (F) and the other under pasture (P), and to verify how the change in land use influences soil attributes, evaluating their inclusion in the Brazilian Soil Classification System (SiBCS). The soils were sampled in pits located at the summit, backslope and footslope positions for morphological, chemical, physical, and clay mineralogy analysis. The results show that the soils are chemically poor and predominately kaolinitic. Sandy and loamy sand soils are in the surface horizons, with an increase in clay content with depth. The highest values of bulk density and lowest values of macroporosity were observed in the Bt horizons due to the change from a granular structure in the surface to an angular and subangular blocky structure in these horizons. The morphological properties observed in the field are strongly influenced by the annual soil water dynamics, the parent material, and the landscape, representing diagnostic characteristics that influenced classification of the soils, such as aquic with episaturation (epirredóxico), saprolitic, and gravelly. These diagnostic characteristics in the Argissolo Amarelo (Hapludults) are important morphological properties that have not been highlighted by the current edition of the SiBCS.

O conhecimento da curva de retenção de água (CRA) é essencial para compreender e modelar os processos hidráulicos no solo. No entanto, a determinação direta do CRA consome tempo, e o custo é alto. Além disso, é necessário grande número de amostras, em razão da elevada variabilidade espacial e temporal das propriedades hidráulicas do solo. Uma alternativa é o uso de modelos, que são chamados de funções de pedotransferência (FPT), que estimam a CRA por meio de propriedades do solo de fácil determinação. O objetivo deste estudo foi testar a acurácia de 16 FPT, pontuais ou paramétricas, existentes na literatura, em diferentes solos do sul e sudeste do Estado do Pará, Brasil. As FPT testadas foram propostas por Pidgeon (1972), Lal (1979), Aina & Periaswamy (1985), Arruda et al. (1987), Dijkerman (1988), Vereecken et al. (1989), Batjes (1996), van den Berg et al. (1997), Tomasella et al. (2000), Hodnett & Tomasella (2002), Oliveira et al. (2002) e Barros (2010). Utilizou-se um banco de dados contendo textura (areia, silte e argila), densidade do solo, carbono orgânico, pH do solo, capacidade de troca catiônica e CRA. A maioria das FPT testadas não demonstrou boa acurácia para estimar as CRA. As FPT paramétricas apresentaram melhor desempenho do que as FPT pontuais em estimar a CRA dos solos na região. Entre as FPT paramétricas, as propostas por Tomasella et al. (2000) obtiveram melhor acurácia em estimar os parâmetros empíricos do modelo de van Genuchten (1980), principalmente, quando testadas na primeira camada do solo.

Knowledge of the soil water retention curve (SWRC) is essential for understanding and modeling hydraulic processes in the soil. However, direct determination of the SWRC is time consuming and costly. In addition, it requires a large number of samples, due to the high spatial and temporal variability of soil hydraulic properties. An alternative is the use of models, called pedotransfer functions (PTFs), which estimate the SWRC from easy-to-measure properties. The aim of this paper was to test the accuracy of 16 point or parametric PTFs reported in the literature on different soils from the south and southeast of the State of Pará, Brazil. The PTFs tested were proposed by Pidgeon (1972), Lal (1979), Aina & Periaswamy (1985), Arruda et al. (1987), Dijkerman (1988), Vereecken et al. (1989), Batjes (1996), van den Berg et al. (1997), Tomasella et al. (2000), Hodnett & Tomasella (2002), Oliveira et al. (2002), and Barros (2010). We used a database that includes soil texture (sand, silt, and clay), bulk density, soil organic carbon, soil pH, cation exchange capacity, and the SWRC. Most of the PTFs tested did not show good performance in estimating the SWRC. The parametric PTFs, however, performed better than the point PTFs in assessing the SWRC in the tested region. Among the parametric PTFs, those proposed by Tomasella et al. (2000) achieved the best accuracy in estimating the empirical parameters of the van Genuchten (1980) model, especially when tested in the top soil layer.

A transformação estrutural entre horizontes B latossólico e B nítico foi estudada numa topossequência sobre sedimentos vermelhos quaternários e diabásio em Piracicaba (SP). Estudos morfológicos e micromorfológicos, análises de imagem, curvas de retenção e o monitoramento do potencial total da água no solo foram utilizados. A presença de cavidades policôncavas, revestimentos de argila e poros fissurais mostrou que a transição vertical e lateral e a transformação estrutural entre os horizontes B latossólico e B nítico ocorrem pela coalescência da estrutura microgranular, devido provavelmente a tensões criadas durante um período mais seco que o atual. A mudança para climas mais úmidos com estação seca bem definida favoreceu a instalação de ciclos de umedecimento e secagem mais frequentes, que resultaram na fissuração do material previamente coalescido, formando agregados e microagregados poliédricos. Ao mesmo tempo em que esses processos estavam acontecendo, a iluviação de argila preenchia o espaço poroso e, junto com a ação de compactação provocada pela atividade biológica, auxiliava na coalescência dos microagregados.

The soil structure transformation from ferralic to nitic horizons was studied in a toposequence on quaternary red clayey sediments and diabase in Piracicaba (SP), Brazil. Morphological and micromorphological studies, image analysis, soil water characteristic curves and monitoring of (total) soil water potential head were used. The presence of polyconcave vughs, clayskins and planar voids shows that the vertical and lateral transition and structural transformation from ferralic to nitic horizons is given by the coalescence of the microaggregates, probably due to tensions created in a drier period in the past. Changes to a more humid climate with a defined dry season and alternate drying and wetting cycles resulted in the fissuration of the previously coalesced material, forming polyhedral aggregates and microaggregates. Simultaneously, clay illuviation filled the voids and together with the compacting action of the biological activity of these soils contributed to the coalescence of microaggregates.

Normalmente a utilização agrícola dos solos é realizada com máquinas e implementos pesados que lhes causam compactação, responsável por afetar a disponibilidade de ar e de água para as plantas. O objetivo deste trabalho foi estudar o comportamento de agregados de Latossolo Amarelo da Amazônia sob compressão mecânica, submetidos a quatro níveis de umidade. Amostras de solo, úmidas e indeformadas, coletadas sob floresta, no horizonte A, a 10 cm de profundidade, e no horizonte B, a 150 cm de profundidade, foram utilizadas neste trabalho. No laboratório, essas amostras foram fragmentadas e peneiradas para obtenção de agregados de 2 a 3 mm de diâmetro, os quais foram colocados em equilíbrio com umidade de quatro potenciais matriciais para, após essa etapa, sofrerem uma compressão uniaxial com pressões de 32 a 1.000 kPa. Os resultados mostraram que a maior compressibilidade dos agregados, tanto para o horizonte A como para o B, ocorreu para o potencial matricial -32 kPa (umidade de 0,38 kg kg-1), enquanto a menor ocorreu para os potenciais -1 kPa e -1.000 kPa (umidade de 0,32 e 0,30 kg kg-1, respectivamente), indicando que esse solo não deve ser trabalhado com umidades próximas a 0,38 kg kg-1. A maior redução de volume dos agregados, tanto para o horizonte A como para o horizonte B, foi obtida para pressões mecânicas inferiores a 400 kPa, indicando que esses solos são bastante sensíveis à compactação, principalmente os agregados do horizonte A, possivelmente por terem melhores condições estruturais decorrentes do maior teor de matéria orgânica.

The agricultural use of soils usually relies on the use of heavy machinery that cause soil compaction, affecting the amount of air and water available for plants. The present study was carried out in the year 2000 and aimed at studying the performance of aggregates of an Amazon Oxisol under mechanical compression and that were subjected to varying degrees of matric potential. Humid and undisturbed tropical forest soil samples were collected from the A horizon (10 cm deep) and B horizon (150 cm deep). In the laboratory, the samples were broken into fragments and sieved to obtain 2 to 3 mm aggregates, which were placed in equilibrium under four matric potentials. Thereafter, they were exposed to uni-dimensional compression with pressures varying from 32 to 1.000 kPa. The highest compressibility of the aggregates for horizon A as well as for horizon B was observed for a matric potential of -32 kPa (soil moisture of 0.38 kg kg-1), and the lowest for the potentials of -1 kPa and -1.000 kPa (soil moistures of 0.32 and 0.30 kg kg-1, respectively), indicating that the soil should not be managed when moisture approximates 0.38 kg kg-1. The largest reduction in aggregate volume occurred under a mechanical pressure below 400 kPa. This indicates that these soils are quite prone to compaction, specially the aggregates of the A horizon (10 cm), possibly because of better structural conditions owing to the higher organic matter content.