ABSTRACT Stony soils have been increasingly used for agriculture production; however, little is known about their hydraulic properties due to problems, such as sample deformation and hydraulic continuity between samples and suction devices when the sampling and measurements are accomplished with traditional techniques. In this study, the traditional ring sampling technique was replaced by the sampling of undisturbed soil blocks coated with paraffin wax to preserve their structure. A saturated paste of fine-grained mineral particles was used to ensure contact and hydraulic continuity between samples and suction devices (sand table and ceramic plates). This allowed us to determine 30 water retention curves for three stony soils with coarse particle contents (> 2 mm) ranging from zero to 69 %. The van Genuchten model was fitted to the measured retention data and the root mean square errors were between 0.0034 and 0.0331 m3 m–3, with no outliers or odd behavior in the retention curves. These results showed that consistent water retention curves for stony soils can be determined with the technique proposed. Fine-grained minerals sandwiched between the surface of suctions sources and sampled blocks improve hydraulic continuity between them. These techniques can be applied to determine water retention properties in structured soil samples with coarse particles where it is unfeasible to collect structured soil samples with metal sampling rings.

ABSTRACT The steady infiltration rate of soil profiles is commonly determined for irrigation and soil conservation planning, but the divergence of methods reduces the reliability of measurements. In this study, the steady infiltration rate measured with a double ring infiltrometer (isr-dri) in different layers of a soil profile was compared between layers and with the steady vertical saturated flow rate estimated by the Richards equation (isr-hy). The measurements of isr-dri at the top of the A, E, and Bt horizons were compared to each other and also compared with the isr-hy to detect the occurrence of lateral flow in double ring infiltrometer measurements. The isr-dri in the A horizon (236 mm h-1) was around 10 times higher than in the Bt horizon (20 mm h-1), which implies in a lateral flow of almost 90 % in the surface horizon. The occurrence of lateral flow in double ring infiltrometer measurements was also shown by comparing isr-dri with the vertical saturated flow rate estimated with the Richards equation, isr-hy. The main conclusion is that isr-dri measured at the soil surface overestimates the steady infiltration rate of soil profiles when underlying horizons are less permeable and more restrictive to water flow. In these cases, the use of an effective saturated hydraulic conductivity of the soil profile would imply inaccurate planning of drainage, irrigation, and soil conservation designs.

A compactação do solo é uma consequência indesejável do uso agrícola do solo, sobretudo em sistemas de cultivo com mínimo revolvimento do solo, como é o caso do plantio direto (PD). Contudo, a compactação que o tráfego de máquinas causa no solo sob PD não inviabiliza a produção das culturas, indicando que mecanismos intrínsecos a ele promovem reversão da compactação. Neste estudo, avaliou-se a influência de ciclos de contração e expansão sobre a densidade do solo (ρ) de um Latossolo Vermelho argiloso (0,57 kg kg-1 de argila e 0,12 kg kg-1 de areia) e a mudança temporal da ρ no campo. Amostras de solo foram compactadas no laboratório até atingirem ρ de 1464 kg m-3 e submetidas a cinco ciclos de contração (secagem ao ar) e expansão (saturação). Durante a contração, foi monitorado o conteúdo gravimétrico de água e a ρ. A ρ foi medida também no campo, nos anos de 2010, 2011 e 2013. O decréscimo do conteúdo de água nas amostras provocou aumento da ρ conforme uma função sigmoide com duas assíntotas, e o aumento da ρ foi expressivo em conteúdos de água menores que o do ponto de murcha permanente (1,5 MPa). Embora tenha havido aumento da ρ durante a contração, os sucessivos eventos de contração e expansão reduziram gradativamente a ρ de 1713 para 1570 kg m-3 (final da contração), e de 1464 para 1385 kg m-3 (próximo à saturação). Em solo compactado no campo também foi verificado a variação decrescente de ρ (de 1406 para 1327 kg m-3) a uma taxa de -26 kg m-3 ano-1. Concluiu-se que a diminuição do grau de compactação no campo está ligada em grande parte ao mesmo mecanismo que diminuiu o grau de compactação das amostras no laboratório. Assim, no solo usado neste estudo, a contração e expansão conseguiram reverter grande parte da compactação que o tráfego de máquinas causa nele.

Soil compaction is an undesirable consequence of land use for crop production, especially under cultivation with minimum soil turnover as in the no-tillage system (NTS). However, the compaction caused by machine traffic on soils under NTS does not make crop production unviable, indicating that some intrinsic mechanisms promote the reversion of soil compaction. In this study, the influence of alternate contraction and expansion cycles was evaluated with regard to the bulk density (ρ) of a Latossolo Vermelho argiloso (clayey Oxisol) (0.57 kg kg-1 clay, and 0.12 kg kg-1 sand) and the temporal change in ρ in the field. Soil samples were compacted in the laboratory to reach a ρ of 1,464 kg m-3 and subjected to five cycles of contraction (air drying) and expansion (saturation). During the contraction process, soil gravimetric water content and ρ were monitored. Soil ρ was also measured in the field in the years 2010, 2011, and 2013. A decrease in soil water content in the samples increased the ρ according to a sigmoid function with two asymptotes, and the increase in ρ was significant at water contents lower than that of the permanent wilting point (1.5 MPa). Although there was an increase in ρ during contraction, successive contraction and expansion events steadily reduced ρ from 1,713 to 1,570 kg m-3 (end of contraction), and from 1,464 to 1,385 kg m-3 (near saturation). In the field, a decline in ρ was observed in compacted soil (from 1,406 to 1,327 kg m-3) at a rate of -26 kg m-3 yr-1. We conclude that the decrease in the state of field compaction is largely linked to the same mechanisms that cause the decrease in compaction in the samples in the laboratory. We conclude that for the soil used in this study, contraction and expansion cycles may reverse most of the compaction that machine traffic causes to the soil.

Foram desenvolvidas funções de pedotransferência (PTFs) para estimar os parâmetros (α, n, θr and θs) do modelo de van Genuchten (1980), utilizados para descrever curvas de retenção de água no solo. Os dados usados foram provenientes de diversas fontes, principalmente de estudos realizados na Região Nordeste pelas universidades, pela Embrapa e Codevasf, totalizando 786 curvas de retenção, que foram divididas em dois conjuntos de dados: 85 %, para desenvolvimento das PTFs, e 15 %, para teste e validação, considerados como dados independentes. Além do desenvolvimento das PTFs de caráter geral para todos os solos conjuntamente, desenvolveram-se PTFs específicas para as classes Argissolos, Latossolos, Neossolos e Planossolos, utilizando técnicas de regressão múltipla, com o uso do procedimento stepwise (forward e backward), para selecionar os melhores preditores. Dois tipos de PTFs foram desenvolvidos: o primeiro inclui todos os preditores, densidade do solo, teores de areia, silte, argila e de matéria orgânica, e o segundo, apenas com os teores de areia, silte e argila. A avaliação da adequação das PTFs foi com base no coeficiente de correlação (R) e índice de Willmott (d). Para avaliar as PTFs, para o teor de água em potenciais matriciais específicos, utilizou-se a raiz do erro médio quadrado (RMSE). A predição da curva de retenção por PTF é relativamente fraca, exceto para o teor de água residual. A inclusão do teor de matéria orgânica como preditor da PTF melhora a predição do parâmetro a de van Genuchten. Não houve melhora de desempenho das PTFs específicas por classe de solo, em comparação com uma PTF geral. Exceto no caso do teor de água do solo saturado, estimado pela distribuição granulométrica, modelos para a predição do teor de água em potenciais matriciais específicos são bons. Predições do teor de água em potenciais matriciais mais negativos do que -0,6 m, usando uma PTF contendo a distribuição granulométrica, são somente um pouco inferiores àquelas obtidas por PTFs, que incluem densidade do solo e teor de matéria orgânica.

Pedotransfer functions (PTF) were developed to estimate the parameters (α, n, θr and θs) of the van Genuchten model (1980) to describe soil water retention curves. The data came from various sources, mainly from studies conducted by universities in Northeast Brazil, by the Brazilian Agricultural Research Corporation (Embrapa) and by a corporation for the development of the São Francisco and Parnaíba river basins (Codevasf), totaling 786 retention curves, which were divided into two data sets: 85 % for the development of PTFs, and 15 % for testing and validation, considered independent data. Aside from the development of general PTFs for all soils together, specific PTFs were developed for the soil classes Ultisols, Oxisols, Entisols, and Alfisols by multiple regression techniques, using a stepwise procedure (forward and backward) to select the best predictors. Two types of PTFs were developed: the first included all predictors (soil density, proportions of sand, silt, clay, and organic matter), and the second only the proportions of sand, silt and clay. The evaluation of adequacy of the PTFs was based on the correlation coefficient (R) and Willmott index (d). To evaluate the PTF for the moisture content at specific pressure heads, we used the root mean square error (RMSE). The PTF-predicted retention curve is relatively poor, except for the residual water content. The inclusion of organic matter as a PTF predictor improved the prediction of parameter a of van Genuchten. The performance of soil-class-specific PTFs was not better than of the general PTF. Except for the water content of saturated soil estimated by particle size distribution, the tested models for water content prediction at specific pressure heads proved satisfactory. Predictions of water content at pressure heads more negative than -0.6 m, using a PTF considering particle size distribution, are only slightly lower than those obtained by PTFs including bulk density and organic matter content.