Guillermo O. Sarli

Sensitivity of fractal parameters of soil aggregates to different management practices in a Phaeozem in central Argentina

Abstract Changes in soil structure often accompany changes in management practices and may affect the effectiveness of these practices. Parameters are needed to quantify these changes. Our objective was to see if fractal dimensions derived from ‘aggregate bulk density–aggregate size’ and ‘aggregate number–aggregate size’ relationships could be applied to quantify such changes. The study was conducted at the Experimental Farm of the School of Agronomy and Forestry Engineering, National University of La Plata, Argentina. A Vertic Phaeozem soil was sampled at seven locations differing in long-term management practices. The ‘aggregate bulk density–aggregate size’ and ‘number of aggregates–aggregate size’ data were obtained for seven ranges of aggregate sizes. Differences in treatments were reflected by the fragmentation fractal dimension but not the mass fractal dimension. The lowest fragmentation fractal dimensions corresponded to plots under long-term no-tillage and the highest to plots with a history of cultivation of rice ( Oryza sativa L.) under water. The fragmentation fractal dimension reflected the differences in soil management whereas the mass fractal dimension appeared to be insensitive to those differences.

Anisotropy of pore size classes' connectivity related to soil structure under no tillage

Abstract No-tillage (NT) systems have been increasing during the last several decades. Development of a near-surface layer of laminar structure under NT has also been reported. The objectives of this work were to determine whether preferential connectivity of different pore size classes in two differently textured soils under NT exists and to propose a new index of pore connectivity based on water flux (Cw). The study was carried out at two sites in the Pampas Region of Argentina under long-term NT. Site 1 had a loamy A-horizon, and Site 2 had a sandy loam A-horizon. Undisturbed samples were extracted in vertical and horizontal directions within the first 8 cm of the soil. Saturated hydraulic conductivity (K s), near-saturated hydraulic conductivity (h: −50, −100, −200, −300, and −500 cm), pore size distribution from water retention curve, and total porosity were determined. The index of pore connectivity based on water flux, Cw, was developed as a ratio between K and pore volume for each pore size class. Site 1 showed laminar structure in the A-horizon, whereas Site 2 showed a blocky structure. We found a preferential connectivity in horizontal direction of macropores in Site 1. The index of pore connectivity was useful to assess the presence of preferential orientation of different pore size classes. In future studies, the connectivity of the different pore size classes should not be neglected.

Using Tension Disc Infiltrometer to Determine Infiltration and Water-conducting Macroporosity and Mesoporosity Relationships in an Agricultural Silty Loam Soil

Measurement of soil hydraulic properties, such as hydraulic conductivity (K) and quantification of water-conducting macroporosity and mesoporosity (&thgr;ma, and &thgr;me), is important to improve the understanding of soil physical behavior. The aim of this study was to assess the effects of soil loosening on &thgr;ma and &thgr;me, as well as on K, and to relate the importance of &thgr;ma and &thgr;me to infiltration. The experimental design was completely randomized, with two treatments: (i) no tillage and (ii) conservation tillage (CnT). The soil was classified as a Typic Argiudoll, with a silty loam A horizon. A tension disc infiltrometer at three ascending soil-water pressure heads (−6 cm, −3 cm, and 0) was used to infer &thgr;ma, &thgr;me, and K at different water potentials (K6, K3, and K0, respectively), after harvest. The K values obtained for each tension were statistically greater for CnT than for no tillage. To summarize, mean values of K varied between 0.50 and 2.06 cm h−1. &thgr;ma and &thgr;me were statistically greater for CnT. Soil loosening created water-conducting macroporosity and mesoporosity that remained active after harvest. Saturated hydraulic conductivity, K0, was mainly influenced by &thgr;ma for both treatments. The influence of &thgr;ma on K0 was greater for CnT. This result indicates that the increment in K0 by soil loosening is mainly due to the creation of water-conducting macropores, supporting the idea that studies of effect of tillage on water movement should focus on macroporosity.

Stabilization of soil hydraulic properties under a long term no-till system

Nos ultimos anos, o sistema plantio direto (NT) tem aumentado. Alguns autores indicam que os atributos fisicos do solo ficam estaveis apos anos cultivando nesse sistema, sendo esse fato discutido por outros autores. O objetivo deste trabalho foi avaliar o efeito do ultimo cultivo apos a rotacao (1o ano: milho; 2o ano: soja; e 3o ano: trigo/soja) sobre a configuracao do sistema poroso do solo e das suas propriedades hidraulicas de diferentes tipos de textura (solo 1: franco; e solo 2: franco-arenoso) da regiao do pampa argentino, em NT por um longo prazo, para determinar se a estabilidade das propriedades fisicas do solo independe do momento da rotacao. Adicionalmente, foram comparados os procedimentos para avaliar a macroporosidade, bem como avaliada a eficiencia da funcao de pedotransferencia ROSETTA, para estimar os parâmetros do modelo de van Genuchten-Mualen (VGM) desses solos. A configuracao do sistema poroso do solo e as propriedades hidraulicas nao foram estaveis e se alteraram em razao da rotacao da cultura e do ultimo cultivo utilizado na rotacao em ambos os solos. Nos dois locais, a condutividade hidraulica do solo saturado (K0), a macroporosidade (ema) e o raio medio ponderado do poro por fluxo (R0ma) aumentaram do 1o para o 2o ano da rotacao de culturas e isso se atribuiu ao aumento de macroporos condutores de agua pra as raizes de milho. O modelo VGM descreveu adequadamente a curva de retencao de agua (WCR) nos dois solos, porem nao foi adequada para as curvas de condutividade hidraulica (K) vs tensao (h). A funcao ROSETTA nao foi eficiente para a estimativa dos parâmetros. Em resumo, os valores medios de K0 variaram entre 0,74 e 3,88 cm h-1. Com relacao as propriedades fisicas do solo no NT, o efeito do cultivo deve ser considerado.

Sedimentación de partículas con distribución de tamaño fractal

Since several years the fractal fragmentation model has attracted the attention of researchers, as a logic way to describe and interprete observed particle size distributions. Textural analysis has shown to be very important because of its usefulness in the dignosis and inferences about soil functioning and use. Most popular methods of textural analysis employ sedimentation of particles in water using the hydrometer or the pipet. Both have the objective of determining the particle fraction remaining in suspension at predetermined time and to relate them with particle diameters. In the present work a new power law relationship between the mass fraction in suspension and the time was developed. Using this relationship it was possible to determine the fragmentation fractal dimension of a set of particles in sedimentation. The new equation has been checked with data obtained in this research by the Bouyoucoss hydrometer, and others published in the literature, using the Robinsons pipet method. The agreement between the model and the experimental data, using non linear regression, was excellent. Resulting fractal fragmentation dimensions ranged from 2.404 to 2.512, for samples from La Plata, Argentina, and between 2.434 and 2.819 for soils from California, USA. Determination coefficients, R2, were always higher than 0.9.