ABSTRACT: The structural strength of soils has been extensively described by physical and mechanical properties evaluated on the mesoscale for different soils and management conditions. However, changes in the organization of the soil matrix at the microstructural level, which can be obtained by rheometry, are still seldom used in soil mechanics. Our aim was to use the amplitude sweep test, a rheometry technique, to investigate the microstructural strength of four subtropical soils (two Oxisols, an Ultisol and a Vertisol) and to discuss difficulties with respect to the samples, water content, soil density and vertical force. The various rheological properties which reveal the soil microstructural strength were determined: deformation at the end of the linear viscoelastic range, LVE range (γLVE), shear stress at the end of the LVE range (τLVE), deformation at yield point, YP (γYP), storage and loss moduli at YP (G′G″YP), maximum shear stress (τmax), and integral z. In general, soil elasticity (γLVE and γYP) and microstructural strength (τLVE and τmax) were greater in the Oxisols and the Vertisol, which both possess high clay content, while the latter also contains expansive clay minerals. The lowest structural strength was observed in the Ultisol which had a high sand content. As rheological properties are related to soil properties such as particle size distribution and carbon content, they can be applied in the evaluation of the microstructural strength of clayey and sandy soils and allows for inferences regarding inter-particle shear strength. However, the test is not applicable to very dry soil samples and sample preparations can affect the results. We suggest a number of approaches to find solutions for these difficulties/problems.

ABSTRACT Rheological parameters have been used to study the interaction between particles and the structural strength of soils subjected to mechanical stresses, in which soil composition and water content most strongly affect soil resistance to deformation. Our objective was to evaluate the effect of water tension on rheological parameters of soils with different mineralogical, physical, and chemical composition. Surface and subsurface horizons of four Oxisols, two Ultisols, one Alfisol, and one Vertisol were physically and chemically characterized; their rheological parameters were obtained from amplitude sweep tests under oscillatory shear on disturbed soil samples that were saturated and subjected to water tension of 1, 3, 6, and 10 kPa. In these samples, the rheological parameters linear viscoelastic deformation limit (γL), maximum shear stress (τmax), and integral z were determined. By simple regression analysis of the rheological parameters as a function of soil water tension, we observed increased mechanical strength with increasing water tension up to at least 6 kPa, primarily due to increased capillary forces in the soil. However, increased elasticity assessed by γL was not as expressive as the increase in structural rigidity assessed by τmax and integral z. Elastic deformation of the soil (γL) increases with the increase in the number of bonds among particles, which depend on the clay, total carbon, expansive clay mineral, and cation contents; however, maximum shear resistance (τmax) and structural stiffness (integral z) mainly increase with clay, kaolinite, and oxide content by increasing the strength of interparticle bonds. A decrease in mechanical strength occurs for water tension of 10 kPa (the lowest water content evaluated) in sandy horizons or in horizons with a high proportion of resistant microaggregates (pseudosand), when associated with low bulk density, due to fewer points of contact between soil particles and therefore lower capillary force.

A retenção e a disponibilidade de água variam com os atributos do solo e determinam o crescimento das plantas. Nesse sentido, este estudo objetivou avaliar a retenção e a disponibilidade de água nos solos de Santa Catarina, em razão da classe textural, classe de solo e litologia. Foram amostrados os horizontes superficiais e subsuperficiais de 44 perfis de solo, em áreas de diferentes regiões do Estado e coberturas vegetais, para a determinação da capacidade de campo, ponto de murcha permanente, água disponível, granulometria e teor de matéria orgânica. A retenção e a disponibilidade de água foram comparadas entre os horizontes por meio de um modelo misto, considerando as classes texturais, classes de solo e litologias, como fatores fixos, e os perfis, como fatores aleatórios. A retenção de água foi maior em solos argilosos ou siltosos e com maior teor de matéria orgânica, especialmente nos Cambissolos Húmicos, Nitossolos e Latossolos desenvolvidos de rochas ígneas afaníticas ou sedimentares, como folhelhos, argilitos e siltitos. A disponibilidade de água foi maior em solos de textura franca, com elevados teores de matéria orgânica, principalmente nos com caráter húmico; e menor em solos da classe textural areia, principalmente nos Neossolos Quartzarênicos, formados a partir de depósitos aluvionares recentes ou em solos cascalhentos, derivados de granito. A maior disponibilidade de água nos horizontes superficiais, com mais matéria orgânica, em comparação aos subsuperficiais, comprovou a importância da matéria orgânica na retenção e disponibilidade de água.

The retention and availability of water in the soil vary according to the soil characteristics and determine plant growth. Thus, the aim of this study was to evaluate water retention and availability in the soils of the State of Santa Catarina, Brazil, according to the textural class, soil class and lithology. The surface and subsurface horizons of 44 profiles were sampled in different regions of the State and different cover crops to determine field capacity, permanent wilting point, available water content, particle size, and organic matter content. Water retention and availability between the horizons were compared in a mixed model, considering the textural classes, the soil classes and lithology as fixed factors and profiles as random factors. It may be concluded that water retention is greater in silty or clayey soils and that the organic matter content is higher, especially in Humic Cambisols, Nitisols and Ferralsol developed from igneous or sedimentary rocks. Water availability is greater in loam-textured soils, with high organic matter content, especially in soils of humic character. It is lower in the sandy texture class, especially in Arenosols formed from recent alluvial deposits or in gravelly soils derived from granite. The greater water availability in the surface horizons, with more organic matter than in the subsurface layers, illustrates the importance of organic matter for water retention and availability.

No Brasil existem extensas áreas com plantio de pínus para a produção de papel e celulose, processo que gera resíduos sólidos alcalinos, como o dregs. O uso desse resíduo em solos florestais é uma alternativa para correção da acidez do solo, fornecimento de nutrientes às plantas e seu próprio descarte. O objetivo deste trabalho foi avaliar o efeito residual da aplicação superficial de dregs, comparando-o com o calcário dolomítico, nos atributos físicos e químicos de um solo ácido e no desenvolvimento de pínus. O experimento foi implantado em 2004 em Bocaina do Sul, SC, em um Cambissolo Húmico alumínico, constituindo da aplicação superficial de doses crescentes de dregs e calcário em uma área de produção de Pinus taeda L. O delineamento experimental utilizado foi em blocos ao acaso com quatro repetições e parcelas com dimensão de 10 x 10 m. Os tratamentos consistiram de níveis de corretivos da acidez do solo correspondentes à recomendação indicada pelo método SMP para atingir pH 5,5 na camada de 0-20 cm, sendo: sem corretivo; dregs nas doses de 5,08 (1/4 SMP), 10,15 (1/2 SMP) e 20,3 Mg ha-1 (1 SMP); e calcário nas doses de 8,35 (1/2 SMP) e 16,7 Mg ha-1 (1 SMP). Amostras de solo em camadas foram coletadas em 2010 para análise de atributos químicos e físicos do solo. O diâmetro à altura do peito das plantas de pínus com 6,5 anos também foi avaliado. A aplicação superficial de dregs melhorou a fertilidade química do solo, com diminuição da acidez e aumento da saturação por bases de forma semelhante ao calcário, principalmente nas camadas superficiais. O dregs, assim como o calcário, diminuiu o grau de floculação da argila, mas não prejudicou a qualidade física do solo. Quanto ao crescimento em diâmetro do pínus, não houve efeito dos corretivos. Assim, a aplicação de dregs em solos florestais para elevar o pH a 5,5 é uma alternativa para o setor florestal, como forma de descarte do resíduo e aumento da fertilidade do solo.

In Brazil extensive areas are covered with pine forests, planted for pulp and paper production. This industry generates solid alkaline waste, such as dregs. The application of this dregs to forest soils is an alternative for soil acidity correction and plant nutrient supply, as well as a solution for its proper disposal. The purpose of this study was to compare the residual effect of surface application of dregs and dolomitic lime on (a) changes in the physical and chemical properties of an acidic soil and (b) pine tree development. The experiment was carried out in 2004 in Bocaina do Sul, Santa Catarina, consisting of the application of increasing dreg and lime rates to a Pinus taeda L. production area, on a Humic Cambisol, in a randomized block design with four replications and 10 x 10 m plots. The treatments consisted of levels of soil acidity amendments corresponding to the recommendations by the SMP method to reach pH 5.5 in the 0-20 cm layer, as follows: no soil amendment; dregs at 5.08 (1/4 SMP), 10.15 (1/2 SMP) and 20.3 Mg ha-1 (1 SMP); and lime at 8.35 (1/2 SMP) and 16.7 Mg ha-1 (1 SMP). Soil layers were sampled in 2010 for analyses of soil chemical and physical properties. The diameter at breast height of the 6.5 year old pine trees was also evaluated. Surface application of dregs improved soil chemical fertility by reducing acidity and increasing base saturation, similar to liming, especially in surface layers. Dregs, comparable to lime, reduced the degree of clay flocculation, but did not affect the soil physical quality. There was no effect of the amendments on increase in pine tree diameter. Thus, the alternative to raise the pH in forest soils to 5.5 with dregs is promising for the forestry sector with a view to dispose of the waste and increase soil fertility.