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Mechanical Behavior of an Ultisol Under Different Sugarcane Management Systems in Brazil

Authored by José Ramon Barros Cantalice

Compressibility and shear strength in agricultural soils are associated with pressures exerted on the soil surface by the intensive use of machines. This study aimed to evaluate the mechanical behavior of an Ultisol cultivated with sugarcane under the application of sugarcane residues (vinasse and filter cake) and compare it with an Atlantic Forest soil. Uniaxial compression tests were performed through the application of increasing with pressures from 12.50 to 1,600 kPa, at three water contents in undisturbed samples from the layers of 0-0.20 and 0.20-0.40 m, and for shear direct test were collected in the layers of 0-0.20 m and, subjected to three water content levels and four levels of normal tensions of direct shear. The higher content of total organic carbon in the soil under native forest (2.42 g kg-1) allowed higher pre-compression stresses (101.21-143.55 kPa) due to an increase in soil cohesion from 22.58 to 61.23 kPa, with the reduction in the volumetric water content. Thus, this natural condition was significantly different from the management systems, with respect to the mean values of cohesion, by Tukey test (p<0.05). The application of filter cake and vinasse for 25 years significantly favored the dissipation of pre-compression stress in the soil, compared with the soil under native forest. The system with filter cake application showed higher shear strength from the tension of 100 kPa on, with values from 120 to 190 kPa, in comparison to the system with vinasse application.

Compressibility and shear strength in agricultural soils are associated with pressures exerted on the soil surface by the intensive use of machines. This study aimed to evaluate the mechanical behavior of an Ultisol cultivated with sugarcane under the application of sugarcane residues (vinasse and filter cake) and compare it with an Atlantic Forest soil. Uniaxial compression tests were performed through the application of increasing with pressures from 12.50 to 1,600 kPa, at three water contents in undisturbed samples from the layers of 0-0.20 and 0.20-0.40 m, and for shear direct test were collected in the layers of 0-0.20 m and, subjected to three water content levels and four levels of normal tensions of direct shear. The higher content of total organic carbon in the soil under native forest (2.42 g kg-1) allowed higher pre-compression stresses (101.21-143.55 kPa) due to an increase in soil cohesion from 22.58 to 61.23 kPa, with the reduction in the volumetric water content. Thus, this natural condition was significantly different from the management systems, with respect to the mean values of cohesion, by Tukey test (p<0.05). The application of filter cake and vinasse for 25 years significantly favored the dissipation of pre-compression stress in the soil, compared with the soil under native forest. The system with filter cake application showed higher shear strength from the tension of 100 kPa on, with values from 120 to 190 kPa, in comparison to the system with vinasse application.

Mosaddeghi et al. [11], using equation (6), concluded that the increase in soil resistance, defined by the pre-compression voltage and, due to soil matrix suction, can be explained in terms of effective stress (σ›). Caputo [12] emphasizes that all measurable effects of variations in soils, such as compression, distortion and shear strength are due to variations in the effective stresses. According to Braida [13], when the effective stress resulting from the application of normal load on the soil surface exceeds the shear strength at the points of contact between the particles, the compression and deformation processes become eminent.

 

Shear strength is the resistance (𝜏) that the soil can offer to shear stresses in its matrix or structure, and it is directly related to the cohesion forces between soil particles and to cohesion parameters and the angle of internal friction. Other intrinsic properties of the soil, such as clay content and organic matter are also directly related to soil resistance to shear [14,15].

 

The effects of soil organic matter on shear parameters are somehow controversial. Some studies [16,17] reported a decrease in shear tension with the increase of soil organic matter due to its capacity to decrease soil density, leading to a reduction in shear strength. On the other hand, soil organic matter increases the cohesion forces between soil particles, which is directly related to soil shear strength [18]. Other studies [13-21] reported increase in soil shear strength with the increase of organic matter in cultivated soils.

Soil mechanical behavior tends to show different values according to the variability of the properties of cultivated soils under different management systems. In Brazil, the types of soil cultivated with sugarcane range from Entisols Quatzipsamments to Oxisols, the latter of which, along with Ultisols, are the most representative soils in the Northeast region [22]. In the state of Pernambuco, the soils of most areas cultivated with sugarcane are Ultisols, Oxisols and, in lower proportion, Gleysols and Spodosols.

Given the above, this study aimed to evaluate the mechanical behavior of an Ultisol cultivated with sugarcane and subjected to the application of vinasse and filter cake for more than 25 years.

The study was conducted in a sugar and ethanol unit, located in the municipality of Sirinhaém, 80 km away from Recife-PE, Brazil. Two sugarcane cultivation areas were selected from thereabout 17 hectares each, both with Ultisol, one of the three most predominant soils in the region (Figure 2). The two management systems were the applications of filter cake and vinasse. These systems were compared with an area of native forest, which were located on the slope of the terrain.

History and management of the sugarcane cultivation areas

The areas have been cultivated for more than 50 years. Vinasse and filter cake have been applied in the areas of the sugar and ethanol unit for approximately 25 years. In the management system with vinasse application, irrigation is performed for 0.5-2 h in each sprinkler, with water depths ranging from 40 to 45 mm in the ratoon from the second cut on. In the management system with filter cake application (industrial organic residues from sugar production), approximately 40 t ha-1 of filter cake are applied in the total area during the planting.

Chemical fertilization with N-P-K in the areas under vinasse application is performed only in the ratoon, using 500 kg ha-1 of the 21-00-00 formulation, exclusively composed of ammonium sulfate. For areas under filter cake application, 500 kg ha-1 of natural phosphate are applied inside the furrows during the planting and, after 60 days, 500 kg ha-1 of the 15-00-26 formulation, which is exclusively composed of urea and potassium chloride, are applied as topdressing.

Determination of pressures of the machines on the soil

The vehicles considered in this study were used in the sugarcane cultivation areas for planting and periodical soil preparation. In these two operations, the characteristics of the vehicles were recorded: five BH-180 tractors, with mean weight of 10 t. The front axle consisted of two 18.4-26 Goodyear tires with inflation pressure of 179 kPa and the rear axle consisted of two 24.5-32 Goodyear tires with inflation pressure of 165 kPa, both ballasted with 75% of water. These data were plotted in SoilFlex, where the vertical distribution of contact stresses has all been simulated with a contact area as a super ellipse, according to Keller et. al [23].

The model proposed by Keller et al. [11], using analytical equations for vertical propagation of stresses developed by Boussinesq [24] and Frohlich [25], and uses the approach Soane’s [26] to calculate the normal stress. The procedure used by Soane [26] for calculate the load applied by the wheels is to divide the contact area of small elements (i) where each element will have an area (Ai) in which an axial force is exerted (σi) therefore, the point charges σiAi = Pi. Therefore, the vertical stress at a given depth z is calculated by Soane’s approch [42] by equation (7):

 

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