Uniaxial compression data
Description
Uniaxial compression data from samples collected at three different depths (0-5, 10-15 and 20-25 cm) from different fields in Southern Minas Gerais state, Southeastern Brazil, annually cultivated with corn for silage production. Samples were collected according to a spatially-stratified design. In each sampling point, six undisturbed soil samples were collected (within metallic cylinders 2.5 cm high and 6.4 cm wide), a replicate pair in each of the sampling depths: 0-5 cm, 10-15 cm, and 20-25 cm. A set of 76 sampling points was distributed in the three fields (field 1A: 22 points in 6.2 ha; field 1B: 29 points in 15.6 ha; field 2A: 25 points in 7.6 ha), totalling 456 samples. The samples were saturated and set to equilibrate at water tensions of 10 or 100 kPa (half the samples at each water potential). These water tensions were chosen because silage harvesting usually occurs during the rainy season and the soil is therefore expected to be moist. The samples were then submitted to drained, confined uniaxial compression tests on electric-pneumatic consolidometers (model S-450, Durham GeoSlope, USA). A stress sequence of 25, 50, 100, 200, 400, 800 and 1600 kPa was applied to the samples for eight minutes per load step without decompression between each step. reference: field, depth and water tension; field: identifies sampling field r1, r2 and ed, respectively 1A, 1B and 2A in the attached figure; depth: sampling depth (0-5, 10-15 and 20-25 cm); psi: water potential (in kPa) as numeric variable (10 or 100 kPa); tensao: water potential (in kPa) as categorical variable (10 or 100 kPa); Dsmax: maximum bulk density from the standard Proctor test (Mg/m3); sample: sample identification number (each undisturbed sample was collected within metallic rings 6.4 cm wide and 2.5 cm high); load: applied loads (in kPa) during the uniaxial compression tests (25, 50, 100, 200, 400, 800 and 1600 kPa); w: weght basis water content (g/g); teta: volume basis water content (m3/m3); db0: initial bulk density (that is, before compression) in Mg/m3; e: initial void ratio (that is, before compression); n: initial total porosity (that is, before compression) in m3/m3; sat: initial degree of satuartion (that is, before compression); air: initial air-filled porosity (that is, before compression), calculated as the difference between total porosity and water content; dbi: bulk density at each applied load level; GC: degree of compaction (dbi/Dsmax) at each applied load level; ei: void ratio at each applied load level; ni: total porosity at each applied load level; strain: vertical deformation at each applied load level; sati: degree of saturation at each applied load level; airi: air-filled porosity at each applied load level; deriv.ro: rate of change in bulk density at each load increment; deriv.e: rate of change in void ratio at each load increment; deriv.n: rate of change in total porosity at each load increment; deriv.str: rate of change in strain at each load increment.
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The studied sites are located in the municipality of Lavras, Southern Minas Gerais state, Southeastern Brazil. Three fields annually cultivated with maize for silage production were selected. The fields belong to two different dairy farms and are managed under conventional tillage (annual disk-harrowing before seeding, with occasional subsoiling). The predominant soils classes in the studied sites are Oxisols (mainly Hapludox) and Inceptisols (Dystrudept) (FEAM - Fundação Estadual Do Meio Ambiente, 2010; Soil Survey Staff, 2014). Local climate is classified as Cwa (mesothermic with rainy summer and warm winter) with mean annual precipitation of 1470 mm and average annual temperature of 22,3 °C (INMET-Instituto Nacional de Meteorologia, 2018). Sampling (Feb-Mar 2017) was performed prior to harvesting, in order to capture the soil physical condition to which the crop was submitted during most of the growing season. The samples were collected according to a spatially-stratified design, aiming to systematically coverage the spatial variation. In each sampling point, six undisturbed soil samples were collected (within metallic cylinders 2.5 cm high and 6.4 cm wide), a replicate pair in each of the sampling depths: 0-5 cm, 10-15 cm, and 20-25 cm. A set of 76 sampling points was distributed in the three fields (field 1A: 22 points in 6.2 ha; field 1B: 29 points in 15.6 ha; field 2A: 25 points in 7.6 ha), totalling 456 samples. Following preparation, a nylon cloth was attached to the samples, which were put to saturate in plastic trays with distilled water. When saturation was reached, the samples were weighed and set to equilibrate at water tensions of 10 or 100 kPa (half the samples at each water potential) in porous plate extractors (Soil Moisture, USA). These water tensions were chosen because silage harvesting usually occurs during the rainy season and the soil is therefore expected to be moist. After equilibrium, the undisturbed samples were weighed and submitted to drained, confined uniaxial compression tests on electric-pneumatic consolidometers (model S-450, Durham GeoSlope, USA) according to Dias Junior and Martins (2017). A stress sequence of 25, 50, 100, 200, 400, 800 and 1600 kPa was applied to the samples for eight minutes per load step without decompression between each step. The resultant deformation was recorded by a dial gauge. This compression time was chosen because it ensures at least 90% of the maximum deformation per load step according to the square-root of time method (Taylor, 1948). Following compression, the samples were oven-dried (48 h at 105 °C) for determination of the soil dry mass, then used to calculate the soil mass/volume ratios and water content.