Abstract
Soil penetration resistance (PR) and the tensile strength of aggregates (TS) are commonly used to characterize the physical and structural conditions of agricultural soils. This study aimed to assess the functionality of a dynamometry apparatus by linear speed and position control automation of its mobile base to measure PR and TS. The proposed equipment was used for PR measurement in undisturbed samples of a clayey "Nitossolo Vermelho eutroférrico" (Kandiudalfic Eutrudox) under rubber trees sampled in two positions (within and between rows). These samples were also used to measure the volumetric soil water content and bulk density, and determine the soil resistance to penetration curve (SRPC). The TS was measured in a sandy loam "Latossolo Vermelho distrófico" (LVd) - Typic Haplustox - and in a very clayey "Nitossolo Vermelho distroférrico" (NVdf) - Typic Paleudalf - under different uses: LVd under "annual crops" and "native forest", NVdf under "annual crops" and "eucalyptus plantation" (> 30 years old). To measure TS, different strain rates were applied using two dynamometry testing devices: a reference machine (0.03 mm s-1), which has been widely used in other studies, and the proposed equipment (1.55 mm s-1). The determination coefficient values of the SRPC were high (R² > 0.9), regardless of the sampling position. Mean TS values in LVd and NVdf obtained with the proposed equipment did not differ (p > 0.05) from those of the reference testing apparatus, regardless of land use and soil type. Results indicate that PR and TS can be measured faster and accurately by the proposed procedure.
Highlights
Soil penetration resistance (PR) is often used as soil quality indicator, to characterize compaction and mechanical limitations of root growth (Barber, 1994) as well as the effects of tillage (Busscher et al 2000) and machinery traffic (Sharratt et al, 1998). Letey (1985) argues that PR is a soil physical property that influences plant growth directly and crop yield (Kirkegaard et al 1995; Beutler et al, 2006)
The use of manual control to determine the tensile strength of soil aggregates is necessary due to varied diameters in samples, i.e. in this case, the limit switch sensors were used only as a safety precaution by limiting the mobile base displacement to a lowest position (≈ 0.5 cm above the machine base surface) and highest position (≈ 1 cm above the soil aggregate) to avoid damage to the proposed equipment
The resulting data set of cone penetration force values varied between 0 and 20 kgf, which confirmed that the motor used in the proposed equipment maintained a constant displacement rate of the mobile base
Summary
Soil penetration resistance (PR) is often used as soil quality indicator, to characterize compaction and mechanical limitations of root growth (Barber, 1994) as well as the effects of tillage (Busscher et al 2000) and machinery traffic (Sharratt et al, 1998). Letey (1985) argues that PR is a soil physical property that influences plant growth directly and crop yield (Kirkegaard et al 1995; Beutler et al, 2006). The magnitude of PR is related to aspects of the cylindrical rod, such as basal diameter, length and angle of the cone, as well as soil physical properties, e.g., bulk density, shear strength, water content, and clay content (Bradford, 1986; Lowery & Morrison, 2002). Penetrometers can be coupled to different mechanical structures that make them more portable, to sensors that measure the rod penetration depth and soil water content (Vaz & Hopmans, 2001), and to computerized devices for data acquisition and storage (Bradford, 1986; Lowery & Morrison, 2002). Althouhg the use of these devices makes results more accurate, they increase costs and restrict the applicability
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