IMPROVEMENT OF A TESTING APPARATUS FOR DYNAMOMETRY: PROCEDURES FOR PENETROMETRY AND INFLUENCE OF STRAIN RATE TO QUANTIFY THE TENSILE STRENGTH OF SOIL AGGREGATES

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 eutroferrico (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 distrofico (LVd) - Typic Haplustox - and in a very clayey Nitossolo Vermelho distroferrico (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 (R-2 > 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.