Quantifying soil physical degradation through the soil penetration resistance curve

Soil resistance to penetration (PR) frequently reduces plant root growth. PR increases with soil bulk density (Bd) and decreases with soil moisture (0), and the mathematical description of the relationships of PR with Bd and 0 determines the soil penetration resistance curve (SRC). The objective of this study was to quantify the SRC of an Oxisol under different land use practices and to use it to describe soil physical degradation. Four areas were selected: (a) native forest; (b) pasture cultivated with Brachiaria humidicola for over 20 years; (c) area cultivated with Citrus plants for 10 years; and (d) soil under annual crops for 15 years. lit each treatment, 48 undisturbed soil samples were collected in the 0-0.10 m layer. Matric potential (Psi) values from -10 to -15,000 hPa were applied to sets of 16 samples (4 per treatment) to simulate soil drying. The variables PR, Bd and 0 of these samples were determined. A non-linear equation was fit to the experimental data to describe SRC (PR = a Bd(b) theta(c)), in which a, b and c are coefficients of the fitted model. The fitted model explained more than 83 % of the PR variability in all treatments. In compacted soils a higher soil water content is necessary to maintain PR at adequated levels for plant growth (RP < 2.0 MPa). The PR values in soils under annual Crops and citrus orchard were excessively high at theta values close to the field capacity (Psi= -100 hPa), indicating greater soil physical degradation.