SOIL PENETROMETER RESISTANCE AND BULK-DENSITY RELATIONSHIPS AFTER LONG-TERM NO TILLAGE

Soil penetrometer resistance (SPR) and bulk density (p) measurements can be used for assessing soil strength or density. Results from this research add to the understanding of residual soil compaction of sandy soils by answering three questions: What are the long term effects of no-tillage on SPR and rho in a double-cropping rotation? Where in the soil profile are maximum SPR and rho-values found? How much of the variations in SPR are accounted for by the regression of rho? Compaction was evaluated at the end of an 8-yr oat (Avena sativa L.)/Bragg soybean [Glycine max (L.) Merr.] double-cropping tillage experiment. The soil was Arredondo fine sand (Grossarenic Paleudults). The four treatments (replicated four times) included no-tillage (NT), conventional tillage (CT), no-tillage plus in-row subsoiling (NTPS), and conventional tillage plus in-row subsoiling (CTPS). Forty days after planting soybeans the soil was irrigated to field capacity. Readings were taken to a depth of 60 cm at five positions in the row and at 15 and 30 cm from the row on both traffic and no traffic sides of four rows. Samples for rho and soil water content (theta) were collected at three positions and 12 depths. The SPR data were analyzed as a split-split-split plot design and the rho and theta-data as split-split-plot design. The most significant overall long-term effect of no-tillage compared to conventional tillage occurred at the 15-cm depth for SPR and in the 5- to 10-cm depth for rho. The increases were 19% for SPR and ll% for rho. Maximum SPR (3.1 MPa) and p (1.6 Mg/m3) occurred in the 25- to 35-cm depth. Wheel traffic increased SPR more than 35% in the upper 25 cm of soil, whereas rho was increased less than 3%. This indicates SPR was 10 times more sensitive as an indicator of soil compaction than was rho. Penetrometer resistance was predicted from an equation, in which rho-accounted for 24%, depth 30%, and depth-squared 25% of the variation in SPR. The R2 for the model was 0.82 (P < 0.01).