Long-term effects of cultivation on particle size and water-retention characteristics determined using wetting curves

Short-term (i.e., seasonal) variations are expected with some soil physical properties, whereas other properties, such as particle size, are considered static. However, less is known about long-term (i.e., decadal) variations in properties, such as particle-size fractions and water-retention characteristics, caused by continuous annual cultivation. The objective of this study was to determine the effect of land use (i.e., undisturbed virgin prairie vs. cultivated agriculture) and years of continuous annual cultivation on soil particle-size fractions and water-retention characteristics determined using soil wetting curves. Particle-size fractions (i.e., 0.05-2 mm, 0.002-0.05 mm, and <0.002 mm representing sand-, silt-, and clay-sized fractions, respectively) were determined on soil samples collected in 1987 and 2001 from the 0- to 10-cm depth along a transect across four adjacent fields representing a range from 0 to 44 years under continuous annual cultivation. Soil wetting curve data, obtained from a dewpoint potentiameter by re-wetting air-dried, crushed, and sieved soil, were fit by nonlinear regression to determine modeled water-retention characteristics for the native prairie and cultivated agricultural soil from samples collected in 2001. Particle-size fractions and water-retention characteristics were significantly affected by land use and years under continuous cultivation. The sand-sized fraction decreased and the clay-sized fraction increased significantly for the agricultural soil that had been annually cultivated the longest. Results indicate that long-term changes in particle-size fractions can occur following decades of continuous annual cultivation and that the effects of tillage on soil-water-retention characteristics can be ascertained after removing the confounding effects of initial differences in soil and pore structure by using soil wetting curves.