Characterization of Tillage and Traffic Effects on Unconfined Infiltration Measurements

Tillage and wheel-traffic compaction alter pore structure and hydraulic properties of agricultural soils. These alterations will affect root growth and movement of water and solutes. This study was conducted to develop new methods needed to characterize the effects of wheel traffic and tillage on pore structure as measured by water flow through macropores. Unconfined (three-dimensional) saturated and unsaturated infiltration measurements (0-, 30-, 60-, and 150-mm water tension) at the soil surface were taken sequentially at field sites to determine the steady-state rate of water flow through different pore-size classes on a Tama silty clay loam (fine-silty, mixed, mesic Typic Argiudoll). A tension infiltrometer was used to obtain the unsaturated infiltration rates. Sites were selected on trafficked and untrafficked interrow positions in two tillage systems (chisel plow and no-till). Steady-state infiltration rates from 2 to 300 µm s−1 were measured. Increasing the tension of applied water resulted in decreasing infiltration rates for both tillages and traffic treatments because, as tensions increased, larger pores emptied. Wheel traffic reduced infiltration rates in both tillages, but caused a greater decrease in infiltration rates in the chisel-plow system than in no-till. Increasing tension caused proportionately smaller decreases in infiltration rates for wheel-trafficked positions. This suggests that larger macropores were transporting a greater proportion of the total water flow in untrafficked soil than in trafficked soil and reinforces the concept that larger, as opposed to smaller, pores are more easily destroyed by wheel traffic. Unconfined infiltration measurements were shown to be useful in quantifying the effects of tillage and compaction on soil macropores