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Although the effects of no‐till on crop yields has been widely investigated, there are virtually no data on its physical effects on clay loam soils like those in the Corn Belt. In order to compare the effects of no‐till with conventional fall plow tillage on a south‐central Minnesota clay loam soil (Aquic Argiudoll), we measured some soil physical properties 6 years after beginning a field experiment with corn (Zea mays L.). Data show that soil under no‐till had significantly greater bulk density, both in spring and fall, for samples in the surface 30 cm. Densities of no‐till soil ranged from 1.24 to 1.32 g/cm3 in in contrast to those of conventional tillage, which ranged from 1.05 to 1.12 g/cm3. Air‐filled porosities of surface samples under no‐till were lower at all potentials measured. At −100 rob, no‐till averaged 0.143 cm3/cm3 compared to 0.198 cm3/cm3 for conventional tillage. Mean number of channels 1 mm and greater created by earthworms and decomposed rootlets were significantly greater for no‐till, ranging from 666 to 1,732/m2 compared to 243 to 1,475/m2 for conventional tillage. Volumetric water contents were also greater for no‐till surface samples, ranging from 0.35 to 0.28 cm3/cm3 vs. 0.31 to 0.25 cm3/cm3 for conventional tillage. Saturated hydraulic conductivities of spring surface samples were lower under no‐till than conventional tillage averaging 14.6 cm/hour and 38.2 cm/hour, respectively. Generally. differences in measured parameters between treatments were smaller for samples collected in fall before harvest, than those in spring. Significant differences for tillage treatment were not found below 30 cm depth. At high water potentials, lower porosity under no‐till may restrict gaseous exchange and create conditions unfavorable for germination and seedling development. However, higher number of biochannels in the no‐till soil may compensate for this reduced exchange.

The hydraulic properties of a Waukegan loam profile were determined by field and laboratory procedures. Pressure‐water content relationships obtained in the laboratory were found to be variable at pressures above −100 cm of water. In this range field data were considered more reliable.Hydraulic conductivity in the field was determined from flux and hydraulic‐head gradient data. Hydraulic‐head gradients were obtained from tensiometric measurements of pressure at various depths. In the soil profile that was subject to both evaporation and drainage, the position of a downward moving “zero flux” boundary was determined. Flux across any depth was obtained by integrating the rate of change of water content with time between the “zero flux” boundary and the depth in question.A modified laboratory technique was used to determine the diffusivity of undisturbed soil cores. Water content vs. distance data were obtained subject to the conditions that evaporation was proportional to the square root of time and the soil core was effectively semi‐infinite. A diffusivity equation developed by Bruce and Klute (1956) was used to calculate diffusivity from the water content‐distance functions. Diffusivities were converted to conductivities.The “zero flux” boundary technique greatly reduced the time needed by covered plot methods to obtain conductivities at high soil‐water pressures. The laboratory procedure required only about 30 min/sample and gave results that compared favourably to field results.At high water contents and to a depth of 20 cm, field conductivities were slightly lower than laboratory estimates. Below the 20‐cm depth field data tended to be slightly higher.

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