ALTERNATIVE SOIL-WATER RELEASE PARAMETERS FOR DISTINGUISHING TILLAGE EFFECTS

Tillage effects on the water-holding characteristics are frequently difficult to evaluate. This may partially be due to the insensitivity of the parameters selected to make the comparisons. This study was initiated to evaluate alternative parameters for assessing tillage effects on water-holding characteristics. Corn (Zea mays, L.) was planted on a Crete silt loam soil (fine, montmorillonitic, mesic Pachic Argiustoll) which was subjected to conventional and no-till management for four consecutive years. Water-release data were obtained from undisturbed soil cores taken from the surface soil three times during the 1989 growing season and at harvest. The following parameters were calculated: the air-entry value, psi(e); the pore-size distribution index, d; the transformed air-entry value, c; the zeroth ordinary moment or porosity (OMO), the first ordinary moment or mean pore size (OM1), the second central moment or breadth of the pore-size distribution (CM2), and the skewness (SKW) of the pore-size distribution; and four volumetric fractions. Except for psi(e) and the volume fraction of pores with radii between 1.5 and 15-mu-m, VF3, all parameters showed an effect of tillage treatments at the 90% confidence limit as determined by an F test. Only the CM2 showed a significant effect of time. The above parameters were also calculated from data on three Iowa soils. For these data, two additional parameters, OMO and VF3, did not show a difference between selected tillage treatments. Stepwise discriminant analysis selected only OM1 as a discriminator for the data from the Crete soil, but several combinations of OM1, OM2, d, and VF3 were selected when the Iowa soil and Crete soil data were combined. Our recommendation for a single characterization of the pore-size distribution is moment analysis.