Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Agricultural and Biosystems Engineering

First Advisor

John M. Laflen


Field experiments were conducted to measure the effect of previous crops and time after tillage on rill and interrill soil erodibility parameters of two Iowa soils. Experiments were conducted on plots that had previously been in soybeans, corn, or sod. Rainfall simulations with flow additions were done immediately and one month after tillage. All plots were moldboard plowed, lightly disked, and had rills formed prior to the initial rainfall simulation;Time after tillage was found to have no significant effect on interrill erodibility, Ki. Previous crop did have a significant effect on Ki with the sod Ki averaging 20% less than the corn Ki and 14% less than the soybean Ki;Rill erodibility was found to change significantly with soil and previous crop, but time after tillage had a much less definite effect. The average rill erodibility, Kr, for the Clarion soil at Ames was 3.4 g/s/N and for the Monona soil at Castana the average Kr was 8.0 g/s/N. This difference was highly significant;The average Kr for soybeans was 7.5 g/s/N, for corn 6.4 g/s/N, and for sod 3.1 g/s/N. The Kr for sod was significantly less than for both the corn and soybeans. The corn and soybean rill erodibility difference was not statistically significant;Time after tillage produced no statistical differences in rill erodibility. Although no statistical difference was found, there was some indication on the Clarion soil at Ames that soil consolidation one month after tillage could reduce rill erodibility. This occurred on three steeper plots where material deposited by natural rainfall was rapidly eroded exposing a consolidated soil. This consolidated soil (that had been tilled one month previously) had a rill erodibility about 50% lower than it had one month earlier. This was not observed on the Monona soil at Castana;Both rill and interrill erodibilities were found to have a significant negative correlation with soil vane shear strength;Soil type, previous crop, and time after tillage had no significant effect on critical shear stress values. Critical shear stresses were generally less than 5 N/m[superscript]2.



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Copyright Owner

Lowell Gregory Brenneman



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118 pages