Soybean yield response to tillage, fragipan depth, and slope length

Soybean (Glycine max) yield from continuous no-till and conventional-till s ystems were measured for 24 years on 12 pairs of 30.5-m long up-and-down-hi ll plots. The plots were located on shallow Loring (Typic Fragiudalf) silt loam soil that was under-lain by a restrictive fragipan. The no-till provid ed minimal erosion and the conventional-till provided excessive erosion, Af ter the first three years, no-till yields exceeded those from conventional- till for the remainder of the study. This report deals with soybean yields that were measured from sequential 7.6-m slope segments designated as A, B, C, and D from the top to the bottom of each plot. The average fragipan dep ths in the spring of 1985 were 42, 38, 37, and 30 cm in the conventional-ti ll plots and 46, 44, 35, and 30 cm for the no-till plots for each of these respective segments. The effective slope length for runoff travel distance on segments A through D were 7.6, 15.2, 22.8, and 30.5 m, respectively. Til lage, year effective slope length, and fragipan depth significantly affecte d crop yield during the 1984 to 1997 study period. Both increase in slope l ength and decrease in fragipan depth produced lower yield in both tillage s ystems with greater yield reduction from the conventional-till practice. An exponential equation fitted to the differences of no-till and conventional -till average yield reflected that no-till soybean yield exceeded conventio nal-till soybean yield by about 70% after 14 years. Predicted erosion per u nit area (RUSLE, version 1.06) within segments B, C, and D for conventional -till increased 54, 85, and 108%, respectively, as compared to that within segment A. The increase was only 12.5% for no-till segments B, C, and D as compared to that within segment A. The estimated accumulated depth of soil loss from each slope segment A, B, C, and D for conventional-till represent ed a net decrease in fragipan depth of about 2, 5, 8, and 10%, respectively : from 1984 to 1997. No-till produced no estimated significant changes to d epth of fragipan during the study, period. The authors believe the data sug gest that greater erosion from conventional-till on the lower slope segment s contributed to a decrease in soil productivity on these soils.