Masters Theses

Date of Award

8-1978

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Frank F. Bell

Committee Members

George Buntlet. M. E. Springer, L. N. Skold

Abstract

Soil erosion is a very serious problem in upland soybean fields in West Tennessee. This study was undertaken for the purpose of determining the effects of degree of erosion and slope characteristics on soybean yields on four West Tennessee soil series - Memphis, Grenada, Lexington, and Loring. In 1976 and 1977, yield samples were taken from areas of various soil mapping units of each soil series in soybean fields on West Tennessee farms and experiment stations. Soil mapping units sampled varied in slope from 1% to 12%, and in degree of erosion from slight to severe. The yield data collected were analyzed by use of a least squares analysis of variance. Least squares mean yields were obtained for each soil mapping unit sampled within each soil series. Erosion and slope effects on yields were determined by comparisons of pairs of least squares mean soil mapping unit yields. Results in 1976 showed no effect of degree of erosion on yield on any soil series. A trend toward reduced yields as slope gradient increased was evident on all soils. Results in 1977 showed significant yield reductions on all soils due to the combined effects of degree of erosion and slope gradient. On Grenada soils, the reduction in yields was due largely to degree of erosion, while on the other soils it was equally due to slope gradient and erosion. A combined analysis of both years' data showed no differences in yield due to degree of erosion and slope gradient on Memphis soils. On the other three soils, yields were reduced significantly by a combination of severe erosion and increased slope gradient. Neither increased slope gradient alone, nor severe erosion alone caused a significant yield reduction. Yields on Lexington and Memphis soils did not differ significantly from year to year, but yields on Grenada and Loring soils declined significantly from 1976 to 1977. The differences in the effects of degree of erosion and slope gradient between 1976 and 1977 were probably a result of lower 1977 growing season rainfall. The drier conditions in 1977 resulted in lower yields on steeper, more eroded sites due to their lower moisture supplying capacity. The greater variability in yields on the Grenada and Loring soils was due to the fragipans in their subsoils, which restricted rooting and led to more moisture stress in a drier year. The overall lack of reduction in yield on Memphis soils was due to their deep, silty subsoils, which were able to supply more water to the soybeans than the less favorable subsoils of the other three soils. Shape of slope, either convex, concave, or smooth, had no significant effect on yields on any soil in either year when included in a model with field and soil mapping unit. The inclusion of linear and quadratic effects of pH and available potassium in the model had a significant effect on yields on Memphis soils in 1977. Adjustment of soil mapping unit yields for pH and available potassium eliminated significant differences between yields. The inclusion of linear and quadratic effects of pH in the model had a significant effect on yields on Grenada soils in 1977. Adjustment of soil mapping unit yields for pH increased the differences caused by slope gradient and degree of erosion.

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