Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Agricultural and Biosystems Engineering

First Advisor

Donald C. Erbach


Tillage plays a major role in food production. In developed countries, farmers practice excessive tillage. Soil scientists are recommending reduced tillage practices known as conservation tillage. In developing countries such as India, farmers practice indigenous tillage which has characteristics of conservation tillage. A review of the theoretical developments in tillage mechanics showed that there exists inadequate knowledge to predict the soil failure mechanisms, to predict the tool forces, or to determine the soil modifications caused by tillage. It is shown that there is a need to develop new theoretical concepts relevant to soil failure observed in agricultural soils, and that observations in soil failure can form the bases for a better understanding of how tillage modifies soil tilth;Agricultural soils are subject to seasonal wetting and drying that create drying stress in soil. Experiments were done in a clay loam soil to investigate the effect of drying stress on soil physical properties and on soil mechanical behavior as influenced by a model tine. A dry soil was subject to three higher moisture treatments and then each of them was dried to the initial moisture content. Results showed that the dried soil physical properties significantly differed from those of the unwetted soil. Soil strength and aggregate size increased with drying stress. Soil aggregate size and tine forces were greater in dried soils as compared to the unwetted soil;Experiments in wet and dry soils showed that the tine performance and soil properties resulting from tillage depended not only on moisture content but also whether a given moisture content was obtained by wetting a drier soil or by drying a wetter soil. Results showed that, at a given moisture content, the wetted soils failed by Fracture mode and offered relatively more draft than the dried soil which failed by "Preferential Fracture" mode. Tine forces were higher for wetted soils than for dried soils, due to hysteresis effect caused by wetting and drying.



Digital Repository @ Iowa State University,

Copyright Owner

Govindarajan Rajaram



Proquest ID


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File Size

204 pages