Tillage refers to the manipulation of soil by an implement powered by humans, animals or machines. Tillage operation generally create two zones: (1) a zone where soil has been fractured and then turned over leading to rough surface conditions; and (2) a zone where soil has been compacted by the weight of the machinery. Thus, modeling tillage effects on soil physical properties involves two separate approaches depending upon the zone under consideration. Modeling tillage systems offers an opportunity to: (1) synthesize the extensive experimental data in the literature; (2) develop tools for site specific management recommendations; and (3) identify areas of research where additional information is needed. Modeling tillage systems involves modeling the soil physical, chemical and biological properties and processes and then linking them with crop growth models to simulate crop yields or environmental impacts. This paper reviews models for predicting tillage effects on state soil physical properties. Specifically, we reviewed models which predict bulk density, surface microrelief, aerodynamic roughness length, water retention characteristics, hydraulic conductivity function, thermal conductivity, volumetric heat capacity and gas diffusion coefficient. Since most of the existing models for predicting soil physical properties are developed for untilled soils, the paper outlines procedures to adapt these models to fractured and compacted zones in tilled soils. The paper also identifies specific assumptions that need both laboratory and field testing.
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