Tilth is a qualitative term describing the physical state of soil. There was no available method to quantify and measure tilth. A quantitative understanding of soil tilth would help scientists, engineers, and farmers. A tilth index was developed to quantify soil tilth. Coefficients based on five soil physical properties (bulk density, cone index, uniformity coefficient, organic matter content, and plasticity index) were used in calculating the tilth index. The index ranged from 0 for conditions unusable by plants to 1 for nonlimiting soil;In a field study at Ames, in 1989, there were positive correlations (r[superscript]2 = 0.78 and 0.86, respectively) between tilth index and crop yield in continuous corn and corn-soybean rotations. In 1990, the effect of tillage systems on tilth index and crop yield was significant. The tilth index significantly changed during the cropping season. The tilth index was increased by tillage and planting operations and then decreased with time until harvest. Best correlations (r[superscript]2 = 0.78 and 0.76, respectively) were obtained between crop yield and tilth index after disking in the corn-soybean rotation and after planting in continuous corn;In a field study at Waseca, in 1989, a chisel plow system provided a significantly higher tilth index and crop yield as compared to no-tillage. The values of r[superscript]2 between tilth index and corn yield were 0.74 and 0.75, respectively, for continuous corn and a corn-soybean rotation. The values of r[superscript]2 between tilth index and soybean yield were 0.64 and 0.80, respectively, for continuous soybeans and soybean-corn rotation;Tilth index was more responsive to tillage and provided better correlations with crop yield as compared to a modified productivity index. This was the first attempt to quantify soil tilth. The relations were tested on a limited amount of data and should be used as an initial guideline. Attempts should be made to generalize the relations by extensive data collection over a wide range of soil, climatic, and management conditions;A decision support system for soil tilth assessment was designed and developed. The system calculates the tilth index of soil, estimates crop yield, reports the tilth status of the soil at a particular time, and provides suggestions for sustaining or improving the tilth.