To understand the effect that soil characteristics have on soil productivity (and the impact of erosion on soil productivity), producers need a working knowledge of soil properties. Iowa's soils range in thickness from a few inches to several feet. Each soil has a unique combination of properties; however, some properties are common to all soils. All soils consist of solid materials and pores. Soil solids consist of mineral particles and organic matter. The pores are filled with air, water, or both. The ratio of soil solids to pores in a defined volume is a measure of soil density.