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'echnical Note 107 The Compaction of Urban Soils Many professionals have an interest in the compaction of urban soils. For example, a structural engineer may need to increase compaction to provide a stable foundation for a road or building. Conversely, an urban forester or landscaper may want to decrease or prevent compaction in order to improve root growth and plant survival. A stormwa- ter engineer must understand soil compaction to accu- rately model the runoff from lawns and landscaped areas, to identify suitable locations for stormwater treatment practices, or to stabilize an embankment or slope. Soil compaction is also an important issue for managers involved in land conservation, erosion and sediment control, watershed education and watershed Tanning: In this note, we examine how soil compac- increases in response to watershed development d the implications it has for watershed professionals. What distinguishes soil from dirt? One of the major factors is the amount of "fluff' within a soil. Undisturbed soils have a lot of pore space. Indeed, air comprises from 40 to 55% of the soil volume (unless it has recently rained, in which case the pore spaces are filled up with water). Scientists and engineers frequently measure bulk density to indicate how much fluff is present in a particular soil. Bulk density is defined as the mass of dry soil divided by its volume, and is expressed in units of grams per cubic centimeter (gms /cc). Bulk density is a useful indicator of the structure of a soil, and can help predict its porosity, permeability, infiltration rate and water holding capac- ity. In general, as the bulk density of a given soil increases, it will produce more surface runoff and allow less infiltration. What distinguishes soil from dirt? The surface bulk density of most undisturbed soils ranges from 1.1 to 1.4 gms /cc, depending on the type of soil present (Table 107.1). Soils that are predominately Downer Soil Bulk Density (grlcc) 1.2 1.4 1.6 1.8 2 4 6 8 10 12 Depth (inches) 14 • 16 18 20 22 24 Forest Pasture ' t Urban • • k 1 1 • • Depth (inches) 1.2 Freehold Soil Bulk Density (prlcc) 1.4 1.6 1.8 2 4 6 8 10 12 14 16 18 20 22 24 1 Pasture• Forest i. • Urban Figure 107.1: Change in Bulk Density in the Soil Profile as a Function of Land Use Watershed Protection Techniques • Vol. 3, No. 2 • January 2000 661