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Technical Notes ainage problems such as surface ponding, since no longer have their water - holding capacity. The second key concern with soil compaction relates to its impact on the roots of trees, shrubs and ground covers. Generally, once bulk density exceeds 1.6 gms /cc, roots are no longer able to penetrate through the soil, and growth is limited. The critical bulk density for root penetration for different kinds of soils is indicated in Table 107.2. The practical consequence of the lack of root growth is that trees, shrubs and grass cover are extremely difficult to establish without extensive soil preparation or plant- ing pits. Since compacted soils hold little water, plants are more prone to drought, and may require supplemental irrigation to survive even in humid climates. Likewise, compacted soils have lower oxy- gen transfer, extreme summer soil temperatures, less nutrient retention, less soil fauna (such as earth- worms) and less mycorrhyizal fungi compared to uncompacted soils (Bethenfalvay and Linderman, 1992 and Craul, 1994). Consequently, urban trees and ground covers tend to be very sparse, short- lived, and disease- prone, unless they are provided with significant irrigation, soil amendments, fertilization and other inputs. Bulk Density Increases in Response to Watershed Development We do not walk very lightly on the earth. Nearly every kind of watershed development compacts the soil and increases bulk density. Soil compaction begins with grazing, as the weight of livestock tramples soils of the pasture. A modest increase in soil bulk density of 0.12 to 0.20 gms /cc has been observed in pasture soils, compared to forest ones (See Table 107.3). Soil compaction, however, is largely confined to the surface, and does not extend more than a few inches into the soil profile. Compaction becomes much more severe when crops are cultivated. As heavy farm machinery passes over the field, soils are compressed up to two feet below the surface. In addition, as topsoil is eroded, more compacted subsoils are exposed. The common practice of tilling the fields does relieve compaction in the upper few inches of the soil profile, but the effect is seasonal and does not extend more than six inches to a foot below the surface. Overall, the effect of cropping is to increase bulk density by an average of 0.25 to 0.35 gms /cc, compared to forest soils, depending on the hydrologic soil group (Table 107.3). Compaction becomes even more dramatic dur- ing the urbanization of a watershed. Soil structure is compacted in three different ways during the con- Table 107.2: A Comparison of Root Limiting Bulk Density for Different Soil Types (after Morris et a1., 1988) Sandy Loam Sandy Loam Silt Loan Clay Loam 1.5 *only soil types which do not limit root growth after 85% compaction by proctor test Table 107.3: Comparison of Bulk Density in New Jersey Soils Average bulk density in first foot of soil (gmslcc) computed from database provided by Smith (1999) NRCS Hydrologic Soil Group Cultivated Soils 1.61 gals/cc HC" Soils Moderate runoff potential This table' provides a conpanson of the bulk density fordifferent hyrtologc soil "groups (HSGs), as classified by the Nature Rear) cesConsovation Service. Hydrologic soil groups are frequently used to define curie nun to characterize runcff potential vfthin; various hyridogic malels The HSG dassifrcatior; is nct stnctlybased on the porosityof the soil „but also includes bother soil crc ernes that govern runoff pdential, such as the infrl f afon rate, depth to watertable and the presence of opnfining layers such as hardpans: and fragipans M re information on HSG can be found in the 'National Resources ConsenrationService,National Engineering Hancbooh'_Chapter 2 . ` Notes pasture categoryincludas grassland, hay and-grazed lards. Watershed Protection Techniques • Vol. 3, No. 2 • January 2000