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Technical Notes 1 2 3 4 5 6 7 CUMULATIVE RAINFALL, IN CENTIMETERS Figure 108.1: Cumulative Rainfall Versus Runoff Coefficients for Different Lawn Age Groups (0.05 to 0.15 gm/cc) when this specialized equipment was used at several Swedish construction sites, com- ared to traditional construction equipment. Even so, specialized construction equipment still resulted in it compaction at the site. Based on current research, it appears that the best construction techniques are only capable of preventing about a third of the expected increase in bulk density during construction. Further, it is evident that the only truly effective technique for avoiding compaction is prevention, i.e., setting limits of disturbance that are capable of physically excluding all construction traffic from portions of a site. Techniques to Reverse Soil Compaction After Construc- tion Once soil is compacted, is there anything that can be done to reverse the process? Many natural processes act to loosen up soil, such as freezing /thawing, particle sorting, earth worm activity, root penetration and the gradual buildup of organic matter. Often, however, these processes take decades to work, and operate primarily within the first foot or so of soil. In addition, many of these natural processes are effectively turned off when soil compaction becomes severe (i.e., bulk density greater than 1.7) because water, plant roots and soil fauna simply cannot penetrate the dense soil matrix and get to work. There is some evidence that the bulk density of sidential lawn soils does gradually recover over sev- eral decades. Legg et al. (1996) monitored the soil and runoff properties of 20 residential lawns in Madison, Wisconsin that ranged in age from one to 70 years. They found that newly established lawns (less than three years old) had the highest bulk density and lowest organic matter content of all the lawns sampled. Subsequent analysis indicated that these younger lawns produced significantly more runoff than their older counter- parts (Figure 108.1). As lawns grew older, bulk den- sity declined modestly and the amount of organic matter increased in the first foot of the soil profile. It was speculated that root penetration, earthworms, and general soil building created more macro pores, and contributed to the improvement in bulk density and soil quality over time. Another long -term approach for restoring com- pacted urban soils is reforestation. Trees and shrubs gradually build soil structure through root penetra- tion, leaf fall, macro pores and associated soil fauna. However, this process may take decades to occur, and usually requires a helping hand in urban watersheds. For example, establishing trees in compacted urban soils often requires the excavation of larger and deeper tree pits filled with special soil mixes to allow tree roots to flourish. Soil Restoration Through Soil Amendments A quicker technique for reducing soil compac- tion involves amending the soil with organic matter that has a low bulk density, such as compost, fly ash, or peat. Patterson and Bates (1994) found that amend- ments of sintered fly ash were able to decrease bulk density by 0.17 gms /cc over a 22 -year period on soil test plots on the heavily used Mall in Washington, D.C. Other researchers have reported decreases in bulk density of as much as 0.30 gms /cc when compost was incorporated into glacial till soils in the Pacific North- west (Kolsti et al., 1995). Clearly, the compost amend- ment technique shows promise in reducing compac- tion in urban soils, and has recently received a great deal of attention as a potential practice for reducing stormwater runoff problems at the site level. Much of the work in this area has been conducted in the Pacific Northwest, and is focused on incorporating compost amend- ments for new or existing resi- dential lawns. The compost amendment practice is fairly simple, and is best started in the very early spring or early fall, during rela- tive dry conditions. For an existing lawn, it begins with a soil test to determine existing bulk density for the yard. If the test indicates that soils are compacted, the next step involves deep tillage of at least the top foot of soil, using a rototiller or ripper. After the sod has had a few months to decompose, compost is incorporated into the soil at the volumetric ratio of one part compost to two parts loose soil (or three to four inches over the lawn). As a rule of thumb, about The compost amendment technique shows promise in reducing compaction. Watershed Protection Techniques • Vol. 3, No.2 January 2000