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34 City Trees veins reach down the full 24 inches (61 cm). Summary Under foot and underappreciated, soil is the support sys- tem for our urban forests. Developing effective strategies for dealing with soil pays off, however, and SPR may be a welcome option to include in your soils toolbox. For more on soil restoration, see the chapter, "Improving Soil Quality for Urban Forests," in the upcoming Routledge Handbook of Urban Forestry due to be released this winter. Literature Cited Bassuk, N.L. and S.D. Day. Chapter 5, Site Design: Soils (pp 247-322). 2012. In The Sustainable Sites Handbook: A Complete Guide to the Principles, Strategies, and Best Practices for Sustainable Landscapes. M. Calkins (Ed.). Wiley & Sons, Hoboken, New Jersey. 545 pp Day, S. D., Layman, R. M., Chen, Y., Rolf, K., Harris, J. R., Daniels, W. L., Wiseman, P. E., McGuire, K. J., Strahm, B. D., Wick, A. F., Mauzy, B., 2012. Soil Profile Rebuilding Specification, Virginia Tech, urbanforestry.frec.vt.edu/SRES/ specification.html, doi: 10.13140/RG.2.1.3503.3049. Day, S.D. and J. R. Harris. "Improving Soil Quality for Urban Forests (in press)." 2016. In The Routledge Handbook of Urban Forestry. C.Konijnendijk van den Bosch, F. Ferrini, and A. Fini (Eds.). Routledge, London. Peter Somerville: Personal communication. dure is relatively simple, but a few procedural details will help contribute to a better outcome. First, conduct a site assessment that ascertains, at a minimum, the compac- tion level of soil, depth of compaction, underlying condi- tions, soil pH, and any soil contamination issues. For those unfamiliar with site assessment, Chapter 5, "Site Design: Soils," in the Sustainable Sites Handbook gives a good overview of the site assessment process (Bassuk & Day 2012). You need to verify that the compacted soils zones will be treated by SPR and that there are not underlying soil chemical problems (see Things to Watch for When Using SPR, below) that may swamp any benefits. Second, implement the SPR procedure: 1. Spread 4 inches (10 cm) of compost; 2. Incorporate it to 24 inches (61 cm) with a backhoe throughout the site working backwards to avoid recompaction (scoop, lift, and drop); 3. Spread 4-8 inches (10-20 cm) of topsoil on surface and rototill; 4. Plant trees or shrubs; 5. Mulch or plant groundcover as desired. For a detailed explanation of the process, see the specification that is available online for your free use or adaptation (Day et al. 2012). Things to Watch for When Using SPR Compost quality can make or break an SPR project. Researcher Peter Somerville at the University of Melbourne found trees planted in SPR-treated soil developed nitrogen deficiencies. The culprit was compost with a high carbon/ nitrogen (C/N) ratio of 34 that immobilized soil nitrogen. Although the trees recovered within 6 months, using high-quality stable compost pays off (see What to Look for in Compost for SPR). Pre-existing contamination can also be an issue. Soils with high salt content or heavy metal contamination can prevent or slow tree growth and SPR will have little or no effect on such pre-existing conditions. Where there is significant concrete or gravel mixed into soils, soil pH can be high, limiting growth of many species. While SPR can be used in such soils to remediate compac- tion, it will not address underlying pH issues and may even raise pH slightly if compost pH is high. Any site with soil pH greater than 6.5 will limit growth of some tree species. While soils can be screened to remove limestone gravel or concrete rubble, this will damage soil structure and may not be cost effective. The compost incorporation depth called for in SPR is far greater than most contractors and equipment operators are used to and clear communication is essential. Work closely with contractors and follow up with a quick soil push tube test to verify that compost What to Look for in Compost for SPR • C/N ratio < 20 • pH between 6 and 7.8 • >50% organic matter • Low salts (EC < 4 mmhos/cm) • Appropriate N and P levels depending upon application • Low phytotoxicity • Stable and mature • Free of weed seeds and pathogens • Supplied by a professional compost- ing operation