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HomeMy WebLinkAboutSoil volume needed to grow a treeHOW MUCH SOIL TO GROW A BIG TREE? Soil volumes depicted in this chart is based on the amount of roots loam soil can support with optimum compaction for root growth. www.deeproot.com Deep Root Partners, LP 530 Washington Street, San Francisco, CA 94111.© 2011 DeepRoot Partners LP. All rights reserved. Printed in the U.S. 1/11 RATIO OF TREE SIZE TO SOIL VOLUME The line on the graph is based on 20% soil water holding capacity in a bioretention soil mix. This is a conservative estimate based on bioretention research7 and soil water properties.8 STORMWATER STORAGE Several studies3,4,5 have calculated a relationship between tree growth and soil volume. Below is an example from one such study, and its soil volume methodology.6 This soil volume methodology indicates that every 1 ft3 to 3 ft3 of soil results in 1 ft2 of projected tree canopy diameter. Field observations indicate that trees that share soil may need less soil volume per tree. For example, 25 -year old street trees sharing soil in Charlotte, North Carolina, with 700 ft3 of soil per tree have grown an average of 16" DBH (diameter at breast height) and have a 98% survival rate. 25-year old trees sharing soil in Bethesda, Maryland with 600 ft3 soil per tree have grown 14"-20" DBH and continue to flourish. Crown projection (drip line area) x Leaf area index x Evaporation rate x Evaporation ratio = Volume of water used by tree daily (water loss) Water loss x Percent water holding capacity of soil = Volume of soil (to hold water used by the tree) Volume of Soil x Rainfall frequency (estimated number of days between rain events) = Volume of soil (to meet demands of the tree for a certain period of time) 1. Urban, J. (2008) Up By Roots, Healthy Soils and Trees in the Built Environment. International Society of Arboriculture, Champaign, IL. 2. DeepRoot Partners, LP. 3. Perry, T.O. (1985) Planting site for a three inch caliper tree with room to grow. Proc Fifth Conference Metropolitan Tree Improvement Alliance. 4. Perry, T.O. (1989) Conditions for plant growth. Proc Fourth Urban Forest Conference, St. Louis, Missouri. 5. Urban, J. (1989) New techniques in urban tree plantings. Journal of Arboriculture. 15, No. 11. 281-284. 6. Lindsey, P. & Bassuk, N. (1991) Specifying soil volumes to meet the water needs of mature urban street trees and trees in containers. Journal of Arboriculture, 17, No. 6. 141-149. 7. Brown, R.A., Hunt, W.F., & Kennedy, S. G. (2009) Urban Waterways Series: Designing Bioretention with an Internal Water Storage Layer. North Carolina Cooperative Extension. 8. Rawls, W., Brakensiek, D., & Saxton, K. (1982) Estimation of Soil Water Properties. Transactions of the American Society of Agricultural Engineers. Vol. 25, No. 5, pp. 1316-1320, 1328. 39' 14' 21' 27' 32' 36' canopy diameter Example: A 16" Diameter Tree Trunk (35' Canopy Diameter) Requires 1000 ft3 of Soil PROJECTED MATURE TREE SIZE300 600 600 600600600600 100 ft 3 748 gal 200 ft 3 1,496 gal 300 ft 3 2,244 gal SOIL VOLUME (FT3) 600 800 1000 1200400200 trunk diameter (DBH) 24" 4" 8" 12" 16" 20" Ratio of Tree Size to Soil Volume1 Stormwater Storage (ft3)2 Example: A 16" Diameter Tree Trunk (35' Canopy Diameter) Requires 1000 ft3 of Soil Example: 1000 ft3 of Soil Stores 200 ft3 of Stormwater 40 80 120 160 200 240 Total soil porosity - Field capacity of soil = Available water storage within soil water storage (ft3 / gallons)