Specifying Soils for Plant Growth Which by coincidence reduces runoff Timothy A. Craul, CPSSc President, Craul Land Scientists
The soil must be plant friendly. The soil is a natural system. All soils are not created equal. The soil is a living thing and needs to be treated as such. Especially if you are going to grow plants in it.
(Brady, 1974) Productivity is based on the limiting factors of the soil. Some of these factors cannot be manipulated by man. Factors such as nutrients, organic matter, water, and mechanical support can be designed or at least specified. Environmental factors most often cannot be changed.
Nature s design is of various layers called horizons that interact together to provide a growth medium. The mineral soil ranges from most weathered at the surface to least weathered at depth. Most all biologic activity occurs at the surface 12 18 inches. The B and C horizons provide most of the structural support and water uptake for trees. Soil density increases with depth. Soil permeability decreases with depth.
Equal Soils??? Forest Agricultural Urban
Forest Agricultural Urban
The stuff that makes a soil the Elixir of Life OR The cause of failure
There are Chemical properties. There are Physical properties. There are Biological properties. These groupings of properties help control the productivity of the soils.
Soil Survey Staff, 1962
Particle size distribution in urban soils is much more important than soil texture. There can be soils with the same soil textures, but extremely different reactions to outside forces usually induced by humans. Particle size distribution must be of the whole soil using USDA particle size breaks if you wish to grow plants.
Well Graded Poorly Graded
If you consider the number of contact points between the various distributions, the more contacts, the more dense the soil can become. Therefore, not all soil textures are created equally.
From Craul, 2006
The moisture content of a given soil at given time directly relates not only to the water holding capacity for plant growth, but how effective compactive forces are when applied to the soil. Typically, most engineers need 95% of Standard Proctor to adequately support their structures. HOWEVER
A and B are soils with well graded particle size distributions C and D are poorly graded particle sizes with significant amounts of sand sized particles. Holtz and Kovacs, 1981
95% of Standard Proctor is not usually conducive for plant growth. Highly dense soils have increased runoff. They have low permeability This is a reason to design a soil that has lower max density at optimum compaction.
Soil Texture Ideal Bulk Densities Bulk Densities at which may affect root elongation Bulk Densities that restrict root elongation Pore Space Range g/cm 3 g/cm 3 g/cm 3 % Sands, loamy sands <1.60 1.69 1.80 32 40% Sandy loams <1.40 1.63 1.75 34 47% Sandy clay loams, loams, clay loams <1.40 1.60 1.70 36 47% Silt, light silt loams <1.30 1.60 1.70 36 51% Heavy silt loams, silty clay loams Sandy clays, sitly clays, clay loams (35-45% clay) <1.10 1.55 1.65 38 58% <1.10 1.49 1.58 40 58% Clays (>45% clay) <1.10 1.39 1.47 44 58% Source: Modified from Protecting Urban Soil Quality, USDA-NRCS
Requirements Soil Resource Min Max Oxygen in soil atmosphere (for root survival) 3% 21% Air pore space (for root growth) 25% 60% Soil bulk density of the surface 24-93.6 lbs/ft 3 (clays) 109.3 lbs/ft 3 (sands) Penetration resistance (moist) 50 lbs/in 2 275 lbs/in 2 (clays) 300 lbs/in 2 (sands) Water content 12% 40% Temperature limits for roots and soil biology 40 F/4 C 94 F/34 C Soil ph 5.5 7.5 Soil Cation Exchange Capacity (CEC) of the surface 6 8 meq/100g >10 meq/100g Soil organic matter content of surface 6 only 3% 10% Soil organic matter content of subsoil - <1% Soil coarse fragment content of the surface 6 (rocks etc. >75mm) - <20% Source: developed from Coder, 2000 and Craul, 2006 see Soil texture table from Urban Soil Quality, USDA-NRCS for greater detail
Water holding capacity is an idealized function of the entire soil profile Water retention curves are used for this. Water retention is based on the depth of soil that has a unbroken water column from the surface to a depth Therefore the soil has a higher tension of water at the surface dropping to lower tensions at depth.
4 x 10 x 150 Tree Trench Common Rooting Tree Trench SUNY ESF, Syracuse, NY
Infiltration rate is different than Saturated Hydraulic Conductivity, and Permeability. Infiltration rate is the amount of water a soil absorbs over a set period of time. It deals with the entire soil profile to the aquifer or to a limiting layer, but is sometimes erroneously applied just to the soil surface. The closer to saturation the soil gets, the slower the infiltration rate becomes.
Ksat is the measure of a saturated soil s ability to transmit water. Based on Darcy s Law which predicts flow through a saturated ideal porous media. Unfortunately, in situ soils are not ideal porous media. In olden times (a whole 40 years ago) permeability and Ksat were interchangeable. Soil Physicists are still wrestling with water movement through soils.
HSG are developed by two main soil characteristics; saturated hydraulic conductivity and depth to limiting layer. Most natural soils range from B to D or some mixture of the two. Almost all urban soils are a solid D.
If one runs calculations of pre-construction conditions, post-construction should use a HSG of D because of soil compaction due to basic construction practices. What if you could design a soil that was HSG A even after construction? What if you could remediate a soil to keep it what it was before construction?
ORIGINAL TREE PIT WINDTHROW HINGED TOWARDS HOUSE (Good-looking turf!!)
Critical Processes to Ensure Functionality
Blossom Music Center, Cuyahoga Falls, OH (2003)
Long Dock Beacon, NY
Penn State, Harrisburg Campus (2010)
Bio-Swale not Installed Carefully Enough Bio-Swale not Protected from Sediment Bio-Swale ended up as a Wetland
Penn Park University of Pennsylvania
Penn Park University of Pennsylvania Basin # 5 min 10 min 15 min Averaged (cm) (cm) (cm) Inches / Hour 1 A 5.2 24.6 B 7.1 33.5 (1) C 4.2 19.8 D 3.3 6.0 14.9 2 A 7.1 33.5 (1) B 2.4 4.6 6.3 10.7 3 A 1.9 3.4 4.5 8.0 (2) B 2.5 4.2 5.2 10.0 (2) 4 A 2.5 4.8 7.0 11.4 B 3.7 6.9 8.8 15.9 7 A 7.3 34.5 (3) B 8.2 38.7 (3) 1. This area was recently disturbed during irrigation installation. 2. Basin #3 density is at its maximum. 3. Basin #7 has insufficient soil material above the gravel layer.
These are some projects that use soils as part of the stormwater management plan.
Chemistry Building Princeton University Before Plugs - 2010 Day after Hurricane Irene- 2011 Michael Van Valkenburgh Associates
Phipps Conservancy - Pittsburgh
Phipps Conservancy - Pittsburgh
Shoemaker Green University of Pennsylvania
Shoemaker Green University of Pennsylvania Penetration resistance of the Bio-retention basin *In lbs/in 2 Bed 0 6 6 12 12 24 24 + 100 150 250 300 125 150 250 250 100 150 225 200 75 150 150 275 Bio- 50 175 275 300 Retention 50 50 50 50 Basin 50 100 175 250 50 125 150 275 50 50 50 50 55 75 150 300 Infiltration Rate of the Bio-retention basin 5 min 10 min 15 min cm/hr In/hr 2.5 5.4 8.2 32.8 12.9 1.9 4.0 6.2 24.8 9.8
Manage the planting soils based on their fragility not according to a schedule. To provide adequate plant production, the soils must be able to bestow a balanced ecosystem. Maintaining traffic exclusionary practices to allow good turf and plant establishment after construction. Give the landscape time before running roughshod over it!!!! Educate the maintenance crews on sound management practices. Just don t build the thing and walk away. Look at the site as a whole. Do not ignore the soil/landscape outside of the project area. Match the soil system to the exposure of the site. Less prominent sites do not need the works. Design the landscape to meet the needs of the maintenance of buildings and management of the plants. Look at storing runoff onsite, whether in cisterns, rain gardens or other subsurface storage designs. Use the soil. Involve the contractor, engineers, and architects early to gain rapport for a better working relationship. Involve the client in the landscape. Prove to them that the building setting can be as important as the building s architecture. Especially, in the LEEDs department.
Soil Design Protocols for Landscape Architects and Contractors ISBN #978-0-471-72107-9
Bringing Science to Stormwater Implementation Email Mark Bowen, Chairman if you are interested in helping out Mark.Bowen@KleinschmidtUSA.com