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  • James Urban FASLA 12:57 pm on May 23, 2016 Permalink | Reply

    Retaining soil structure to improve soil 

    Unscreened soil harvesting image: James Urban

    Unscreened soil harvesting
    Photo credit: James Urban

    Soil structure (how soil particles are held together to form larger structures within the soil) is recognized as an important property of a healthy soil. Grading, tilling, soil compaction and screening soils during the soil processing and mixing process damages structure.  Structure makes significant contributions to improving root, air and water movement thru the soil. Soil screening is extremely damaging to structure but is included in most soil specifications.

    Why do we screen soils and what happens if we do not? Prior to the mid 1970’s soils were rarely screened and landscape plants performed quite well.  Installed soil was moved with clumps or peds throughout the stockpile. In the last 15-20 years farmers who have stopped tilling their soil have found significant improvements in soil performance. Several new research projects suggest that elimination of the screening and tilling processes in favor of mixing techniques or soil fracturing that preserve clumps of residual soil structure may improve landscape soils.

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    • Aris W. Stalis 10:49 pm on July 6, 2016 Permalink | Reply

      This topic is near and dear to me – regardless of how much we put in specs, this is often ignored. At times, it is because earth moving is in the Civil scope, or the requirements are simply ignored. Construction managers and contractors are concerned with building construction, and ignore the site. My plan is to request photographic documentation of the process – photo, and send via email. Quick, easy, no need for reports so it will not drive up cost. Additionally, the need to be free of weed seed in compost is also a bit of a challenge – I look forward to hearing from others on that topic.


    • Paul Josey 3:58 pm on January 5, 2017 Permalink | Reply

      Great article! How can we measure soil clumps or peds and include such requirements in specification language?


    • Jim Urban 10:15 pm on January 5, 2017 Permalink | Reply

      Paul: Right now more work is needed to add metrics to ped or clump sizes and limitations particularly in specifications. We need to look at a number of soils across the soil texture range to determine what is possible. The biggest problem is that different soils types have different ability to hold peds together during harvesting, shipping and grading. Tthe greater the clay the stronger the peds, while loamy sands and sands have almost no peds. This is further compounded by moisture. In clay soils more moisture generally results in weaker peds but they can reform in the process as the clay is sticky. Less water results in stringer peds. In sandy loans what peds that exist will be stronger with more water. Silt loans are somewhat in between. finally greater soil organic matter will strengthen peds, but the peds tend tend smaller. In my area of the mid Atlantic coastal and piedmont the soils are pretty good at maintaining peds as long as you do not screen the soil. I suspect that in the central plains areas you would have the same results. The rest of the country can key off this basic understanding.
      As far as specifications, I am still using a fairly vague ped language that is primarily there to recognize if the soil has been screened. My spec reads: “Topsoil and Planting Soil shall NOT have been screened through any screen smaller than 2” square and shall retain soil peds or clods larger than 2 inches in diameter throughout the stockpile.” I would like to use a larger minimum screen size say 3″ or even 4″ but it seems that 2″ is what most soil suppliers have and the ped idea is not mature enough to expect the industry to buy different equipment. Some times other factors make some minimum screening a valid part of the process. I try to control the overall screening requirements by controlling the source stock pile approvals and looking for soil with more clay. 15- 35% clay is a good target number for a soil that will hold up under grading and handling.
      Hope this helps.
      Jim Urban

      Liked by 1 person

  • Lauren Mandel 1:10 pm on May 17, 2016 Permalink | Reply
    Tags: , ,   

    Urban Plants & Soil as Stormwater Management Workhorses 

    Shoemaker Green at the University of Pennsylvania, designed by Andropogon Associates with stormwater engineering by Meliora Design. Photo credit Barrett Doherty

    Shoemaker Green at the University of Pennsylvania, designed by Andropogon Associates with stormwater engineering by Meliora Design.
    Photo credit Barrett Doherty

    When high-intensity rainfall events roll through cities, particularly those with combined sewer systems, peak flows increasingly overwhelm grey infrastructure, compromise water quality, and induce sedimentation and erosion. New research suggests that engineered soil and purposely selected plants within green infrastructure may help offset these flows by offering more benefit than most stormwater engineering models and municipalities acknowledge.

    A handful of progressive entities – like the Chesapeake Stormwater Network and the Commonwealth of Virginia – now award extra stormwater credit for management approaches that deploy high-performance engineered soils, dense and varied planting palettes, or an inter-connected series of green infrastructure elements. More research is needed, however, to mobilize engineers, designers, and policy makers to rely more heavily on the “green” in green infrastructure.

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    • Lauren Mandel 2:38 pm on July 21, 2016 Permalink | Reply

      Thank you, Jim! We think this topic is relevant to urbanites and urban stormwater policy alike.


    • Trevor Kimball 9:01 pm on October 7, 2016 Permalink | Reply

      I’m not sure if this is the right place to post this, but I wanted to get some feedback on issues related to stormwater infiltration/management in planting beds along a roadway, especially in the Intermountain West region. I figured this could be a good forum to possibly get some answers.

      1. What is the right way to deal with high clay content in soil for infiltration areas? What are appropriate procedures for amending the existing soil in these types of situations?
      2. How do you deal with chemicals from roadway runoff, especially high salt concentrations from winter road maintenance on a busy street? If irrigation is in place (for use when needed), is there procedure for “flushing” these types of contaminants through the soil? Or is it simply handled by selecting the appropriate plants for the situation, with the expectation that they will be resilient enough to withstand such chemicals?

      I apologize if these seem to be simple or naïve questions. I have read a lot of information concerning green infrastructure, but don’t have a lot of experience with it and am trying to become more educated so I can have the correct approach when working on these types of projects. I would appreciate anyone’s insight who may have expertise in this area.



      • Jim Urban 5:35 pm on October 26, 2016 Permalink | Reply

        Trevor On clay soils, There is emerging research that suggest that fracturing these soils to a depth of about 24 inches and then tilling the top 8″ really increases the permeability and reduces runoff of these soils. in your area I would also apply compost when tilling. If you send me an email to jimtree123@gmail.com I will send you the papers. On salt, your biggest issue is low rainfall means less flushing, and flushing along with good soil in the bed is critical. Be sure to include under drainage lines in all beds is the best recommendation along with using plants that have a good track record of salt tolerance.


    • Jim Urban 4:17 pm on November 28, 2016 Permalink | Reply

      Trevor: Sorry it took me so long to respond. Here is my take on your questions.
      1. Clay soil: Simply fracturing, deep tilling, ripping, lofting what ever we should be calling it will make clay soils excellent soils for infiltration. My method id to use a back hoe and dig into the soil to about 24 inches and then drop the soil back into place. There is an excellent paper on using clay soil that was presented at the ASLA conference in Chicago “Amending Site Soil to Enhance Infiltration on Compacted Urban Sites” The actual paper reference is “Controlling Storm water Quality and Quantity by amending soils for enhanced infiltration on construction sites in North Carolina Richard McLaughlin et all April 28, 2014. This study only looked at runoff and they were not looking at plant growth. I would add a 3” layer of compost to the fractured soil and surface till it to improve plant growth if the soil is low in organic matter (less than 2% in my areas but in the intermountain areas a much lower SOM may be fine. A second study Below Ground Matters: Urban Soils rehabilitation increases tree canopy and speeds establishment, Lyman et all Urban Forestry & Urban greening, 16 (2016) 25-35, looks at a similar soil fracturing concept on tree growth. This paper did not look at infiltration but there is a strong link between infiltration and plant growth. If you get one you will get the other. There are several other research projects that examine reuse of existing soils on other soil types.
      The easiest way to get the papers is to send me an email to Jimtree123@gmail.com and i will send you copies ( and anyone else who responds to this post).
      2. Chemicals from road ways particularly salt. There is a section in my book “Up by roots” on salt (pare 64-65 and 314) that should guide your thinking on a wide range of salt issues. The most simplistic answer I can offer is that you need to identify the extent of the problem. I find for example that in dense retail environments where normal road salt and sidewalk salt are combined with additional salt spread by business owners the problem is the most severe and a multi range approach is needed. Road soil by itself is not too severer if the soil is well draining an not compacted. This can most often be felt with by better plant choices. But road salt that becomes airborn and drifts can impact a wide range of plants.

      Hope this helps.
      Jim Urban


      • Trevor Kimball 6:09 pm on January 11, 2017 Permalink | Reply

        Thanks for your reply, I appreciate your input! I will send you an email to get the papers you mentioned, that should be helpful.


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