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

    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.

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    • 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?

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    • 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

  • James Urban FASLA 2:56 pm on May 19, 2016 Permalink | Reply
    Tags: James Urban   

    Nursery Standards – Rootstock Problems and Specification 

    Roots properly pruned with no roots above the root collar Photo credit: James Urban

    Roots properly pruned with no roots above the root collar
    Photo credit: James Urban

    Stem girdling roots, kinked roots, J roots, T roots, and root collars buried deeply in the root package are one of the principle reasons whey trees and large shrubs fail to recover from transplanting or decline and even die at a young age after planting. These problems are typically created in the nursery by practices that do not produce plants with radial root architecture and place the root collar close to the surface of the soil. As a plant moves thru the production process from propagation to delivery at the site, there are many opportunities for root problems to develop in the plant.

    Most plants are started in small containers and then gradually moved into larger containers. If the plant is sold in a container there may be three or four different container sizes. Each of these containers may result in a series of roots circling around the edges of the pot forming circling roots. Any of the circling roots above the root color can eventually choke the tree. Other roots may be deflected from the bottom of the container and grow upward to the surface forming a sharp kink in a root that may eventually become an important structural root. If these misshapen roots are not pruned at each shift in pot size they form an imprint of constricting roots in the next container. As trees are repotted they are also often placed too deeply in the next pot. Trees lined in the field may also be buried in the soil. This places the roots too deep in the soil where oxygen is less available at a critical point in the trees development.

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    • Aris W. Stalis 10:17 pm on July 6, 2016 Permalink | Reply

      One thing about this, is I am trying to upload image associated with discussion, and not working to well, or I just am missing something. Instead of new post, I wish to add to the discussion.

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    • Aris W. Stalis 10:21 pm on July 6, 2016 Permalink | Reply

      Further trying of this tool – we find we cannot be on site for plantings. The result is a need to dig up the plants. Messy, since we disturb the “perfect mulch circle” hiding problems, but that is what we have to do to see if it was installed properly.

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    • James Urban FASLA 1:12 pm on July 21, 2016 Permalink | Reply

      Bringing these ideas into the work of LA’s is going to be difficult but is not impossible. Using the referenced specifications and details will give you the basis to reject plants and get plants modified. But getting the time and fees to actually do the field inspections is the tough part. In the end it will fall to how committed is the designer to delivering sustainable quality products to their clients. But atlas the above is a start in the process and an new tool to use.

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