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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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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Urban Design and Tree Planting Spaces

Failed project with inadequate soil volume and tree grates image: James Urban
Failed project with inadequate soil volume and tree grates
image: James Urban

Trees are important to the composition of urban design proposals. Drawings and sections show healthy, mature trees lining streets and punctuating plazas. There is an unspoken conclusion that a street without trees is not a complete street. Yet there is a critical component missing from most of these renderings.

Drawings almost always show the tree magically rising out of the ground plane with no means of support. Typically the sidewalk paving is shown right up to the trunk of the tree, the critical swelling of the trunk flare at the base of each tree above ground is not drawn. Also unspoken is the assumption that the trees will somehow find rooting space. The messy details of how the tree grows are left to the next phase of the design process. To be fair, urban design drawings, particularly the ubiquitous “typical” sections, also omit the building and light pole foundations. These omissions in the beginning of the planning process are to be added as the project moved forward. It is reasonable to assume the engineers and architects will put foundations under buildings and light poles, unseen structures typically built into the very first cost estimates. But sadly and all too often, the tree’s requirements and cost are ignored throughout the entire process.

There are two basic elements of the tree that urban designers must incorporate into their drawings, reports, and cost estimates. These are (1) sufficient soil volume to support the size tree expected to grow and (2) acknowledging the structural requirements of the tree where it meets the ground.

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