Monday, July 9, 2007

IS AHK/China --- Review of the book 'Urban Soil: Applications & Practices'

*** Station Mail is for the information of Station Members only, but Station Mail has given up copyright & can be freely circulated. For administrative reasons, comments from outsiders are usually not entertained & may be circulated within our system locally & overseas.***

Dear Station Members,

Recent ISA arbor research frequently referred to the book:

"Urban Soils: Applications & Practices by Phillip Craul, Senior Lecturer in Landscape Architecture, Harvard University (1999), 366 pages."

This book is said to be a standard text for Bachelor Degree students in Landscape Architecture (BLA) classes in the USA. It is also in the recommended reading list for arborists in ISA publications & can be bought on-line at ISA website (

The name of the book Urban Soils is actually rather deceiving. The Station Manager has found that this is an excellent book on more than just soil texture, soil profile & soil structure. It actually touches on many principles of landscape design such as Sun-Shade Relationship, Calculation of Potential Evapotranspiration on site, Dynamic Water Relations of designed soil, Xeriscape Design, Rooftop Applications, Drainage Design & Techniques, Irrigation Design & Techniques, Tree Planting Techniques, Project Erosion & Sedimentation Control with case studies. It is giving knowledge to the BLA students to understand what is happening on site & be able to talk in similar language to other parties involved in the project, e.g. engineers & architects.

The mathematical models so presented in the book could only be barely understood by the Station Manager who was a trained engineer in the UK. Examples include (all units in imperial units) :

1. Bulk Density Calculation Method (Rawls 1983):

p = 100 / (x / pa) + (100 - x / pm) , where p = soil bulk density
x = % by weight of organic matter
pa = bulk density of organic matter
pm = bulk density of mineral matter

2. Discharge Flow Rate for parallel Relief Drains:

Q = qS(L + S/2) / 43200 , where Q = relief drain discharge
q = drainage coefficient
S = drain spacing
L = drain length

3. Sun's energy absorbed by soil or plant:

Sa = Sb(1 - A)sin q , where Sa = radiation absorbed by the surface of soil or plant
Sb = incident radiation
A = albedo of the surface
q = angle of the Sun to the object

There are many more graphs & calculations for Fluid Mechanics, Energy Transfer & Particle Dynamics which would take an engineer in various disciplines to slowly digest of what is going on in every chapter. Yet this is supposed to be a BLA reading text for US Landscape Architecture students ! Is it really true that they have to be put through all these disciplines ?? If so, US Landscape Architects have high salute from the Station Manager to talk parallel to any qualified engineers, architects, & of course not forgetting we arborists.

The chapter on Tree Planting in this book has been of particular interest to the Station Manager. Some extracts are quoted as follows:

1. Moisture requirement is about 1 inch per week for most plants in the humid temperate climate.
2. Plant nutrients supplied by soil must be evaluated on the basis of native nutrients contained in soil with rate of recycling of plant debris in the system.
3. Depth of tree pit should be slightly less than the height of the rootball.
4. Amendment of backfill is usually not necessary in well structured medium to fine textured soil. Whitcomb (1987) & others have found that amending backfill had no effect as compared to unamended backfill.
5. A properly specified & prepared landscape backfill does not require Hydrogels or absorbents as amendment.
6. For sites covered by impervious materials like sidewalk or traffic way, "The tree survives if the infrastructure fails; the tree fails if the infrastructure succeeds ! "
7. Experience shows that a sidewalk placed over the framework (structural) roots of an established tree, without severing the roots, has little effect on the root system since most of the absorbing roots may be beyond the sidewalk & are in the yard.
8. Fertilization should be done after the first growing season. Fertilization salts of quick release materials can be injurious to roots on direct contact. Nitrogen fertilizer applied at planting may even suppress root growth (Warren 1993).
9. Broadcasting of the fertilizer over the soil surface for open-grown tree is by far the best method of application (Good 1989).
10. If sufficient rooting space is provided in the planting design, root barriers should not be necessary. Their utilization becomes necessary only when the roots are to be confined into an insufficient space for elongation & expansion, which must be considered a design or planning failure.
11. Infiltration rate of design soil should exceed 1 inch per hour after soil saturation.
12. Bulk density of backfill should be in the region of 1.7 to 1.8 MT/cu.m .

Some of the research in this book appear to be in direct contradiction to the General Specifications for landscape work in our Govt Depts & Consultancies. It would depend on whether anyone would doubt the research & experience of this Senior Lecturer of Landscape Architecture at Harvard University & world renown Soil Scientist. The Station Manager wouldn't. A good arborist is supposed to be humble in learning, fact-finding & truth-telling, always bearing Integrity, Professionalism & Participation into our society in mind. This Station is trying to live up these principles constantly, one tree at a time.

Urban Soils is an excellent book to be recommended for study by our Station Members working in the landscaping field. It serves not only as an excellent guide but also can be used for Court reference if ever standards & guidelines are required to be presented in a Court case. After 1997, British laws & standards are no longer binding like before. It will be up to the Court Judges to make decisions in accordance with the most credible facts & reliable evidence presented. Urban Soils, with all its research & National Standards quoted, can be a very acceptable presentation in Court.

The study of an ISA Arborist would stop for nothing if good...

best regards,

Sammy Au
Station Manager


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