Tree Soil Failure
Arborists in Des Moines, Iowa place tree failure into 3 categories - Branch, Trunk and Root. (No soil category)
Even the Tree Risk Assesment Form (under “root” conditions effecting the likelihood of failure) has only 1 check box for “soil weakness”. Tree Soil Failure is fascinating. The line between the tree or the soil being the weakness leading to failure is very thin and challenging to determine.
The 2 most common types of soil failures effecting trees are lubrication failures and soil shear failures.
Lubrication failures are caused by poorly drained soils retaining moisture, which in turn decreases friction to the small fibrous roots.
(This failure the Arboriculturist at Olson Tree Care did not find in their 40 mile survey of the Winterset, Iowa Tornado damage. This is due to the fact there was no rainfall before the event, although Olson Tree Care is sure there was some lubrication failures in low lying areas by water)
Shear failure is the other Tree Soil Failure common with trees. Deficient O, A and E horizons are to blame, leaving the B horizon close to the tree with limited friction between horizons.
(This failure was evidenced in the Arboriculturist from Olson Tree Cares Tornado survey, but not frequently, due to the Tornado staying out of urban forests which have a higher likelihood of poor soil structure)
Soil failure is complex study independent of trees. Often the soil can be the obstacle causing tree defect. In these cases, the tree defect is the contributing factor to failure, not the soil. When tree defect is absent, soil failure is just that….. soil failure.
Tree Soil Failure
It is important to note that small broken roots do not indicate root failure. Even in Lubrication Failures when whole sections of roots are displaced, the roots on the side of the fall break because of the fall but are not the cause of the fall.
It is equally important to note that compacted Soil can also be an obstacle to root plate development. In these cases Tree Soil Failure would not be the contributing factor, but tree defect.
Soil compaction is a huge obstacle to root development. Most recently the Arboriculturist from Olson Tree Care in Des Moines, Iowa looked at declining trees that suffered due to compaction from livestock. The common problem Tree Services face in residential urban forests is that we all start with bad soil compositions.
In Des Moines, Iowa they used to have a law that there needs to be a minimum amount of topsoil graded onto each construction site. This was passed due to a problem that developed from them taking the good soil and profiting from it while grading the sites with clay. Not a good future for anyone doing that. Unfortunately they reversed that law in 2015 leaving it to discretion. Most likely this change was made due to corporate pressure, not representing the interests of the public.
The best way to prepare a site for tree development would be by excavating multiple soil samples to determine site candidacy. Factoring drainage would be secondary. Often the horizons vary in Soil Structure on the same site.
Olson Tree Care has found that improving the soil that is next to impossible without replacement. Soil amendments are just that, they only add to the existing but do not change it. In Olson Tree Cares experience over time you end up with the same soil you had in the beginning.