Soil Smearing & Tree Mortality

I was going to title this blog ‘Big Buckets of Piss’, which whilst true is somewhat unsavoury to any British audience and is perhaps why this issue is somewhat off the agenda.

There are many huge threats facing trees in the urban environment and these threats are in constant competition with each other in terms of publicity in the twitter /blog o sphere and certainly in the general media. The percentage of young trees dying in their infancy is a statistic yet to be officially realised. But it is high, easily more than 50% when thinking about trees getting to an age where they can ‘go it alone’ and with an inherent ignorance of good maintenance procedure it is a percentage set to increase – perhaps as with most things until it is far too obvious and too late to reverse established funding the media will remain uninterested.

Soil smearing remains an issue that is hardly ever considered even within the development industry when considering the planting of trees and in an age which has replaced “if humans cannot do it a machine will” to a habitual use of machines for the most mundane of tasks, it is an issue which will remain a very common cause of infant tree mortality despite being easily solved.

The problem is simple; when using a machine or sometimes by hand digging without much care and attention the walls of the tree pit are smeared by a thin layer (sometimes quite thick) of clay. It is quite common to find at least a thin layer or deposit of clay in urban soils and the smallest quantity can be a problem if not identified. The result is an almost impermeable layer surrounding the walls of the tree pit. If the tree survives into maturity root systems can become compacted, roots unable or unwilling to penetrate the clay and this man made shear zone creates a greater risk of windblow. However many trees are unlikely to reach this stage.

Up to 20 litres of dog urine a week can be deposited at the base of a single urban tree. In times of drought the sheer quantity of urine is too much. The tree can be quite literally sat in a bowl of urine, the water content of the urine is rapidly evaporated or taken up by the tree leaving an increasing amount of deposits of the other ingredients of urine, which include: urea, creatine, uric acid, carbohydrates, enzymes, fatty acids, hormones, sodium, potassium, chloride, calcium, ammonia and magnesium in varying amounts. It is a slow death, which unfortunately has above ground symptoms similar to other recognised threats to trees. And the increasing volume of urine is an added stress for all trees in all urban and peri urban public locations.

It is not just dog urine that is a problem – some trees are unfortunately sited where the same damage occurs by excess human urine. Once when inspecting a replacement tree sited between a cafe and a car park, we were lucky that a local resident approached us and said that the tree was used as pissoir so frequently she kept her curtains facing the spot permanently closed. In this situation the cafe owner stepped forward to help by advising all customers as to the location of his WC. And British city centre trees face a weekend torture as revellers head home – A tree planted behind a taxi rank stands little chance of reaching full maturity.

The widely held myth that urine is actually good for trees and plants has to be changed.

Inadvertently the use of tree stakes and heavy mulching can help in alleviating this problem, but more often simply prolongs the inevitable. The gritting of roads and the depositing of other pollutants will hasten the demise of the tree and can lead to a misdiagnosis of the cause of death, which little helps if replanting is desired.

Soil is to a large extent prepared and able to accept urine deposits on a relatively large scale. There is evidence that urine actually ‘kickstarts’ chemical processes by certain organisms and altering populations within the soil – and having pockets of ‘urine tolerant’ soil zones provides a buffer against more extreme pollutants. Even in arid areas such as the Mediterranean countries, soil is able to mitigate against urine and further actually clean it and create a by product of worth to vegetation using the same soil. In the UK rainfall is considered enough to flush out urine and even salt from roads into the wider soils – but the massive problem is that the soil structure from modern development is simply no longer suitable to allow this.

And the solutions are easy, soil smear is certainly a problem accelerated by using non-professionals instead of qualified arboriculturists, landscapers or horticulturalists, because ‘good spadesmanship’ is the key. And together with providing methods of dispersing the urine by simply planting more trees for a start or the very effective ‘Canisite’ which concentrates and treats urine and faeces in situ, there is every chance of preventing any more damage or death to our trees.

The canisite is a little more complex than the more often built and frankly useless sandpit dog toilet, and the best designs incorporate phytoremediation planting to create a complete treatment system, with little maintenance requirements. (I cannot due to copyright explain the construction method here but please email for details).


Filed under Trees and Woodlands

3 responses to “Soil Smearing & Tree Mortality

  1. Something I had never thought about!

  2. Pingback: Soil Smearing & Tree Mortality | Forest Communication |

  3. Pingback: Tree Mortality | Landscape Architect | Liz Lake Associates |

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