Fertilising Trees

It is virtually impossible to write a brief summary of fertilisation requirements and nutrient deficiency with regards trees. The subject could amount to a 300 page book with regards one species and still be non exhaustive. It is however one of those issues which any arborist or silvicultural surveyor needs to understand at a glance and as with so many aspects of tree care this combines experience with a knowledge of the location. As such based on my experience and the locations I have worked in the following statements can be ascertained:

The vast majority of trees in an urban setting suffer from some form of nutrient deficiency.

The vast majority of trees in a suburban (garden landscape) suffer from too many nutrients, particularly Nitrogen.

The vast majority of rural broadleaved native trees, including those in semi natural woodland have an equilibrium nutrient base – although very often trees bordering pastoral intensive farmland suffer from too much Nitrogen.

Riparian trees habitually have too much nitrogen but riparian species are not only able to cope well with this over surplus but actually remediate the situation.

In rural settings it is important to note I am ignoring plantation trees. Forest fertilisation is often a necessary evil to enable (at a formative stage) growth to provide the essential base for a competitive crop. There is much present debate with regards planting trees that will cope with climate change but would require fertilisation techniques that would alter the soil ecosystem for ever. Such issues are a scary reality for climate change models and may be necessary if the threats of diseases and pathogens which are spreading rapidly due to recent climatic fluctuations continue.

There are some very basic guidelines with regards visual identification of nutrient deficiency:

Lack of Phosphorus: This is generally a reduction in size of both leaves and tree form. In more severe cases the leaves can be discoloured at the margins with a reddish or purplish hue.

Lack of Potassium: A yellowish discolouration in between the veins of the leaves, often mistaken for exposure – as more protected leaves are less affected and in more severe cases the leaves appear scorched.

Lack of Nitrogen: This is actually a rare problem for broadleaved trees except in gardens which have seen severe disruption to soil, (new builds). Complete discolouration of the leaves occurs over the whole tree and can rapidly lead to death for young trees. It is easy to remedy however.

It is far more common to witness the effects of too much Nitrogen and is an increasingly common problem for trees in a suburban setting. The signs are dieback of the canopy, which if severe can happen quickly with complete browning of the leaves, followed by the tree ‘shedding’ branches. It is unfortunate that often this behaviour encourages a concerned owner to increase feeding directly to the tree.  As a ‘rule of thumb’ it is prudent to allow a buffer zone around the base of garden trees which matches the canopy in which no fertiliser or compost should ever be applied.

Never, ever apply fertiliser to semi natural or ancient trees, woodland or hedgerow.

For urban trees, parkland trees and other ornamental trees of worth, it is vital to call on the expertise of an arborist. The actual costs of fertilising a tree of high value to increase its longevity are small in comparison with any pruning or surgery operations but as each tree has different requirements according to species and location (and given the somewhat dubious quality of available fertilisers, often as a mulch), a trained arborist or forester is essential. That all too familiar phrase to tree specialists with regards fertiliser ‘’I have a friend who has stables so there is a limitless supply of fresh horse manure’’ never fails to produce an exasperated groan – the truth is that such a mulch together with ‘home made’ composts are rarely fit for purpose and nearly always counterproductive.

The maintenance of a tree to ensure its longevity and thus accrue a financial value of worth within an urban setting is solely down to an able practitioner.

’In some parts of inner London, for example, each tree is calculated to be worth as much as £78,000 in terms of its benefits.

I might make the tree surgeons in Smith Square prune with a little more sensitivity next time!’

RT Hon Caroline Spelman, Kew July 26th 2010.

Thus the above quote by Caroline Spelman was an insult to those practitioners whose work had enabled the value to be so high.

An urban tree can be fascinating to a tree specialist as the variety of possible stresses and pollutants allow for a forensic study to determine the cause of physical defect.  Unfortunately budget rarely allows for this and thus a rapid decision is needed and yet the risk implications of wrongful diagnosis can be serious indeed. The use of ‘mycospikes’ or similar mychorrizal / fertiliser based treatments can be highly effective in the short term. I am personally of the opinion that outside of an urban environment mychorrizal treatment is yet to be proved either useful or safe.


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