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.
Comments
An excellent article Pip. But please could you explain why home made compost is unfit for purpose? I am sure there are some people who know what they are doing as regards producing a good home made version.
Hi Stuart. Thanks for bringing this topic up in discussions. An interesting article by Pip.I have a particular interest in trees and value them greatly.
I'm going to have a stab at answering your query and I'd be interested to know what Pip's answer is.
I guess that mulching with home made compost (esp. if rotted manure is included) is proportonately high in nitrogen which according to the above is not required in the majority of (suburban) gardens and would be more likely detrimental to the tree in the long term. Also I expect that say annual mulching around the tree under the canopy would slowly raise the soil level above the root zone which is not desirable.
I await Pip's thorough comments!
Pip, I'd be interested if you know of the original reference which quotes an approximate monetary value on trees as Ive heard this kind of thing before but can't remember where. Thanks.
Also pip - How would you factor in the increasing usage of Legume trees for hedging and in Silviopasture / Arable to provide Nitrogen for a crop - With alley cropping at combine width of around 50-75 trees per Ha to provide from 50-300kg per year to Nitrogen. Not only does this have massive impacts on our Landscape as many more trees will mean from a distance arable fields, while still growing Wheat, Triticale and Rape, may appear from west or east views to actually be a woodland!
Further more these trees, although adapted to producing and tolerating higher nitrogen levels, will be treated as a crop, but will also affect their neighbouring hedgerow and timber trees with a Sustained increase in Nitrogen levels, rather than a Human application "spike" twice a year.
I know this digresses from your emphasis on Urban trees - but It the logic behind this appears in my mind to contradict some of what you (Correctly) point out are the problems trees are facing.
Also using mulch and fertilizer near a tree - how do you factor in the removal of nitrogen in Grass clipings and shrub prunings removed from site? Which tend to be the plants for which the fertilizer applications were targeted at.
Hi Stuart, Melissa's stab at answering for me is spot on. This blog was written with a layman in mind and following on from site visits where I had witnessed damage to tree basal areas by the increasingly frequent 'good healthy dose of manure' mentality. Mulching around trees is a fine balance between combining the roots need for oxygen and a much needed feed - a lot of home produced compost is of frankly dubious quality and all too frequently used before it is actually ready - thus trapping significant heat which can be very damaging to the basal area and dramatically disturbing in particular the immediate top layer of soil so vital as a conduit for the root system and disturbing any equilbirum for existing mychorrizal fungi and a host of other micro and macro soil organisms, this can weaken the soil structure thus leading to a weakened root plate itself, even increasing the shear plate and thus the tree topples. But the main and most common factor is the damage created by mulching higher than the collar, which I have seen in RHS gardens no less! This causes immense stress to the tree.
I don't doubt that there are many professionals and public who produce great compost, but I have even seen compost used on television which was clearly not ready!
Trees have to be treated separately to all other garden plants, including turf and they rarely are. I am currently trying to write a piece on drought and trees for SOW as a lot of the garden advice for drought rarely factors this in.
In reference to the monetary value, the Spelman quote was based on a CAVAT valuation - http://www.islington.gov.uk/DownloadableDocuments/Environment/Pdf/g...
But of course both the Helliwell and iTrees systems are very much in use also.
David, this is a much more site specific technique - and one which should be the sole realm of a professional in my opinion. With slow drainage techniques both traditional and modern there is huge potential in the use of legume trees. But a full survey is absolutely essential, particularly in determining the underlying soil strata. My main gripe is the over use of both compost in a confined area, (the increasing amount of 'farm' allotment schemes fall guilty of this!), and the habitual 'feed first' mentality, usually extolled by well meaning garden centre staff when placed on the spot. The importance of garden trees is undervalued and we should be much more hot on issues like fertilising if we are to avoid being labelled as part of a problem in environmental terms rather than part of the solution. There are a lot of techniques born from good science in tropical climates being advocated for use in temperate zones and this concerns me - there is a lot of 'jumping on a bandwagon' leading to all too hasty application based on psuedo science. SIte specifics and the empowerment of localised landscaping / forestry knowledge is critically important.