Thursday, 20 September 2018

Nearly myth, you can visually determine your plant’s nutrient status and fertiliser requirement from examining the leaves


He has not got a clue why the chaenomeles on the wall below me is dieing
All this interveinal chlorosis tells him is that it is having a bad time!
My hastily assembled definition off a ‘nearly myth’ is something that is sometimes true but is either grossly inaccurate or only achievable by someone with rare expertise. I scurry to add that such a person is not me.

Perhaps I can illustrate what I mean with an example. Suppose a tomato grower has years of experience of growing this single much researched crop and has had the benefit of the whole paraphernalia of cultural and scientific advice available to growers throughout his lifetime. Such is his experience he can look at his tomatoes and immediately read the tea leaves and give you chapter and verse of his plants nutritional needs. (An unfortunate analogy as tea leaves tell you nothing)
Not only do we not have his experience we have to deal with thousands of different plants that all behave differently


Leaves tell you a lot - but what?
Brenda asked me recently why one of our agapanthus had a few yellow scars at the tip of the leaves. I received a scathing reply when I announced I did not have a clue. I had not noticed or cared!
This set me thinking as to what you can tell from a plant’s leaves. An awful lot really and not just about nutrition. Such things as effects of weather, temperatures, drainage, light levels, pests, diseases such as virus, root disorders, watering or merely senescence. The trouble is these very diverse sources of leaf colourations, markings and distortions often look the same.
You really have to read the runes and consider your plants history. The longer you have gardened the more likely that you will come up with a correct diagnosis of any problem


Just the fungus disease powdery mildew but has it been predisposed by dry weather or lack of water or wrong position  or varietal susceptability or...?

You might take your plants to a so called expert and get a correct diagnosis. Alternatively a sufficiently beguiling snake oiler might confidently tell you a whole lot of gobbledegook and send you away happy even though your plants may be destined to die. Indeed it gives you a kind of satisfaction to ‘know’ what has killed your plant.
Life is too short to worry about every blemish on your plants.

Difficulties of diagnosis

It is tempting to think that certain symptoms will tell you whether a plant lacks a specific nutrient and that this can then be corrected by soil fertilisation, manuring, liquid or even foliar feeding. Dream on


My tomatoes turned out ok
Even where a leaf symptom is correctly interpreted as a shortage of a particular nutrient it is not necessarily deficient in the soil. 
For example Peter Williams was flabbergasted at the phosphate deficiency in my newly planted tomato seedlings last year. An extended period of cold, not enough to kill them had reduced phosphate take up and the leaves where so very purple - classic phosphate deficiency - that you could scarcely visually pick them out from the soil. Warm weather returned and they grew away a healthy green and gave me one of my best crops ever. Cold had inhibited phosphate uptake as Peter well knew. (In fact I did not know until he told me)


Just old age
Robert Pavlis wrote a very fine post on my subject today presenting reliable leaf diagnosis as a myth and gave the illustration of correctly diagnosed phosphate deficiency symptoms due to shortage of nitrogen! One nutrient may be essential to take up adequate amounts of another.

I have already emphasised that there is a myriad of causes of apparent deficiencies not least toxicity caused either by excess of nutrient, or maybe herbicide. 



Temporary low temperature chlorosis on Acanthus spinosa
Iron uptake is frequently inhibited by cold soil early in the season and especially when excessively wet. A perennial plant I know well, Acanthus spinosa invariably goes through a spell of chlorotic young leaves due to this deficiency as it bursts into Spring growth. I remember at horticultural college every year a young lecturer would take the students out to see this ‘herbicide damage’!  I did not have the courage to tell him. The boot is usually on this same foot in gardens where herbicides are used - nutrient deficiency is wrongly blamed on a weedkiller.

A very common cause of nutrient deficiency is ‘induced’ by some factor such as soil pH or excess of a different nutrient. The nutrient is not short in the soil, it is just that the plant cannot absorb it.


I had to pinch myself to really be sure I was looking at rhubarb under the chlorotic vine
Lets face it the real problem of using visual leaf symptoms as indicators of deficiency is that amongst thousands of garden plants there is just too much variability 
And as a final salvo, some plants might carry yield reducing deficiencies for which there are no symptoms at all.

A more positive view


Multi nutrient deficiency (I think not) or is it just senescence or is it merely damaged stem....
What a gardener can recognise is multi nutrient deficiency where his plants are generally starved. A hotch-pot of chloroses and discolourations.
Most often these will be plants in containers that are subject to leaching and not fertilised at all. Many modern amateur composts have abysmal nutrient reserve.

I have discussed fertiliser practice on numerous occasions. It is too big a subject for today but readers will know I take the view that if you use a modern plant fertiliser that is formulated with all the important plant nutrients; major, minor and trace element most (but not all) deficiencies will be automatically corrected.
Plants take up the nutrients they require and there is no need for special formulations tailored to individual plant needs My Yara Mila general fertiliser satisfies virtually all my plant nutrient needs although I always emphasis there are many garden situations which need no fertiliser at all.
It might surprise you that for most gardeners including myself I do not advocate soil analysis. It’s just playing at being a professional grower. 
Do not use amateur ‘kits', they are completely useless perhaps with the exception of pH widgets that provides a very crude guide.

Chlorosis


An unusual chlorosis caused by sun scorch on one of Peter's shade loving woodland plants
The description of lack of chlorophyl is much bandied around. The most usual cause is lack of nitrogen although there are a plethora of alternatives. 
Interveinal chlorosis is sometimes seen where the veins stand out green. Occurring in new leaves at the growing point it is generally associated with lack of iron; if in old leaves it might show lack of magnesium although where leaves which are naturally becoming senescent it is of no significance at all.

There are some acid loving plants such as rhododendrons that if grown at too high a pH show interveinal chlorosis around the growing point. It might not just be iron deficiency It might equally be lack of manganese.
The good news here is that fertilisers dubbed sequestrene or chelate are usually cocktails of those chemical molecules that can be absorbed from alkaline soils. (If your rhododendrons are in really alkaline soil they just die).
Against huge odds I hit the jackpot this year when I chucked some old rusty looking chelate which had mouldered unused on my shelf for thirty years over a sick plant and watered it in. It revived in a week.
I looked at this years wonderful crop of tomatoes today, (August) and noticed the new growth was a little chlorotic and sprinkled on a further top dressing of my Yara Mila all nutrient fertiliser

What is wrong with my chaenomeles?


These symptoms are caused by stress but what?
They have performed wonderfully for twelve years and before this season had grown eight foot high. One on a west facing wall, one one facing north. They are now dieing back and are showing apalling symptoms of interveinal chlorosis as well as complete yellowing and partial leaf fall.
Is it magnesium defficiency? Almost certainly not and even if so it is only one symptom of a much more serious cause.
I used to lecture on pest and diseases (and other topics I spout about) and ought to know. In fact I do not have much of a clue.

There is no classic disease of chaenomeles although Swedish research shows that stressed plants - and mine are certainly stressed - succumb and die back from all manner of combinations of very common diseases such as botrytis!

Has my chaenomeles outgrown their position and dehydrated all the soil under the house and garage? On the contrary did last Winter’s repeated deluges cause root death by flooding? 
It has been a very long hot dry Summer.
Have they just exhausted the soil’s supply of essential nutrients - after all the roots are exclusively under buildings or paved (but porous) drive.
Why have two separate varieties both started to die? I think in this case I can absolve this year’s beast from the east!
There is one clue. There are just a few scattered healthy dark green shoots emerging from the chlorosis.

I have taken them in hand. It has been necessary to prune back the dieback. I have generously applied Yaramila  ‘complete nutrient’ fertiliser and heavily watered on a couple occasions. I have scattered a dose of magnesium limestone (My soil is acid, although foundations might be alkaline).
It looked better after I blasted the plants with a heavy deluge from the hosepipe and the dead leaves fell like confetti.
I will report in six months if my false quinces are alive or dead.

Fingers crossed
Links
Robert Pavlis was much more forensic discussing this myth
I wrote about the water scorch myth a long time ago
If you want to know about Yaramila all nutrient fertiliser use my search box

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