Wednesday, 30 July 2014

Soil and garden compost as a bulky ingredient in seed and potting compost.

Amateur innovation versus professional exactitude

All the plants pictured today are grown in containers of soil that have no extra ingredients other than fertiliser and occasionally lime.

I am playing with fire today when I suggest unsterilized garden soil might be used in a pot! For forty years in my lectures I declared that you must never grow pot plants in soil from the garden! When we moved to Seaton Ross fourteen years ago I found that my soil to be an extremely fine water retentive sand. I tentatively started to use it as a substitute for regular potting compost and liked what I saw. It is very rare now that for other than very special purposes I use anything else.
When I started blogging I wrote a timid little post about this. It was called ‘Pot compost, breaking the rules’. My posts were edited then and I am grateful to Cathi who struggled a long time to make that piece intelligible. I don’t even know if pot compost is a proper name, but I rather like it! I will be more forthright today!
First for my American readers I must explain that strange word ‘compost’. In the UK we use it with two separate meanings. Most of us are besotted with ‘garden compost’ from garden waste that has decayed in a heap, unsightly wooden/wire structures, or in a bin. We also use the term ‘compost’ for mixtures in our seed trays, pots and containers and call them either ‘seed composts’ or ‘potting composts’.
Of course in ancient times garden compost and potting compost were exactly the same thing.
For my purposes today I am treating ‘garden compost’ and ‘soil’ as synonymous terms for these two bulky ingredients in a seed or potting compost. This is not quite correct as both can be separately used and are not the same! Garden compost is bulky material in various stages of late decay and might contain a little soil from the roots of composted plants. The organic matter will eventually decay away to carbon dioxide and water, beneficially releasing its nutrients, but not lasting forever! 
Actual soil can be used and reused many times. Unless garden compost is very well decayed it will be far too rich to be used exclusively as a bulky ingredient. My first choice material is organic-matter-rich fertile garden soil with a suitable texture.
(texture is the technical term that refers to the proportions of sand, silt and clay)

Loam in John Innes composts
Soil in compost has a very long history. John Innes compost has a fine pedigree and was the ascendant compost in horticulture for perhaps sixty years. More than 50% of the volume of this compost is sterilized loam. Loam was defined as a soil with suitable proportions of sand, silt and clay and fibrous material and it was prepared from lifted, stacked and decayed grass swards. This soil was sterilized with steam before making up the compost. Properly prepared John Innes was a wonderful compost. Modern versions are often a pale shadow of its former self although we do have the advantage  today of slow release fertilizers unavailable to its inventors Lawrence and Newall at the John Innes Institute almost a hundred years ago.
Loam for John Innes was sterilized with steam to control pest, disease and weeds. Because it killed almost all soil bacteria, the new micro-organism profile which developed in consequence enhanced the release of soluble nitrogen and this was at the time considered an advantage.
My comments today are about unsterilised soil. Some modern thinking is that an undisturbed micro-organism profile of fungi and bacteria in a natural harmony might be a good thing. Probably misguidedly, many gardeners today add mycorrhiza from a packet! 
The physical structure of specially prepared loam is not usually suitable when used alone for potting compost and the John Innes formula adds sharp sand  (more latterly grit) and granulated peat for water retention and aeration. Both are still superb ingredients for gardeners who make up their own soil based compost.
The slow release nitrogen fertilizer in traditional J.I. compost was hoof and horn meal which as it broke down acidified the soil. In addition with peat as a component, lime was required. This is less relevant now and if the pH of the soil in the ground is satisfactory the same applies when it is used in a pot. In my own all-soil compost if I judge to need lime I add dolomitic limestone.

Problems with amateur composts today
John Innes composts went out of fashion as they were expensive to prepare and soil supplies were limited and variable. Also, with the then available fertilizers, and as a result of the nutrient boost which was a consequence of sterilization, they deteriorated in storage. 
They were superseded by wonderful peat based composts which in particular were superior for propagation. I do not share many folk’s fashionable reservations about peat. I will not explain the reasons for my opinion today but I fully concur with the opinions here. Amateurs and some professionals never properly understood the water management of peat based compost and I do not miss dried out hydrophobic compost when I buy plants from  the garden centre.
There are some really excellent peat free composts nowadays but others are rubbish and present numerous management difficulties. As I never use them I am completely unable to advise which to buy. Many  potting composts today are based on commercially composted green wastes and the product is very variable. Perhaps you get what you pay for. Composted green waste has many legitimate garden uses but only that produced by very well controlled systems is suitable for potting compost.
I do not approve of shipping compost materials half way round the world to find substitutes for peat.

So what heresies am I advocating?
I cannot promote my own methods carte blanche to use soil in small pots and seed trays. My own soil, almost uniquely, has a satisfactory texture. Most soils, especially clays are completely unsuitable. Where I do part from conventional opinion is for growing plants in large containers, perhaps those greater than ten litres. A lady at the bridge club the other day asked me to recommend a compost for her bourgeoning olive. I was completely at a loss to give her sensible options but after quizzing her closely, discovered she had highly organic well drained garden soil. Eureka!
Visiting my son in Sorrento I noticed all garden ‘planters’ were filled with their native volcanic soil. 
Not all gardeners have suitable soil to use as the sole bulky ingredient in a large container, but in my opinion, very many do.
My comments today are relevant to any gardener who uses unsterilized soil alone as I do, or as an ingredient in a mixture.

The case against my methods

My ferocious cacti need some care when weeding! I use my secateurs as tweezers! The lily seedlings are also in soil.

Unsterilized soil may contain pests, diseases and weed seed.
Most soil is not as water retentive as traditional compost ingredients such as peat.
Many soils will be insufficiently aerated in shallow containers when wet.
Too much clay content will lead to unacceptable hardening, shrinking and cracking when dry.
Soil is highly variable in texture, structure and nutrient content and in no way is suitable for mass production methods that require uniformity.

We have rather exotic pests in Yorkshire. Poppy is nicking the nectar
So why do it?
Because I can!  My own garden soil has properties such as excellent water retention consistent with good aeration.

Where large containers with a deep profile are used the physical properties of soil in a container are similar to those in the ground.

I use huge quantities of compost and my method is almost free!

Soil can be recycled and is not wasted. If you must, you can return the soil to your garden and replace it with some more. Personally I am completely cavalier in reusing refreshed old soil-compost many times over.

If you are using your own soil you are not scouring the environment, driving to the garden centre for sometimes fairly dubious materials transported from all over the world.

Years ago, we used to compare the then new peat composts with loam based John Innes. The prevailing view, which I admit was disputable, was that JI compost was better for long term culture because of the loam. I tend to concur for plants that are to remain undisturbed for many years. Even peat decays and slowly oxidises away and containers need to be topped up. Not so with soil.

Yes I have weeds! I enjoy hand weeding my pots although in the case of large containers of certain plants I can lift up my nozzle and accurately spray them. My own methods in my garden attempt to stop weeds seeding and weedy perennial roots are not present in my soil. I can imagine some soils have so many weed seeds and are wick with wicken (bindweed) that my methods would be completely unsuitable. Plants bought from the garden centre in originally sterile compost are frequently full of weeds and users of ‘proper’ compost have the same problems as I do with liverwort and moss.

It would be foolish to use soil contaminated with for example, club root or white rot of onions - especially to grow onions and brassicas! Subsequent planting would spread these diseases around the garden. I have described how ‘damping off’ is not a problem for me and anyway as a disease of seedlings and not of mature plants, is irrelevant to our discussion today. I personally suffer no more or less with disease, unexplained plant death and sickness than I previously did when I used conventional compost. My plants in pots are just as healthy as those in the ground!

There are even more flowers on my agapanthus this year

There are a number of pests that do not like soil but love peaty and soft organic ingredients. The dreaded mushroom fly and vine weevil, a plague with organic composts, are unknown to me.

Although my soil is hydrophobic, if I water my compost when it has become dry, because of the container’s retaining rim the water soaks in and fully rewets the soil. This is in contrast with hydrophobic materials like peat that shrink when dry and water ‘runs through’.

Heaven forbid that I should neglect the long term nutrition of plants in my containers. Should I be so foolish, my plants, will survive better than those from the garden centre when their slow release fertilizer runs out after very few months.

Using mole soil
never thought I would write this! My fur coated helpers kindly leave neat piles of lovely soil on my lawn. I know my soil is of a suitable sand/silt texture and coming from turf will be high in organic matter. Tongue in cheek, I might add that my grey coated friends have carefully picked out the grubs. I am only too grateful to accept their bounty.
A word of warning, for most gardeners the suitability of mole soil is illusionary. The moles might appear to be magicians when they appear in our gardens from nowhere, but their magic is insufficient to turn clay into sand. It might look nice and crumbly but if the texture is wrong and without divine intervention it will turn rock hard in a pot!

And finally, a story!
My dear friend and colleague, Tony Thompson, would every year build, as part of his teaching, several loam stacks with the students for future use in making up John Innes compost. The method was to stack sods of turf in layers with manure. The principle was that after decaying for a year or so the decayed turf could be chopped up to a wonderful fibrous loam.
The lovely man who was in charge of compost could tell you the year each stack was constructed and we all accepted that when he lovingly described each vintage it was like ‘putting down’ wine.
This of course completely misses the point!  After all those years the benefits of root fibre and manure  would be completely lost.

If you have laboured this far today you will sympathise with my students when I tell you that it used to take me three forty-five minute lectures to cover compost!

You can read more about how I grow the plants in the pictures on these links

Friday, 18 July 2014

Garden myths discussed. Does bonemeal have any horticultural value?

Every time I used to read a gardening article written for amateurs it recommended bonemeal to encourage new roots and fertilise the plants. Whatever the plant, be it grass, bulbs, vegetables, fruit, shrubs, trees and herbaceous perennials it was a panacea and almost mandatory to use it when planting and sowing. 

Whenever I looked at any professional fertiliser recommendations for each one of these plants there was never a mention. Be it nurserymen in Boskoop, farmers at home, fruit growers in Kent, bulb growers in Spalding or vegetable growers in Lincolnshire they all used  fertilisers, usually compound -  but bonemeal.....? Not a word!

Why was this so? It's not much better now. I have agonised over this conundrum for many years and I have concluded it is so widely recommended because it does very little harm. Gardeners (wrongly) feel it is their duty to put a little something into the planting hole to give the plants a good start. Most gardening journalists apart from believing the myth themselves - it must be right because it is repeated ad nauseam - do not want to give any cause to the  gardener to damage or kill his plants. There are people who imagine that if a slack-handful of fertiliser is recommended, either they might have very big hands or are of the mind that if one is good, ten is better. Proper fertilisers contain highly soluble nutrients and too much will damage or even kill plants.

I often recommend Growmore to amateurs, it is a good compromise between too much and nothing (bonemeal). Growmore contains the major nutrients, 7% nitrogen, 7% phosphate and 7% potassium. Better and more concentrated fertilisers are available and these I have recommended in previous posts but clearly the stronger they are the more room for abuse.

A general fertiliser far superior to bonemeal and excellent for amateurs. I remember when it was called NATIONAL  growmore but am too young(!) to have dug-for-victory
Just one final salvo before I try to analyse this myth. Bonemeal smells a little acrid and highly organic. It must be doing good! When I say bonemeal is harmless I believe this to be true. The last time I was in Eire (fifteen years ago), bonemeal was banned - something to do with anthrax as far as I know. Thinking about flinging the powder of crushed denatured bones around it amazes me that in the reaction to the very real  BSE scare they did not ban bonemeal in the UK. There is a complication here, if you check things out on the net, bonemeal can be used in animal feeds where their are different regulations.

Superior to bonemeal, perhaps equivalent to growmore for organic gardeners unless in some formulations its enhanced potash is from an inorganic source. Perhaps you are troubled with taking fish from the sea?
So what's wrong with bonemeal?
Other than;
it contains only phosphate, a negligible amount of nitrogen and normally unnecessary calcium; 
that the phosphate only becomes available in minute quantities even over several years; 
that most garden soils already contain more than enough phosphate; 
that there is little evidence that roots require phosphate more than any other nutrient;
that it is immobile and remains where you put it - and even if roots really do grow towards phosphate, because it remains in the planting hole it will NOT encourage roots to grow out into the soil; 
because extra phosphate might inhibit mycorrhiza and even render other nutrients insoluble; 
it is a very expensive way to buy phosphate when a cheaper compound fertiliser is better;
then I have no objection to it at all.

If you learnt on your mother's knee (she will be well over a hundred now) that bonemeal is an excellent fertiliser events have overtaken you. Modern methods extract all the goodness from bones and you buy the debris.
Bonemeal needs to be mineralised - that is worked on by bacteria - before any soluble phosphate is available at all. If the pH is more than seven it will be immediately locked away and even when the soil is acid, one study showed the nutrients took more than ten years to be released. One enterprising vendor took a positive view and advertised ten years supply in one go! 
Mike Ashford my botanist friend described adding bonemeal to the soil as a form of pollution. I know no professional colleague who ever uses it. I would not put it on my garden if you gave it me for free! You have perhaps by now have decided I might be a little biased. 

About the nutrient phosphate
I hope I have not given the impression that I have  something against phosphate. It is a major nutrient required for the development and function of every plant cell. It is contained in almost all compound fertilisers but it is not usually necessary to provide it separately as a single nutrient fertiliser. It is particularly important for seedlings and young plants. It is not for nothing that superphosphate is the sole fertiliser ingredient of John Innes seed compost - not used now and superseded by loam-less seed composts, it gave fabulous service to amateurs and professionals for more than fifty years. 
Although most phosphate fertilisers (other than bonemeal) are more or less soluble when they are used on the soil, in a matter of weeks they become chemically 'locked up' and much less available to plants. An equilibrium exists between ‘locked up’ phosphate and small amounts dissolved in the soil water. The available nutrient is effected by acidity and alkalinity, is optimally available between pH 6 and 7 and where temperatures are sufficiently high. Phosphate in the soil is like capital in a bank that remains in place (dream on), and small withdrawals are made.
The consequence of this is that in the soil, phosphate, in effect, acts as it's own slow release fertiliser. Perhaps only 20% of phosphate added as fertiliser, is used by the current crop. The remainder remains as capital in the soil 'bank' for future years. Most UK soils have more than enough phosphate and need no more for several years!
Because phosphate becomes locked up in the soil, it is not very mobile and tends to stay where you put it. That's one in the eye for none diggers like me who top-dress only and do not work fertiliser in. My numerous worms relocate the absorptive soil and the roots come to the surface without being chopped away in my undisturbed soil.
Another consequence of the low mobility of phosphate is that not only does it not readily wash into the soil, it does not leach out. Unfortunately this does not always prevent water pollution if soil particles are eroded by wind or water, or phosphate from decaying organic matter left on hard surfaces runs straight to the to the drains.

In early May, Peter William's commented that he had never before seen such intense purple-leaf symptoms of phosphate deficiency on my tomatoes! (sorry no picture). After being planted in my greenhouse at the end of April there had been a spell of rather cold weather and the air temperatures in my greenhouse were almost zero centigrade. Warm weather returned and my soil supply of phosphate became available and now in late May my tomatoes are fully recovered.

In mid July we are within a few days of picking. The initial phosphate deficiency has done little harm
A little known fact in the UK, is that many Australian soils are naturally deficient in phosphate and some plants from that continent have evolved with a low phosphate requirement. In a very limited number of cases in UK soils, such plants have suffered from phosphate toxicity.

Have I been fair to bonemeal?
Probably not. It was a cheap jibe about it locking up other nutrients. That is a property of phosphate and not a specific fertiliser. Indeed the fertiliser superphosphate is sometimes used to lock up heavy metals that are potentially toxic.
As to it's effects on root growth I was perhaps over-anxious to counter the usual baby talk that phosphate is for roots when I fully admit that phosphate is good for the development of young plants (pity that bonemeal provides so very little).
Although the release-rate of phosphate from bonemeal is hardly perceptible that is not little different to the effect of normal phosphate when it is locked up in the soil.
It is generally recognised that fertilisers prevent mycorrhizal associations forming. The release rate of phosphate from bonemeal is hardly enough to make any difference!

And finally a story about bonemeal
No, not the one about West Hartlepool Parks Department making their own version of John Innes compost with bonemeal when I was an apprentice.
No, not the one about nearly coming to blows in a pub.
No, not the one about plenty of bonemeal in my cemetery gardens.

In the early seventies we took the students to see the turf at Headingly cricket and rugby stadium. The new groundsman was scathing about his predecessor's use of bonemeal. He had had the soil analysed and rhetorically declared that there was enough phosphate stored in the soil to last to the millennium. Wonder what happened in 2001?

With trace elements superior to growmore

My current professional fertiliser. Brother-in-law Dave recently bought 25kg from my sundries supplier for £22 to share with fellow allotmenteers. As a hard prill formulation it stores very well.

My previous  fertiliser posts

Monday, 7 July 2014

A visit to Kew Gardens

It was a beautiful day in London
It was meant to be Chelsea! Julie Williams advance-purchased train tickets for the five of us before finding Chelsea was fully booked! So we went to Kew. 

To assuage Julie's embarrassment when Brenda subsequently tried to arrange a different jaunt to London Open Squares Gardens, she purchased the tickets for the gardens, booked Waterloo Travelodge on a none-refundable deal before finding the gardens were open the following week. We did the Jubilee Walk instead! You just can't get the staff!

We arrived At Kew Tube Station and ate a hearty breakfast outside in the morning sunshine before confidently wandering down to the gardens secure that each one of us knew the way. We walked about half a mile 180 degrees in the wrong direction. A perfect start to a wonderful day! As we entered the garden Peter and I reminisced how fifty years ago we went in for a penny. Not anymore!

Before I embark on describing some scenes in this fantastic and wonderful garden I ought to explain why some of my comments are rather ungracious and carping. It's more that I want discuss a few interesting technical considerations rather than complain. To manage such a huge and sophisticated garden as well as they do is no inconsiderable achievement.
Take labelling. Another (unnamed) botanic garden I visited recently had a number of misplaced labels. Spring bulbs had died down to be replaced by the new growth of Summer plants. It was very confusing. One could also see a bulb labelled miscanthus when this tall grass was actually emerging two metres away. It's very difficult when untrained staff and worse Joe Public rearrange labels. An absolute nightmare. 
For Kew it was a minor sin, when in a display bed I found a corydalis wrongly labelled as the related adlumia. Shades of schadenfruede, I am ashamed to say. I felt a frisson of satisfaction.

My camera work is worse than their naming
This is a real Adlumia fungusa
Pictures of Kew

The ducks think the flower urns are nests.

Is tattooing trees a new trend?

Cathi, Julie and Brenda all cook with pine nuts and were fascinated by the bark of this historic tree

Fascinating story how neglect in a Victorian nursery determined the tree’s shape. It gives me hope for my own misdemeanours!
A wonderful place to bring children for stimulating  projects. Hard work for the teachers.

These diligent students are being very good not to walk on the soil. Last time I went to Oxford Botanic Garden, Marilyn, Dave and Brenda shouted at me and gasped in astonishment when I walked on the fluffed up soil to examine a plant.

They don't give my own sophora the same degree of attention.

When you build a new rock garden use generous sized stone
Two of of the things wrong with traditional herbaceous borders. Over-staking and weeds. I find it difficult to believe in a hundred years they have not eliminated common bindweed. On reflexion perhaps I malign them, maybe it’s the more innocuous annual black bindweed?

Dicentra formosa oregona 'Silver Beads is a perfectly nice dicentra but if there is any difference to Dicentra 'Pearl Drops' and 'Langtrees' it is beyond me. Perhaps it is a dwarf form or just growing poorly? I wonder if it is characteristic of 'Silver Beads' to have those pink flowers or is it reverting? Better try it again in my National Collection.

They sometimes say garden maintenance in public places is boring. How could it be so in such a beautiful place?

Did the automatic humidifiers or overhead watering system break down on a very hot day? I sometimes think that plants should be displayed, worts and all, as they really are in the jungle!

What wonderful markings on this aquatic plant. It was fascinating to learn from a display that some of the floating tropical aquatics have on their underside the equivalent of thorns - to deter predators

The water lily house was our favourite place

There are many fine historic buildings in the grounds.
The old orangery now serves as a tea room. I love the shape of that border

They want to make sure that the 'fossil tree', the Wollemi Pine  does not escape. Does it walk like a triffid? There are many fine specimens of those other two fossil trees in the grounds, metasequoia and ginkgo. They are a bit big to get through the gate.
Taking a walk in the daisies

See that plant with the orange tint on the leaves. Is it the biggest weed, an epilobium, that we will see today?  Mea culpa too, you should have seen the disdainful look when Brenda recently pulled out a sow thistle taller than herself in our garden at home

By the time we got here we were a little tired and failed to investigate. Wonder what we missed? 

This tree would be a very good visual aid for a lecture on tree compartmentalisation. You can see how the branch is a very distinct structure from a trunk and where on the shoulder a pruning cut should be made.

Tribute to Kew

What a wonderful garden! What beautiful spacious parkland and grounds. The further you walk away from the gate and explore into the hinterland the more natural it seems to become, albeit, or should I say because of, the plants of the world  are growing together. As a result of the artifices of skilled management and careful planting the plants seem to belong there.
Tens of thousands of different plants are preserved in the grounds. This is just the tip of the iceberg with regard to the conservation work done by Kew.  Worldwide their staff seek out endangered plants and habitats and by advice, education and persuasion help to preserve them. Sometimes natural habitats no longer exist and plants can only be preserved by moving them around the world and growing them in gardens and new ecologies.
Moving plants around the world is what Kew has been good at for the past 250 years. Think of all the plants, food crops and commodities that have passed through their hands and literally have changed the world.
You do not have to be a gardener to appreciate Kew. The garden contains historic buildings, monuments and greenhouses. The buildings provide for administration, research laboratories, preserved plant collections, gene banks of seeds and educational facilities. Kew educates and trains gardeners from all over the world. 
The plants in both the historic and new state of-the-art greenhouses are just amazing.
It is tragic that Kew’s grant from the state has been reduced yet again. The value of trade in plants that Kew have given the world would pay for the annual maintenance of the garden hundreds of times over.
I wish the British public appreciated Kew more. It should be top of any list as a garden to visit in London. ‘Other places’ might display huge skill in short term display, publicity, hype and television coverage. British horticulture is magnificently represented at Kew!

My previous two London garden visits London Squares Gardens 

Wednesday, 25 June 2014

Probing roots

Roots absorbing water and nutrients.

Any gardening encyclopedia will tell you that the function of roots is to absorb water and nutrients, provide anchorage and sometimes store food. The first two are my subject today.

Root spread
Years ago I would present my gardening article to the editor of the village magazine in my own ineligible hand. In a piece about roots I wrote that roots might penetrate down to as much as five feet. The editor, in her wisdom, printed ‘five inches’. Just what people think when they yank a plant out of the ground and imagine what they see is all there is!

Although this bog plant may have less extensive roots when growing near water, here in a drier part of my garden I expect its roots will go as deep as two meters
In suitable conditions it is not uncommon for herbaceous plants to have roots two meters down. Some desert-tree roots have been shown to penetrate down as far as fifty meters. It is well known that the horizontal spread of tree roots is frequently the height of the tree and sometimes much more. Perhaps not unsurprisingly people imagine that the bulk of a mature tree’s roots are deeper than they actually are. In exceptional circumstance they may be as shallow as 400mm but perhaps more routinely between one and two meters. The thing to appreciate about tree and shrub roots is the huge distances they grow horizontally.

Even in the severest of droughts the deep roots of my vine enable it to grow luxuriantly (too luxuriantly for my liking!)

Taking trees as an example, their woody roots are only the actual tip of the iceberg. Fine feeding roots develop in all directions and might penetrate deeply. The life of these delicate roots might be as little as a few days but the volume of soil reached may be quite enormous. Mycorrhizal links with fungal hyphae normally enable tree roots to exploit an even greater soil profile and to extract water and nutrients from every crack and crevice.
The root systems of large plants are huge because they need massive amounts of water and they need to explore a long way to find nutrients. I am not aware that they have any special mechanism to find nutrients, they just grow towards water and well aerated moist soil.

Perhaps I should mention that all the soil’s water is not completely available to plants. As the plant extracts water the remaining amounts are more tightly held by capillarity. A clay soil for example has an extremely high water holding capacity with a great deal of available water and yet as it dehydrates to the point that the plant can ‘suck out’ no more, it might retain almost 50% of its original capacity.

The bog iris has the capacity to grow in boggy soil and sometimes actually in the water. The floating aquatic plant’s roots are exclusively in water.

Although roots grow towards wet soil rather than to dry, typical plant roots do not survive in saturated conditions. Indeed in very wet weather when water tables rise to saturate the soil and displace oxygen, roots cease to function and soon die. This puzzles some gardeners as they observe roots of aquatic plants thriving in water! The difference is that open water contains a much greater amount of dissolved oxygen absorbed from the air or produced by the photosynthesis of aquatic plants. Soil water lacks sufficient dissolved oxygen although rain itself is well oxygenated but is rapidly depleted. Root death due to lack of oxygen is swifter when temperatures are high. Flooding is more serious in Summer but fortunately less common.
My scientist friend Peter reminds me of a more subtle point about root death due to waterlogging. Ethylene is a gas produced by plants as a natural hormone. Surplus cannot freely escape from waterlogged soil and the build up is toxic to plants and this prevents them from absorbing both water and nutrients. Think of this when the houseplant you left standing in water wilts and dies!

In heavy rain sub-surface water runs down the sandy field at the bottom of our garden. Look at the effect on the farmer’s wheat! In contrast see how the aquatic grasses in the natural drainage pond thrive!

The perched water table was extremely high in Winter 2012/2013 and only surface roots survived. A severe drought in Summer 2013 consequently caused very severe dieback. Now in 2014 our catalpa is starting to recover.

Uptake of nutrients
Any gardening book will tell you that plants require nitrogen, phosphorus and potassium and significant amounts of sulphur, calcium and magnesium. Plants also require smaller amounts of trace elements. These are copper, molybdenum, boron, zinc, manganese and very significantly iron. A few more are sometimes listed as some plants can use extra trace elements such as sodium, silicon and chlorine. In nature all plant nutrients are normally absorbed direct from the soil.

1. A farmer’s list of trace elements is longer than that of the gardener. Absorbed elements such as cobalt and selenium are essential to his livestock. Such nutrients passively absorbed by the plants are of interest to fruit and vegetable growers’ customers too.

2. Nutrients are only absorbed by the plant when they are dissolved in water. They are usually absorbed in very simple inorganic forms although dissolved complex molecules such as natural chelates supply nutrients that otherwise might be unavailable. The fact that systemic pesticides can be absorbed show that certain manufactured chemicals can be taken up too. With the aid of mycorrhiza organic molecules are also exchanged.

3. It used to be thought that absorption of nutrients was exclusively passive. In fact plants have a measure of control over the nutrients they absorb as long as they are available in soluble form.

4. Too few nutrients lead to plant nutrient deficiencies. Too much of certain elements can be toxic to plants and  much more rarely to consumers too. Toxicity might occur if plants are grown on polluted ground.

Some interesting facts from the recent scientific press.

Toxicity from industrial pollution, especially in the form of heavy metals such as nickel and lead, can render soil completely inhospitable to normal plants. Fortunately the pool of genetic variability in populations of native plants can mean that natural selection within surprisingly few generations can lead to resistant forms. Peter Williams showed me remarkable data showing how natural vegetation down-wind from former lead mines in Wales grows healthily, whereas exactly the same species transferred to the polluted ground from elsewhere die.

Patents regarding exploiting plants ability to extract heavy metals through their roots are about to run out and release for commercial use this exciting technology. There is potential to grow certain plants that have an astounding capacity to take up heavy metals and if cropped and disposed of can be used to clean up polluted sites. A variation of this technology is to grow such plants in mineral rich soils, to harvest the plants and extract useful metals such as nickel from them. A new kind of mining! Perhaps not a good thing?

Another remarkable fact is that many plants have the ability to passively absorb soluble metal ions and thereby indicate the presence of metals below. Apparently eucalyptus trees have directed miners to nickel deposits one hundred meters down. Don’t ask me if the roots get so deep!

How knowledge of root action can be helpful to the gardener

1. The volume of soil in a container is tiny compared to what roots can find in the ground. Apart from considerations of water retentive compost and enriched nutrient supply, the frequency of watering needs to be more often than for the same plants in the ground. Except in really wet periods, the amount of water supplied by rain is not nearly enough. Last year I wrote how my bougainvillea in my conservatory needed oodles of water and was hyper-sensitive to drought. In a post about Madeira, luxuriant bougainvillea scrambled over the cliffs for many months without rain!

Where watering is skilled, frequent and includes liquid feeding it is actually quite amazing how healthy large plants will grow in very small pots.  Growing in excessively small pots is not recommended!

2. When new plants are planted into dry ground, establishment watering might be needed. Once root growth is underway plants will frequently not need watering ever again. Other than generous watering in, I almost never water my own vegetable garden.

3. It is important to avoid severe compaction such as rotavator pans that might prevent roots penetrating deeply.

4. Too frequent watering encourages roots to grow at the surface rather than deeply. This is fine as long as watering continues but it is a heck of a bind to water bedding plants every day. My bedding plants in the open ground, once established, are almost never watered.

5. Because deep roots of perennials might die in waterlogged ground, especially in winter, surface roots surviving in more oxygenated surface soil should not be cut away by digging!

6. Because nutrients might leach out of containers, extra nutrition such as liquid feeding is usually needed whereas the same plants in the ground may need no fertilizer at all.

7. Beware planting delicate plants near where aggressive tree or hedge roots roots will severely dehydrate the ground. Sometimes in such conditions planting plants in a tub might be a good tactic. This keeps any watering exclusive to your plants and shade from the trees will reduce evaporation.

In the Summer drought of 2013 our Pagoda dogwood tree suddenly showed extreme stress. We needed to leave the hose pipe running for more than an hour over about four square meters to save it!

 8. Sometimes newly planted trees and shrubs will thrive for many years as they grow. As they get bigger their water demand hugely increases and they compete with each other for limited supplies of water. Roots can explore extensively but if they eventually reach already dehydrated soil or depleted water reserves under tarmac or buildings they will suffer in dry periods. Often the weakest trees or shrubs will die.

9. Sometimes an established plant might die for no apparent reason. A very rare occurrence that I have sometimes witnessed and once personally experienced was something toxic buried deep in the ground. Most of us do not know the long term history of our site. Goodness knows what might have been buried down there and roots can go very deep.
Oh dear, I have found another reason for digging!

The roots of the royal fern do not have far to go to find water

The large fern in a large pot has a good reserve of water…..The osmunda must not dry out…. The gesneriad will lose water very quickly when it is a metre high….The Scots flames must not be watered if still very wet….
All to be considered when I go round with my hosepipe!

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