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. It is well known that the horizontal spread of tree roots is frequently
the height of the tree and sometimes very 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 between fungal hyphae 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. 2107 update Apparently they do!
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 eventually 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 excess 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 near the pond thrive! |
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 explore 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 to containerised plants 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 need 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 wedding cake 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! |
9. Sometimes an
established plant might die for no apparent reason. A very rare occurrence that
I have sometimes witnessed and recently personally experienced was something toxic
buried deep in the ground. Most of us do not know the long term history of our garden. Goodness knows what might have been buried down there and roots can go
very deep.
Oh dear, I have
found another reason for digging!
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The roots of the royal fern next to the pond do not have far to go to find water
|
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