Transcript Document

Macronutrients
and plant nutrition
Peter Shaw
This version of the file contains very few pictures – less
pretty but takes up MUCH less disc space and download
time. To see the images in their full glory, make sure you
come to the lecture!!
Overview
Today I want to look at all aspects of plant nutrient
uptake. This will cover the plants’ viewpoint, but also
soil conditions and management.
Topics to cover:
The biology of nutrient uptake by roots
The soil chemistry affecting this uptake.
Root uptake
It has long been appreciated that roots in plants are like guts in animals
– the site where nutrients are taken up. Because of this plant roots
usually have an immense surface area caused by repeated divisions.
Much of this area is due to root hairs, and it is these which are the main
sites of entry into a plant for water and nutrients.
Root hairs adhere tightly to soil particles, which is where soil water
tends to be bound.
Water enters through the epidermis of root hairs into the apoplast of the
root (extra-cellular space). Here is is gradually taken up by cells and
enters the symplast, from where it passes the casparian strips into the
xylem vessels of the stele.
The flow of water into roots is controlled by a band of corky, waterimpermeable cells lining the root cortex which force water to flow into
the main vessels symplastically. This band of corky tissue (suberin +
lignin) is the casparian strip, and is present in the endodermis of the
root systems of most vascular plants.
The casparian strip ensures
that all water entering the
stele of the root (thence up to
the main stem) has passed
through a plasma membrane
so has been regulated by
transport proteins.
Casparian
strip
stele
Note that the tracheids and vessel elements of the xylem
are dead and lack protoplasts, hence their lumen is
apoplast, not symplast.
Minerals and water enter the xylem proper by being
actively pumped from the walls of the endodermal (and
stele parenchymal) cells. This way the xylem contents
have been filtered through the plasma membranes of
many cells, and are highly purified (of bacteria, mineral
debris etc).
Mycorrhizas
Root hairs also get infected by mycorrhizal fungi, which act as
extensions of the root system and collect nutrients (especially P) for the
plant. This is the normal state for most wild plants. In exchange the
plant supplies sugars to the fungus. (This sugar drain can be a detectable
cost, if nutrients are supplied as chemical fertilisers).
The mycorrhizal condition has evolved at least 4 times, probably closer
to 10, and can be over-simplified by shoehorning mycorrhizas into 2
groups: Sheathing (ecto) mycorrhizas, which envelop roots in a coat of
hyphae, and endomycorrhizas which penetrate inside the cells inside a
hosts’ root.
In both cases the increase in uptake area is huge – as much as 3m of
hyphae from 1cm root. There are many trials showing how much better
plants do with mycorrhizal infection in poor soils.
VAM
Or endomycorrhizas
vesicles - these are large and stain well.
Arbuscules are harder - really needing
EM. It is not clear that all endos have these,
casting doubt on the name VAM.
Arbuscules die and are absorbed by the
plant, only to have new arbuscules re-form.
This seems to be how P is transferred to the
plant.
TS of an ectomycorrhizally infected root
Varieties of:
What you actually see
Sheath
Hartig Net
(between root
cells)
The minerals
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One of the classic stories in environmental
science is that of Von Helmut’s willow.
He grew a willow tree in a pot for some years,
adding only water.
During this time the plant gained weight, while
the soil lost only c. half an ounce. He concluded
that plants were composed of water (forgetting
one key component: air)
It is true that given light, water + CO2 plants can
grow for a while – but eventually growth will
become stunted for lack of other nutrition.
All living things need certain vital
elements
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Analyse a plant, and you will find that it contains
classes of biochemicals which require specific
elements.
Class:
Contains:
Sugars
fats
carbohydrates
proteins
nucleic acids
Chlorophyll
cytochromes
cytoplasm
CHO
CHO
CHO
NS [CHO]
NP [CHO]
Mg [CHO]
Fe [CHON]
K, Ca [+lots else!]
Of these nutrients:
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CHO are “free”, from air and water.
All the rest must be taken up (usually by roots from soil),
and are vital nutrients.
The actual importance of these must be determined
empirically by trials.
150 years research has shown that crops routinely respond
well to one set of elements – these are the macronutrients.
A 2nd set of elements don’t usually matter, but can cause
specific deficiencies. These are the micronutrients.
Note that the relative importance of different elements is
NOT indicated by their concentration in ashed material.
Plants do not excrete wastes but accumulate them in old
tissue, so that plant ashes contain metals such as
aluminium which have no metabolic use.
The macronutrients are:
N, P, K [big 3]
not forgetting Mg, S
The micronutrients
Ca, Fe, Mn, Mo, Na, Co, Si
Crucially important point: It is not enough that a soil contains an
element. What matters is its availability to plants. Thus
acid digests of heathland soils show ample levels of nitrogen,
but almost all of this is locked up in biomass or humic
materials, and in fact such soils are acutely deficient in
nitrogen.
Nitrogen
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This is the most important plant nutrient, at least in terms of
producing a reliable and large increase in growth.
Most of the advances in agricultural production since the 1940s
have been won by vast increases in nitrogen application.
More N => more growth, typically soft fast sappy growth, deep
green foliage. Our crops have been described as “nitrogen
green”, referring to their force-fed status and colour.
Deficiency of N => stunting, yellowness and general poor
growth.
Downside of N
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N-rich plants are vulnerable to infection (pathogenic +
saprotrophic fungi, sap suckers and herbivores all
respond well to N). In part this explains the way that
intensive farming is addicted to pesticides.
They are also mechanically softer so open to wind
damage etc, and delay winter hardening so suffer
more frost damage.
In Holland high levels of N pollution are causing forest
dieback and heathland degeneration. This is a form
of pollution called eutrophication (meaning “overfeeding”), leading to growth of weedy species at the
expense of scarce forms.
Modern thatch lasts less well than victorian thatch, as
modern plants experience higher levels of N in the
environment.
N is chemically complex
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It is everywhere, all the time, but in
the useless form of an inert gas N2
It also comes in 2 useful forms:
ammonia NH3 ( ammonium
NH4+), and nitrate NO3-.
These take part in a well known
cycle, that you are strongly advised
to learn:
N2 gas
A simplified
diagram
After exposure to
combustion flames
of the nitrogen
NOx
cycle NO, N O and N
2
Atmospheric NH3
2
Atmosphere
Soil
Denitrification
NO2-
Biological
N Fixation
Nitrosomonas
Nitrobacter
NO3
-
Nitrification
Release of H+
Biological reduction
By nitrate reductase
Organic nitrogen
and NH4+
Phosphorus, P
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This is 2nd in effect after nitrogen.
Is always applied as phosphate PO43P deficiency leads to dark, stunted growth - often
purple tints on the leaves, also causing poor root
growth, so limited uptake of other nutrients as
well.
P promotes cell division, mechanical strength,
maturation/seed set, disease resistance.
Legumes need no N (are fixers) but do need P has been suggested that a major conservation
issue in UK is phosphate eutrophication, which
leads to N enrichment.
Phosphate is tightly linked to the
origins of the fertiliser industry
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Traditionally the only source of phosphate was ground
up bones and teeth (=forms of calcium phosphate).
William Buckland (notable Oxford Don) used guano to
write “GUANO” on his college lawn. The letters
stayed visible as deep green patches for years!
Richard Lawes in the 1840s discovered that bones
dissolved in sulphuric acid made superb phosphatic
fertiliser - is still used, called superphosphate. Bones
+ H2SO4 = £
He used his £ to found Rothamsted experimental
station in 1840s - still running trials 150 years later.
A feature of P:
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Is that it is immensely immobile in soil. This is because many
of its salts are highly insoluble, ie bones! Iron, aluminium
phosphates are if anything even less soluble.
Fe3+ + PO43- => FePO4, insoluble
One practical consequence is that archaeological sites often
show up as phosphate anomalies: P residues in tombs show
where bones used to lie. Neolithic middens are detectable as P
enriched.
P fertilisers once applied can be locked up in the soil without
plants getting any benefit. Acid solutions (such as
superphosphate) are the problem: acidity mobilises Fe, Al
which them immobilise P. This process is confusingly called
phosphorus fixation.
Managing Phosphate
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The issue with P is not so much supply as availability.
Pouring on P is no great help is the soil locks it up.
(Some golf courses have such high levels of P that their
soil could be sold as fertiliser under EU regulations!)
Availability
P in pure solution
4
5
pH
6
7
8
Availability
Availability
Calcium level
Fe, Al in soil solution
4
5
P in soil
pH
6
7
8
Managing phosphate 2
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This is achieved by controlling soil pH,
which should be held around pH 6.5.
A prime function of liming acid soils is to
elevate the pH to the 6-7 range.
Similarly, chalky soils may need to be
acidified somewhat (perhaps as (NH4)2SO4)
to increase P availability, though this is less
usual.
Soils rich in Fe(OH)3 such as tropical clays
will immobilise P whatever the pH
Potassium, K
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Compared to the clear metabolic uses of P
and N, the importance of K is rather surprising.
Despite this, trials consistently show K to be a
major determinant of plant growth.
It determines turgor, frost hardiness and
resistance to wind.
K deficient plants are stunted and yellow often the older needles / leaves are yellowest
as this mobile element is translocated to the
newest tissue.
Toadstools are remarkably high in K: 23% of
the entire K in lawn soil may pass through the
“fairy ring” toadstools Marasmius oreades
The problems with K..
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Are related to its mobility.
Left alone, K will rapidly leach out of soils or litter.
It is low on acid sandy or organic soils, highest in
clays.
Organisms take K up avidly, so that (uniquely
among metals) its distribution is ecosystems is
defined by biological not chemical processes.
K experiences luxury consumption - this means
that plants take up more K than they need if it
applied. It does them no harm, but does cost the
farmer money. Best fertilise little but often.
Magnesium Mg2+
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This is one of the lime minerals, along with
calcium.
It is needed in chlorophyll, so is vital for plant
growth.
Its removal from acid forest soils leads to Type 1
German forest dieback, and magnesium
deficiency can occur in UK crops.
In practice the UK rarely experiences much Mg
deficiency as our rain is enriched with Atlantic
seawater.
Magnesium deficiency..
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Causes yellowing, especially of
older needles / leaves.
Is often caused by waterlogging - a
physiological problem not a
chemical one.
Is cured by adding dolomite
(MgCO3+CaCO3) or purer
magnesium salts.
Sulphur, as sulphate
SO42
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This element is used in proteins due to the
amino-acids cysteine and methionine.
Is prone to deficiencies on sandy soils low in
OM, especially when put to grass or
brassicas. (Old cabbage smells nasty
because of the S in its oils).
Is a new problem - old fertilisers were so
impure as to act as S supplements, and
ambient SO2 levels used to be so high the S
deficiency was unknown.
Now add as (NH4)2SO4 - but sparingly, as this
is a strongly acidifying fertiliser.
Some micro-nutrients
are of academic interest
only – can be an issue in
hydroponics with very pure
chemicals, but not in real soils.
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The ones that matter on a regular basis are:
calcium
 Iron
 Manganese
 molybdenum
 copper
 zinc
 boron
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Calcium Ca2+
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Is a major ingredient of plant ashes and is a
principal inter-cellular signalling element.
Hence one might expect it to be a major nutrient.
In fact calcium deficiency is rare in crops, indeed
virtually unknown for forests.
This is because of the link between Ca and soil
pH – soils low in Ca are so acid that phosphorus
is immobile (due to Fe3+/Al3+/Mn3+), which is the
deficiency to show up first.
Ca problems are of course readily solved by
addition of lime.
The commonest: Fe3+,
Mn3+
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Iron and Manganese (don’t confuse Mn3+
with Magnesium Mg2+) are common soil
elements used in metallo-enzymes
(cytochromes).
In most plants, deficiencies of these
elements is unusual. It occurs on chalky
soils (pH>8) and is known as limeinduced chlorosis. It shows up as
interveinal yellowing, with green veins on
a yellow leaf.
The problem with
Ericas:
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There is one group of plants for which iron is a serious issue.
These are the ericaceous plants: heathers, rhododendrons,
Camellias and allies. These are almost invariably plants of
acid (pH<5) soils, where iron is always readily available.
These plants suffer when pH exceeds c. 5.5ish, due to iron’s
immobility leading to deficiency.
Exactly why these plants need high levels of Fe is, I believe,
unclear. The fact remains that they do, and garden centres
never tell you this when you buy one!
A common tale of woe:
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Is for an expensive new Rhododendron or Camellia to
flower well in its first season in a new soil, then to lose
colour, lose leaves, go yellow, sicken and die.
This is almost always due to the new soil being
inadequately acid, hence iron deficiency.
Two solutions are on offer.
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Plant it in a suitable soil!! (the only long-term solution)
Buy chelated iron (sequestrone etc). This contains
EDTA = ethyl di-amine tetra-acetate, which reversibly
locks up iron inside its molecule like a ball in a cage.
Now the iron is isolated from water it dissolves whatever
the pH, so is available to plant roots. Ericas are able to
take up organically complexed iron, so stay alive a few
more weeks..
EDTA – ethyldiamine tetra-acetate
H
H
On paper
O
O
O
O
C
C
C
C
N
O
C
C
N
C
C
C
O
C
O
O
H
H
Fe
How it actually works