Download LAST LECTURE THIS LECTURE ESSENTIAL ELEMENTS

LAST LECTURE
The nitrogen cycle and nitrogen in the soil
THIS LECTURE
Phosphorus and Potassium and their cycles
ESSENTIAL ELEMENTS
macronutrients
Macronutrients (with content ranges in plant tissues)
Carbon (C) 40-45%
Potassium (K) 1-3%
Hydrogen (H) 5-6%
Calcium (Ca) 0.2-3%
Oxygen (O) 45-50%
Magnesium (Mg)0.1-1%
Nitrogen (N) 0.5-5%
Sulfur (S) 0.1-0.2 %
SOIL PHOSPHORUS AND POTASSIUM
Why is phosphorus so important?
1. Essential component of ATP
(adenosine triphosphate)
Molecular currency of
intercellular energy
transfer
Used as energy source
during photosynthesis and
cellular respiration
Consumed by many
enzymes in metabolic
reactions and during cell
division.
Phosphorus (P) 0.1-0.4%
2. Incorporated into nucleic acids
3. Phospholipid bilayer
Cell membranes,
composed of a
phospholipid bilayer,
control what goes
into and out of a cell
DNA and RNA
Genetic instructions for
the development and
functioning of all living
organisms
* Notice the 5 N and 3 P
in the ATP molecule
Phospholipid
Active transport across
the cell membrane
requires ATP
Sugar-phosphate backbone
Phospholipid bilayer
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PRIMARY NUTRIENT (N-P-K) FUNCTIONS
THE IMPORTANCE OF PHOSPHORUS
Nitrogen (N).
• Gives green colour to plant.
• Induces vigorous, rapid growth in plants.
• Increases protein and yield.
• Aids and promotes seed and fruit development.
• Plants cannot use N as a gas, it must be combined with other elements and in
the correct forms.
Phosphorus (P).
• Important to germinating seedlings.
• Contributes to early maturing crops.
• Necessary for seed and fruit formation.
• Essential for N fixation.
• Stimulates root growth.
P is involved in:
P deficiency:
Photosynthesis
Nitrogen fixation
Flowering
Fruiting & fruit quality
Maturation
Root growth
Tissue strength
Stunting
Thin stems
Bluish-green leaves
Delayed maturity
Sparse flowering
Poor seed quality
• Similarly to nitrogen deficiency, the older
leaves are often first affected
Potassium (K).
• Necessary for production and translocation of carbohydrates.
• Produces plumper seeds.
• Controls water intake and respiration.
• Stiffens straw and stalks.
• P deficiency is often difficult to diagnose,
as visual changes are subtle
The
Problem
in Soil
Fertility
THEPhosphorus
PHOSPHORUS
PROBLEM
IN SOIL
FERTILITY
1. The total P content of soils is low.
200-2000 kg/ha in uppermost 15 cm (topsoil)
2. Phosphorus compounds found in soils are often
highly insoluble (it does not occur as a free element, but is
bound in phosphates)
3. When soluble sources are added (fertilizers and manure)
they often become fixed into insoluble compounds
•
10-15% of P added is taken up by crop in year of application
•
Overfertilization for decades has led to saturation of the
P-fixation capacity (large P reserves in N. American soils)
•
In contrast, P deficiency is a serious problem in sub-Saharan
Africa (removal repeatedly has exceeded addition)
http://www.efma.org/EPUB/easnet.dll/ExecReq/Page?eas:template_im=000BC2&eas:dat_im=000C23
N, P and K Fertilizer Use in USA
IMPACT OF PHOSPHORUS ON ENVIRONMENTAL QUALITY
1. P deficiency: Land degradation
Little P is lost in natural ecosystems as P cycles between
living biomass and soils
Once cleared for agriculture:
(i) Soil erosion loss
(ii) Biomass removal
•
P-supplying capacity decreases, even if total P is sufficient
•
Nodulation is affected by P-deficiency, thereby promoting
N-deficiency
Most problematic in most highly weathered soils
•
Warm, moist environments of the tropics
•
Oxisols & Andisols
•
Low availability of P when in association with Fe & Al
•
Lots of P needs to be applied to Andisols (Fig 14.20)
Figure 14.1
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Combined P & N deficiency limits biomass and promotes
further erosion
WATER QUALITY DEGRADATION DUE TO EXCESS P (& N)
Point sources
Sewage outflows (phosphates in soaps)
Industries
Non-point sources
Runoff water
Eroded sediment from soils in affected watershed
“Too much of a good thing”
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ECONOMIC AND POTENTIALLY ECONOMIC
PHOSPHATE DEPOSITS OF THE WORLD
THE PHOSPHORUS CYCLE
Phosphorus in the soil solution
• Very low concentrations (0.001 to 1 mg/L)
• Roots absorb phosphate ions, HPO42- (alkaline soils)
and H2PO4- (acid soils)
Uptake by Roots
•
•
•
•
•
•
Slow diffusion of phosphate ions to root surfaces
Mychorrizal hyphae extend outward several cm from root surface
P can then be incorporated into plant tissues (Fig 14.9)
Soil P replenished by plant residues, leaf litter, and animal waste
Soil microorganisms can temporarily incorporate P into their cells
Some soil P gets tied up in organic matter (storage & future release)
Available P seldom exceeds 0.01% of
total soil phosphorus
Forms of Soil Phosphorus
PHOSPHORUS REMOVAL FROM PONDS & SLOUGHS
Organic phosphorus
Calcium-bound phosphorus (alkaline soils)
Iron-bound phosphorus (acid soils)
Aluminium-bound phosphorus (acid soils)
A small amount of phosphorus is removed via
uptake through the plants’ roots.
•
•
•
•
More phosphorus settles to the bottom of
ponds and binds to the clay soil.
Low solubility – not readily available for plant uptake
P is slowly released from each of these types of compounds
Leaching loss is low, but can play a role in eutrophication
Unlike N, P is not generally lost in a gaseous form
Gains and Losses
• Losses from plant removal, erosion of P-containing soil particles and
dissolved P in surface runoff water
• Gains from atmospheric dust are very limited, but a balance is
established in most natural ecosystems
Leaching of P after saturation of fixed pool
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POTASSIUM
Soil Solution K:
• Immediately available to plants.
• The nutrient 3rd most-likely to limit productivity
• Present in soils as K+ ion (not in structures of organic compounds)
• Soil cation exchange and mineral weathering dominate its
exchange and availability (as opposed to microbiological
processes)
• Causes no off-site environmental problems
• Amount varies with fertiliser application, weathering and cropping history
• Generally not enough to meet the requirements of the cropÆ fertilizer needed
Exchangeable K:
• K+ retained by negatively charged exchange sites on OM and clay minerals.
Readily available: moves backwards and forwards at ok CEC
Slowly exchangeable K:
• Igneous rocks are a good source – alkaline soils keep it.
• Activates certain enzymes.
• Regulates stomatal opening
• Helps achieve a balance between negatively and positively charged
ions within plant cells.
• Regulates turgor pressure, which helps protect plant cells from
disease invasion.
• Promotes winter-hardiness and drought-tolerance
Potassium deficiency
• Potassium adsorption to wedge sites on micaceous (2:1) clays.
Lattice K:
• K found within the lattice structure of clay minerals and is released slowly into
exchangeable forms as these minerals weather.
Leaching:
• Amount of K added in fertilisers or manures exceeds the CEC of the soil,
potassium can be lost by leaching. Æ higher risk on coarse sands.
WORLD POTASSIUM CYCLE
• Leaves yellow at tip (chlorosis) and then die (necrosis)
ÆThe leaves appear burnt at the edges and may
tear, leaving a ragged edge
• White, necrotic spots may appear near leaf edges
• Oldest leaves are most affected
The Potassium Cycle
• High concentrations in micas and feldspars
Æ K between 2:1 crystal layers becomes available
• Returned to soil through leaching from leaves and from plant residue
decomposition
• Some loss by eroded soil particles and leaching
• Replenishment required in most agroecosystems (1/5 of
plant K is typically removed in product). Excess in plants
can cause a dietary imbalance in ruminants
FACTORS AFFECTING K FIXATION
Almost entirely inorganic
1) Nature of soils colloids
2) Wetting and drying
3) Freezing and thawing
4) Presence of excess lime
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FERTILIZER AND MANURE K
READING FOR THURSDAY
The principal K sources are manures or muriate and
sulphate salts.
In animal manures, K not biologically fixed to other
compounds, unlike N and P, and thus is readily available to
plants.
Chapter 15: Calcium Magnesium and Trace Nutrients
(aka Micronutrients)
Common fertilisers use the muriate (chloride) and sulphate
salts of P. Manure and fertiliser potassium contribute to
potassium in soil solution.
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