Soil Fertility and Nutrient
Bioavailability
Chapter 8
Soil Fertility
Soil fertility is the characteristics of soil that
enables it to:
? Provide nutrients
? in adequate amounts
? in adequate time
? in appropriate balance
? for growth of particular plant(s)
? Depends on the:
? Forms of the nutrient in soil
? Processes of nutrient release to soil
solution (weathering,decay,dissolving,
and desorption)
? Movement of nutrients to the absorbing
surfaces (of plant/mycorrhiza)
? Mechanism of absorption by roots
Soil Fertility
Topics
? Cycling of different elements
? Soil processes and the supply of nutrients
– relative importance of organic and inorganic pools/biological
activity
– effects of pH on nutrient availability
– effects of soil moisture on nutrient availability and
movement
? Movement of nutrients in soil to roots
? Importance of soil structure in soil fertility
? Bioavailability of soil nutrients
Plants absorb nutrients from the soil solution
Concentrations in soil solution (μM) at sites in UK
Na K Mg Ca Fe S P
Grassland/arable
(24 soils)
465 390 135 2120 3.4 327 64
Woodland
(5 soils)
335 384 104 592 24.1 398 17
from Tinker and Nye,2000
values for P can be much lower than those quoted < 10 μm
Major nutrient pools and pathways of nutrient transfers
soil
solution
stable
inorganic
labile
inorganic
microbes
plants
stable
organic
labile
organic
leaching erosion
crop atmosphere
The importance of different pools and
transfers varies between nutrients
? Relative importance of global pools
? Significance of inorganic and organic pools and
biological cycling
? Solubility of inorganic forms of nutrients
? Buffering capacity
? Movement of nutrients in soil solution
? Replenishment of nutrients in soil solution
? Concentrations of nutrients in solution close to roots
Importance of pools varies with nutrients
Nitrogen (N)
– Almost entirely in organic form in soil(500-
5000kg/ha); exchange NH4+ 0-400kg/ha
– Soil solution,anion and cation (NO3-,0.1-
5mM) and NH4+,0.01-0.5mM); very soluble;
low buffering capacity
– Large inorganic pool in atmosphere (N2); very
inert (unreactive)
? Potassium (K)
? Inorganic pools in soil and plants
? Crystal-lattice K,10-10,000kg/ha
? Soil solution,cation (K+),0.05-2mM; soluble
? Exchange K+,10-500kg/ha
? Atmospheric pool negligible
Importance of pools varies with nutrients
? Phosphorus (P)
? Both inorganic and organic pools in soil
? Organic P,5-100kg P/ha
? Absorbed soil phosphate (inorganic P),20-
500kg
? soil solution,anions (H2PO4-/HPO42-,depends
on pH); 0.1-2μM
? High buffering capacity
Importance of pools varies with nutrients
Global terrestrial N cycling,estimates of pools and transfers
(transfers in 106 tonnes)
atmosphere (mainly N2) 39 x1015 tonnes
N2 fixation
biological 139
non-biological 50
‘ nitrate’ 32
rivers and then oceans
denitrification
~150
Nitrate leaching 18-33
plants,animals,microorganisms 1.3 x 1010 tonnes,
soil,peat and litter 30 x 1010 tonnes
Main forms of organic N in soil
? amino acids
– used quickly by soil microorganism
? proteins
– variable availability to soil micro-organisms
? complex polymers
– Chitin(几丁质 ),lignin(木质素 )
– not very available
N is dominated by biological processes
erosion
soil
solution
stable
inorganic
labile
inorganic
microbes
plants
stable
organic
labile
organic
leaching (NO3-)
crop atmosphere
N2 fixation
soil
N availability
? NO3- and NH4+ are available to plants
? Low buffering capacity in soil
? Biological cycling of N between organic matter and
inorganic forms in soil solution
? Mineralisation(矿化作用 ) depends on soil moisture
and temperature and C:N ratio of organic inputs
? Biological fixation of gaseous N2
? (fertilizer applications)
Decomposition of organic N produces NH4
organic
N
NH4
Ammonifying
organisms
NO3
Nitrifying
organisms
NO2/NO/N2
Dentrifying
organisms
atmosphere
nitrogen
fixation
soil solution
K+ pools and pathways 1
? Almost entirely inorganic
except:
– organic matter holds K+ because it is negatively
charged
– living organisms act as sink because they need K+
? Redistribution between available inorganic pools
is rapid so solution K+ is replenished rapidly
? K released only slowly during weathering of
minerals
K+ pools and pathways 2
soil
solution
stable
inorganic
labile
inorganic
microbes
plants
stable
organic
labile
organic
leaching erosion
crop atmosphere
P cycling
? P is scarce and most of P is insoluble
? P is present in soil solution as H2PO4- and
HPO42-
? P cycling involves both inorganic and organic
pools
? Replacement of P lost to oceans is very,very
slow
? Accessible deposits of phosphate rocks will
run out in ~ 80 years;
P pools and pathways
soil
solution
stable
inorganic
labile
inorganic
microbes
plants
stable
organic
labile
organic
leaching erosion
crop atmosphere
Chemical availability of soil nutrients
? Soils differ widely in their fertility.
? So understanding the soil fertility or the capability of
soils to produce good crop growth has therefore been of
interesting for a long time.
? Most plant nutrients come from soils,and total amount
of plants nutrients in soil are not of primary importance,
but rather the content of soluble and easily accessible
nutrients.
? How to determine this available nutrients fraction?
Intensity,quantity and buffer power
? Nutrients availability depends not only on the
nutrient concentration of soil solution at any given
time,but also on the ability of the soil to maintain
the nutrient concentration---buffer,
? This capability of a soil to,buffer” the
nutrient concentration of the soil solution is a
further important factor in nutrient availability.
Nutrients relationships of soil-plant systems
Nutrients
in Soil bulk
Nutrients in
soil solution
Nutrients
in roots
Nutrients
in shoot
The uptake of Nutrients is directly related with the nutrients
concentration in soil solution
The nutrients is related with the nutrients bounded to soil
bulk phase
Intensity and quantity of plant nutrients
? The intensity factor (I) is directly
available and represents by the
concentration of soil solution,
? The nutrients in the soil solution is ready
to be adsorbed by plant root,but they
can’t supply all nutrients required by the
plants,
? Soil solution P,0.5-1.0 kg/ ha,K 10-30 kg/ha
? Plant required about 20 kg P/ha and 100 kg
K/ha
Plant uptake nutrients via different transporter systems
in the high and low concentration of growth medium
Annual average concentration of mineral nutrients in the
soil solution (topsoil0-20cm)of an arable
soil(Luvisol,pH7.7).(Peter,1990 and Marschner,1995)
n u t r i e n t C o n c e n t r a t i o n ( mm o l / m
3
) n u t r i e n t C o n c e n t r a t i o n ( mm o l / m
3
)
K 510 SO 4 - S 590
Ca 1650 PO 4 - P 1, 5
Mg 490 Zn 0, 4 9
NH 4 - N 48 Mn 0, 0 0 2
NO 3 - N 3100
The effects of P concentration in nutrition medium on
the P uptake（ R.S.Russel and R.P.Martin）
P concentration in
the solution
32P content in the
plant
（ P,PPm） ㎎ /plant
5.1 0.573
1.6 0.161
0.51 0.0573
0.16 0.01676
0.051* 0.0048
0.016 0.00137
0.0051 0.000434
0.0016 0.000120
Intensity and quantity of plant nutrients
? The quantity or capacity factor (Q)
represents the amount a potentially
availability nutrients.
? It is usually a much larger labile pool of
easily exchangeable or desorbable
nutrients.
? It represents the main component of the
quantity of available nutrients
Intensity and quantity of plant nutrients
? Apart from the processes of nutrients release from labile
pool,the quantity is related with the nutrient release from
more slowly available sources,such as mineralization of
organic nutrients,witch are pertinent with(相干，相关）
the pH,temperature,aeration and moisture etc,
? The quantity factor is therefore very much dependent on
weather and soil conditions as well as the prevailing climate,
? The root density in soil related to the amount of nutrients
uptake by the roots and the nutrients transformation in the
soil.
土壤溶液中的养分
Nutrients of concentration
in soil solution
贮库（ labile pool）
作物生长过程中释放的养分
Nutrients released during growth
土壤矿物和有机质中的养分
bulk mineral and organic reserves
田间根系体积
field rooting volume
很慢
very slow
慢 slow
快 rapid
强度因素
intensity
容量因素
quantity
? Why is that the field root volume is included in
the quantity?
? The plant’s demand for immobile nutrients,such
as P,is unlikely to be satisfied unless the plant
shortens the travels distance,by extending its
root system.
? So in the P deficient soil,the plant with high root
density could uptake more nutrient than that of
plant with low root density.
Intensity and quantity of plant nutrients
? Buffer power or capacity (Bk) is the
ability of the soil to maintain the nutrient
intensity,Or capability of replenish(补充，补
偿） of solution nutrients
? In quantity terms the buffer capacity is
expressed as the ratio ?Q/ ?I.
? Bk= ?Q/ ?I
? It is very important to nutrients availability.
Buffer power or capacity
Methods of measurement of soil nutrient availability
? Chemical methods
? Sampling
? Extraction
? by chemical extractant and Elctro-ultrafitration
methods
? Tracer techniques:
? E value (exchangeable value)
? L value
Relative of chemical availability of plant nutrients
? The value of the nutrients is dependent
on the extractants
? The relationships of the value and crop
growth and yield should be set up
before using the methods
The means of 15 kinds of soil P availability determined
by different extractants
M e t ho ds E xt r a c t a nt s A va i l a b l e P ( m g/ kg )
O l s e n m e t ho d 0, 5M N a H C O 3 24, 9
M a c hi q i n m e t ho d ( N H 4 ) 2 CO 3 23, 9
Al - A bb a s m e t ho d N a O H + N a 2 C 2 O 4 30, 4
D ou bl e a c i ds m e t ho d H 2 SO 4 + H C I 29, 4
M e h l i c hI I I m e t ho d H A c + N H 4 NO 3 + N H 4 F+ H N O 3 + E D T A 70, 1
S ol t a np ou r m e t ho d NH 4 H C O 3 + D T P A 14, 8
M e ug e pq ko B ( NH 4 ) 2 C 2 O 4 + ( NH 4 ) 2 H C O 4 46, 9
Bioavailability of plant nutrients
? Plants have important effect on the soil nutrient
availability,especially to the immobile nutrient or less
mobile nutrients
? Nutrients availability is not only related to physical-
chemical factors,but also to biological factors,such as
root parameters and microbial activity in soils.
? Although estimation and prediction biological factors
are difficult,it should be remembered that the
determination of the physico-chemical factors can only
give an indication of nutrient availability in the soil-
plant-root system.
Effects of soil pH on soil fertility
Usually indirect due to effects on:
? Nutrient availability
? Toxicities
? Biological activity
? Direct effects of H+ or OH- are only observed at
extreme pH values
pH affects the availability of nutrients in soil
Effects of soil pH on soil fertility
Low pH
? deficiency of K,Mg
? toxicity of Al (Al3+) and Mn
High pH
– deficiency of Fe,Mn
– toxicity of B and Na
P and N most available at moderate pH
– biological activity important for N & P mineralisation is
inhibited at very low and very high pH
– P is immobilised at both high and low pH
Effects of pH on retention of inorganic P in soil
4 5 6 7 8 9
high
med
low
insoluble Fe &
Al phosphates
sorption to clays
and oxides
insoluble Ca
phosphates
pH
P retention in soil P most
available
Biological activity involved replacement and
removal of nutrients from soil solution
? Biologically active soil is important
? Requires organic matter
? Depends on moisture,temperature,aeration,pH
soil solution
fungi
bacteria
immobilisation =
removal
mobilisation =
replacement
Concentration in solution at root surface
Depends on:
? rate of uptake (early lectures)
? rate of replacement in solution and movement to
roots
? uptake>replacement ? depletion at root surface
P,Zn,NH4+
? uptake<replacement ? accumulation at root
surface SO42-
Replacement of nutrients in soil solution
replacement in solution N P
Mineralisation from organic pool +++ ++
Dissolution (inorganic sources) - ++
Desorption (inorganic) - ++
Movement of nutrients to roots
? Root growth towards nutrients
influenced by soil structure and conditions and by nutrient
concentrations
? Nutrient movement through soil
mass flow and diffusion
both are influenced by the physico-chemical
properties of soil and by soil structure
Mass flow of nutrients
? Soil solution (containing dissolved nutrients) moves down
gradients of water potential
? Wet soil ? dry soil
? All nutrients move in the same direction
? Rate of nutrient movement depends on
– concentration in solution - affected by uptake and replacement
– volume of solution - affected by soil moisture and by soil pore sizes
– rate of flow - affected by transpiration,evaporation and drainage
NO3- > K+ > P
Diffusion of nutrients
? Movement in solution but independent of direction
of flow of solution
? Nutrient moves down concentration gradient
? Rate for each nutrient depends on
– concentration gradient – replacement/uptake
– diffusion in soil
– buffering capacity of soil
– tortuosity of pathway in soil
– soil moisture (continuity of water-filled pores)
Ds = rate of diffusion of ions in soil (m2 s-1)
ion wet soil
-10 kPa
dry soil
-1000 kPa
NO3-
(low buffering capacity)
10-9 10-11
K+ 10-11 10-13
H2PO4-
(high buffering capacity)
10-13 10-15
values are much lower than for diffusion in pure water due to,
? tortuosity of pathway
? increased viscosity close to surfaces
? exclusion of ions by surface charge on particles
Processes involved in nutrient replacement
replacement at root surface N P
diffusion rapid slow
mass flow +++ (+)
N has low buffering capacity and high
concentration in soil solution
N is VERY MOBILE (easily gets to roots; easily
leached out of soil)
P has high buffering capacity and low
concentration in soil solution (<10μM)
P is VERY IMMOBILE
Soil structure and pore-size distribution
influence many aspects of fertility
pore-size distribution
? air filled pore space
? water filled pore space
? nutrient movement
? aeration
? biological activity
? accessibility of pores to roots,
microorganisms and animalsarrangement of soil
particles in aggregates
large particles - large pores
small particles - small pores
after Oades,1993
Water and air-filled pore space
wet soil
? larger pores filled with water
? continuity of water-filled pore space
? air-filled porosity lower
? tortuosity lower
dry soil
? small pores filled with water
? low continuity of water filled pore-space (tortuosity higher)
? air-filled porosity higher
from Griffin 1972
Effects of compaction on pore-size distribution
and continuity
Uncompacted soil
Soil compacted by traffic
po
re
dia
me
ter
(μ
m)
>300
100-200
30-100
10-30
3-10
1-3
0.2-1
<2
contribution to total porosity (%)
20 400
1.1 Mg m-3
1.6 Mg m-3
from data of Habib Nadian and Liz Drew
Summary
? Plants absorb nutrients from the soil solution
? There are major differences between N,K and P in
– cycling in the biosphere
– inorganic and organic pools in soil
– processes involved in replenishment of soil solution
? Soil nutrient availability
– Density
– Quantity
– Buffer capacity
– Chemical availability and bioavailability
? Nutrient availability and movement in soil are influenced by
– pH
– water content
– pore-size distribution
– organic matter
Bioavailability
Chapter 8
Soil Fertility
Soil fertility is the characteristics of soil that
enables it to:
? Provide nutrients
? in adequate amounts
? in adequate time
? in appropriate balance
? for growth of particular plant(s)
? Depends on the:
? Forms of the nutrient in soil
? Processes of nutrient release to soil
solution (weathering,decay,dissolving,
and desorption)
? Movement of nutrients to the absorbing
surfaces (of plant/mycorrhiza)
? Mechanism of absorption by roots
Soil Fertility
Topics
? Cycling of different elements
? Soil processes and the supply of nutrients
– relative importance of organic and inorganic pools/biological
activity
– effects of pH on nutrient availability
– effects of soil moisture on nutrient availability and
movement
? Movement of nutrients in soil to roots
? Importance of soil structure in soil fertility
? Bioavailability of soil nutrients
Plants absorb nutrients from the soil solution
Concentrations in soil solution (μM) at sites in UK
Na K Mg Ca Fe S P
Grassland/arable
(24 soils)
465 390 135 2120 3.4 327 64
Woodland
(5 soils)
335 384 104 592 24.1 398 17
from Tinker and Nye,2000
values for P can be much lower than those quoted < 10 μm
Major nutrient pools and pathways of nutrient transfers
soil
solution
stable
inorganic
labile
inorganic
microbes
plants
stable
organic
labile
organic
leaching erosion
crop atmosphere
The importance of different pools and
transfers varies between nutrients
? Relative importance of global pools
? Significance of inorganic and organic pools and
biological cycling
? Solubility of inorganic forms of nutrients
? Buffering capacity
? Movement of nutrients in soil solution
? Replenishment of nutrients in soil solution
? Concentrations of nutrients in solution close to roots
Importance of pools varies with nutrients
Nitrogen (N)
– Almost entirely in organic form in soil(500-
5000kg/ha); exchange NH4+ 0-400kg/ha
– Soil solution,anion and cation (NO3-,0.1-
5mM) and NH4+,0.01-0.5mM); very soluble;
low buffering capacity
– Large inorganic pool in atmosphere (N2); very
inert (unreactive)
? Potassium (K)
? Inorganic pools in soil and plants
? Crystal-lattice K,10-10,000kg/ha
? Soil solution,cation (K+),0.05-2mM; soluble
? Exchange K+,10-500kg/ha
? Atmospheric pool negligible
Importance of pools varies with nutrients
? Phosphorus (P)
? Both inorganic and organic pools in soil
? Organic P,5-100kg P/ha
? Absorbed soil phosphate (inorganic P),20-
500kg
? soil solution,anions (H2PO4-/HPO42-,depends
on pH); 0.1-2μM
? High buffering capacity
Importance of pools varies with nutrients
Global terrestrial N cycling,estimates of pools and transfers
(transfers in 106 tonnes)
atmosphere (mainly N2) 39 x1015 tonnes
N2 fixation
biological 139
non-biological 50
‘ nitrate’ 32
rivers and then oceans
denitrification
~150
Nitrate leaching 18-33
plants,animals,microorganisms 1.3 x 1010 tonnes,
soil,peat and litter 30 x 1010 tonnes
Main forms of organic N in soil
? amino acids
– used quickly by soil microorganism
? proteins
– variable availability to soil micro-organisms
? complex polymers
– Chitin(几丁质 ),lignin(木质素 )
– not very available
N is dominated by biological processes
erosion
soil
solution
stable
inorganic
labile
inorganic
microbes
plants
stable
organic
labile
organic
leaching (NO3-)
crop atmosphere
N2 fixation
soil
N availability
? NO3- and NH4+ are available to plants
? Low buffering capacity in soil
? Biological cycling of N between organic matter and
inorganic forms in soil solution
? Mineralisation(矿化作用 ) depends on soil moisture
and temperature and C:N ratio of organic inputs
? Biological fixation of gaseous N2
? (fertilizer applications)
Decomposition of organic N produces NH4
organic
N
NH4
Ammonifying
organisms
NO3
Nitrifying
organisms
NO2/NO/N2
Dentrifying
organisms
atmosphere
nitrogen
fixation
soil solution
K+ pools and pathways 1
? Almost entirely inorganic
except:
– organic matter holds K+ because it is negatively
charged
– living organisms act as sink because they need K+
? Redistribution between available inorganic pools
is rapid so solution K+ is replenished rapidly
? K released only slowly during weathering of
minerals
K+ pools and pathways 2
soil
solution
stable
inorganic
labile
inorganic
microbes
plants
stable
organic
labile
organic
leaching erosion
crop atmosphere
P cycling
? P is scarce and most of P is insoluble
? P is present in soil solution as H2PO4- and
HPO42-
? P cycling involves both inorganic and organic
pools
? Replacement of P lost to oceans is very,very
slow
? Accessible deposits of phosphate rocks will
run out in ~ 80 years;
P pools and pathways
soil
solution
stable
inorganic
labile
inorganic
microbes
plants
stable
organic
labile
organic
leaching erosion
crop atmosphere
Chemical availability of soil nutrients
? Soils differ widely in their fertility.
? So understanding the soil fertility or the capability of
soils to produce good crop growth has therefore been of
interesting for a long time.
? Most plant nutrients come from soils,and total amount
of plants nutrients in soil are not of primary importance,
but rather the content of soluble and easily accessible
nutrients.
? How to determine this available nutrients fraction?
Intensity,quantity and buffer power
? Nutrients availability depends not only on the
nutrient concentration of soil solution at any given
time,but also on the ability of the soil to maintain
the nutrient concentration---buffer,
? This capability of a soil to,buffer” the
nutrient concentration of the soil solution is a
further important factor in nutrient availability.
Nutrients relationships of soil-plant systems
Nutrients
in Soil bulk
Nutrients in
soil solution
Nutrients
in roots
Nutrients
in shoot
The uptake of Nutrients is directly related with the nutrients
concentration in soil solution
The nutrients is related with the nutrients bounded to soil
bulk phase
Intensity and quantity of plant nutrients
? The intensity factor (I) is directly
available and represents by the
concentration of soil solution,
? The nutrients in the soil solution is ready
to be adsorbed by plant root,but they
can’t supply all nutrients required by the
plants,
? Soil solution P,0.5-1.0 kg/ ha,K 10-30 kg/ha
? Plant required about 20 kg P/ha and 100 kg
K/ha
Plant uptake nutrients via different transporter systems
in the high and low concentration of growth medium
Annual average concentration of mineral nutrients in the
soil solution (topsoil0-20cm)of an arable
soil(Luvisol,pH7.7).(Peter,1990 and Marschner,1995)
n u t r i e n t C o n c e n t r a t i o n ( mm o l / m
3
) n u t r i e n t C o n c e n t r a t i o n ( mm o l / m
3
)
K 510 SO 4 - S 590
Ca 1650 PO 4 - P 1, 5
Mg 490 Zn 0, 4 9
NH 4 - N 48 Mn 0, 0 0 2
NO 3 - N 3100
The effects of P concentration in nutrition medium on
the P uptake（ R.S.Russel and R.P.Martin）
P concentration in
the solution
32P content in the
plant
（ P,PPm） ㎎ /plant
5.1 0.573
1.6 0.161
0.51 0.0573
0.16 0.01676
0.051* 0.0048
0.016 0.00137
0.0051 0.000434
0.0016 0.000120
Intensity and quantity of plant nutrients
? The quantity or capacity factor (Q)
represents the amount a potentially
availability nutrients.
? It is usually a much larger labile pool of
easily exchangeable or desorbable
nutrients.
? It represents the main component of the
quantity of available nutrients
Intensity and quantity of plant nutrients
? Apart from the processes of nutrients release from labile
pool,the quantity is related with the nutrient release from
more slowly available sources,such as mineralization of
organic nutrients,witch are pertinent with(相干，相关）
the pH,temperature,aeration and moisture etc,
? The quantity factor is therefore very much dependent on
weather and soil conditions as well as the prevailing climate,
? The root density in soil related to the amount of nutrients
uptake by the roots and the nutrients transformation in the
soil.
土壤溶液中的养分
Nutrients of concentration
in soil solution
贮库（ labile pool）
作物生长过程中释放的养分
Nutrients released during growth
土壤矿物和有机质中的养分
bulk mineral and organic reserves
田间根系体积
field rooting volume
很慢
very slow
慢 slow
快 rapid
强度因素
intensity
容量因素
quantity
? Why is that the field root volume is included in
the quantity?
? The plant’s demand for immobile nutrients,such
as P,is unlikely to be satisfied unless the plant
shortens the travels distance,by extending its
root system.
? So in the P deficient soil,the plant with high root
density could uptake more nutrient than that of
plant with low root density.
Intensity and quantity of plant nutrients
? Buffer power or capacity (Bk) is the
ability of the soil to maintain the nutrient
intensity,Or capability of replenish(补充，补
偿） of solution nutrients
? In quantity terms the buffer capacity is
expressed as the ratio ?Q/ ?I.
? Bk= ?Q/ ?I
? It is very important to nutrients availability.
Buffer power or capacity
Methods of measurement of soil nutrient availability
? Chemical methods
? Sampling
? Extraction
? by chemical extractant and Elctro-ultrafitration
methods
? Tracer techniques:
? E value (exchangeable value)
? L value
Relative of chemical availability of plant nutrients
? The value of the nutrients is dependent
on the extractants
? The relationships of the value and crop
growth and yield should be set up
before using the methods
The means of 15 kinds of soil P availability determined
by different extractants
M e t ho ds E xt r a c t a nt s A va i l a b l e P ( m g/ kg )
O l s e n m e t ho d 0, 5M N a H C O 3 24, 9
M a c hi q i n m e t ho d ( N H 4 ) 2 CO 3 23, 9
Al - A bb a s m e t ho d N a O H + N a 2 C 2 O 4 30, 4
D ou bl e a c i ds m e t ho d H 2 SO 4 + H C I 29, 4
M e h l i c hI I I m e t ho d H A c + N H 4 NO 3 + N H 4 F+ H N O 3 + E D T A 70, 1
S ol t a np ou r m e t ho d NH 4 H C O 3 + D T P A 14, 8
M e ug e pq ko B ( NH 4 ) 2 C 2 O 4 + ( NH 4 ) 2 H C O 4 46, 9
Bioavailability of plant nutrients
? Plants have important effect on the soil nutrient
availability,especially to the immobile nutrient or less
mobile nutrients
? Nutrients availability is not only related to physical-
chemical factors,but also to biological factors,such as
root parameters and microbial activity in soils.
? Although estimation and prediction biological factors
are difficult,it should be remembered that the
determination of the physico-chemical factors can only
give an indication of nutrient availability in the soil-
plant-root system.
Effects of soil pH on soil fertility
Usually indirect due to effects on:
? Nutrient availability
? Toxicities
? Biological activity
? Direct effects of H+ or OH- are only observed at
extreme pH values
pH affects the availability of nutrients in soil
Effects of soil pH on soil fertility
Low pH
? deficiency of K,Mg
? toxicity of Al (Al3+) and Mn
High pH
– deficiency of Fe,Mn
– toxicity of B and Na
P and N most available at moderate pH
– biological activity important for N & P mineralisation is
inhibited at very low and very high pH
– P is immobilised at both high and low pH
Effects of pH on retention of inorganic P in soil
4 5 6 7 8 9
high
med
low
insoluble Fe &
Al phosphates
sorption to clays
and oxides
insoluble Ca
phosphates
pH
P retention in soil P most
available
Biological activity involved replacement and
removal of nutrients from soil solution
? Biologically active soil is important
? Requires organic matter
? Depends on moisture,temperature,aeration,pH
soil solution
fungi
bacteria
immobilisation =
removal
mobilisation =
replacement
Concentration in solution at root surface
Depends on:
? rate of uptake (early lectures)
? rate of replacement in solution and movement to
roots
? uptake>replacement ? depletion at root surface
P,Zn,NH4+
? uptake<replacement ? accumulation at root
surface SO42-
Replacement of nutrients in soil solution
replacement in solution N P
Mineralisation from organic pool +++ ++
Dissolution (inorganic sources) - ++
Desorption (inorganic) - ++
Movement of nutrients to roots
? Root growth towards nutrients
influenced by soil structure and conditions and by nutrient
concentrations
? Nutrient movement through soil
mass flow and diffusion
both are influenced by the physico-chemical
properties of soil and by soil structure
Mass flow of nutrients
? Soil solution (containing dissolved nutrients) moves down
gradients of water potential
? Wet soil ? dry soil
? All nutrients move in the same direction
? Rate of nutrient movement depends on
– concentration in solution - affected by uptake and replacement
– volume of solution - affected by soil moisture and by soil pore sizes
– rate of flow - affected by transpiration,evaporation and drainage
NO3- > K+ > P
Diffusion of nutrients
? Movement in solution but independent of direction
of flow of solution
? Nutrient moves down concentration gradient
? Rate for each nutrient depends on
– concentration gradient – replacement/uptake
– diffusion in soil
– buffering capacity of soil
– tortuosity of pathway in soil
– soil moisture (continuity of water-filled pores)
Ds = rate of diffusion of ions in soil (m2 s-1)
ion wet soil
-10 kPa
dry soil
-1000 kPa
NO3-
(low buffering capacity)
10-9 10-11
K+ 10-11 10-13
H2PO4-
(high buffering capacity)
10-13 10-15
values are much lower than for diffusion in pure water due to,
? tortuosity of pathway
? increased viscosity close to surfaces
? exclusion of ions by surface charge on particles
Processes involved in nutrient replacement
replacement at root surface N P
diffusion rapid slow
mass flow +++ (+)
N has low buffering capacity and high
concentration in soil solution
N is VERY MOBILE (easily gets to roots; easily
leached out of soil)
P has high buffering capacity and low
concentration in soil solution (<10μM)
P is VERY IMMOBILE
Soil structure and pore-size distribution
influence many aspects of fertility
pore-size distribution
? air filled pore space
? water filled pore space
? nutrient movement
? aeration
? biological activity
? accessibility of pores to roots,
microorganisms and animalsarrangement of soil
particles in aggregates
large particles - large pores
small particles - small pores
after Oades,1993
Water and air-filled pore space
wet soil
? larger pores filled with water
? continuity of water-filled pore space
? air-filled porosity lower
? tortuosity lower
dry soil
? small pores filled with water
? low continuity of water filled pore-space (tortuosity higher)
? air-filled porosity higher
from Griffin 1972
Effects of compaction on pore-size distribution
and continuity
Uncompacted soil
Soil compacted by traffic
po
re
dia
me
ter
(μ
m)
>300
100-200
30-100
10-30
3-10
1-3
0.2-1
<2
contribution to total porosity (%)
20 400
1.1 Mg m-3
1.6 Mg m-3
from data of Habib Nadian and Liz Drew
Summary
? Plants absorb nutrients from the soil solution
? There are major differences between N,K and P in
– cycling in the biosphere
– inorganic and organic pools in soil
– processes involved in replenishment of soil solution
? Soil nutrient availability
– Density
– Quantity
– Buffer capacity
– Chemical availability and bioavailability
? Nutrient availability and movement in soil are influenced by
– pH
– water content
– pore-size distribution
– organic matter
