Soil Fertility and Nutrient Bioavailability

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Soil Fertility and Nutrient Bioavailability

characteristics of soil that enables it to:• provide nutrients• in adequate amounts• in appropriate balance• for growth of particular plant(s)

depends on the:• form of the nutrient in soil• processes of nutrient release to soil solution• movement of nutrients to the absorbing surfaces

(plant/mycorrhiza)• mechanism of absorption by roots

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

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

stableinorganic

labileinorganic

microbes

plants

stableorganic

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– large inorganic pool in atmosphere (N2); very inert (unreactive)– soil solution: anion and cation (NO3

- and NH4+); very soluble; low

buffering capacity

Potassium (K)– inorganic pools in soil and plants– soil solution: cation (K+); soluble– rapid exchange between pools– atmospheric pool negligible; organic pools negligible– organic pool negligible

Phosphorus (P) – both inorganic and organic pools in soil– atmospheric pool negligible– soil solution: anions (H2PO4

-/HPO42-,depends on pH); insoluble; high

buffering capacity

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

N is dominated by biological processes

erosion

soil solution

stableinorganic

labileinorganic

microbes

plants

stableorganic

labile organic

leaching (NO3-)

crop atmosphere

N2 fixationsoil

Main forms of organic N in soil

• amino acids – used quickly by soil microorganism

• proteins– variable availablity to soil micro-organisms

• complex polymers– chitin, lignin– not very available

decomposition of organic N produces NH4

organic N

NH4

ammonifying organisms

NO3

nitrifying organisms

NO2/NO/N2

dentrifying organisms

atmosphere

nitrogen fixation

soil solution

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 depends on soil moisture and

temperature and C:N ratio of organic inputs

• biological fixation of gaseous N2

• (fertilizer applications)

K+ pools and pathways 1

• almost entirely inorganicexcept:– 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

stableinorganic

labileinorganic

microbes

plants

stableorganic

labile organic

leaching erosion

crop atmosphere

P cycling

• Plants can use soluble inorganic P

• P is scarce and insoluble

• P is present in soil solution as H2PO4- and HPO4

2-

• P cycling involves both inorganic and organic pools

• P often limits productivity of ecosystems and crops

• fertiliser applications are often required to attain good crop yields

• 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

stableinorganic

labileinorganic

microbes

plants

stableorganic

labile organic

leaching erosion

crop atmosphere

Main forms of P in different pools

organic PP in organisms (biomass)

organic P

sugar phosphates

nucleic acids

phytate (~ 80%)

inorganic P

soil solution (H2PO4

-/HPO42-)

insoluble

Ca, Al and Fe phosphates

Effects of soil pH on soil fertility 1

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 2

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 (later 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 SO4

2-

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 animals

arrangement 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

pore

dia

met

er (

µm

) >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

• Nutrients reach roots by– root interception– mass flow– diffusion

• Nutrient availability and movement in soil are influenced by– pH– water content– pore-size distribution– organic matter

Useful references

Griffin, D.M. 1972. The ecology of soil fungi. Chapman Hall.

Killham, K. 1994. Soil Ecology. Cambridge University Press, Cambridge, UK.

Oades, J. M. 1993 The role of biology in the formation, stabilization and degradation of soil structure. Geoderma. 56: 377-400.

Paul, E.A. and Clark F.E. 1996. Soil microbiology and biochemistry.2nd ed.

Academic Press, San Diego, USA.

Tinker, P.B. and Nye, P.H. 2000. Solute Movement in the Rhizosphere. Oxford University Press, Oxford.

Comerford, N. B. 1998. Soil phosphorus bioavailabiilty. pp 136-147 in Phosphorus in Plant Biology Vol 19, J. P. Lynch, J. Deikman J. Eds. American Society of Plant Physiologists, Rockville, Maryland, USA.