Soil Fertility and Nutrient Bioavailability Sponsored by the DEST program China Higher Education Strategic Initiatives © The University of Adelaide

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 NaKMgCaFeSP grassland/arable (24 soils) woodland (5 soils) 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 leachingerosion 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 (N 2 ); very inert (unreactive) –soil solution: anion and cation (NO 3 - and NH 4 + ); 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 (H 2 PO 4 - /HPO 4 2-,depends on pH); insoluble; high buffering capacity

Global terrestrial N cycling: estimates of pools and transfers (transfers in 10 6 tonnes) atmosphere (mainly N 2 ) 39 x10 15 tonnes N 2 fixation biological 139 non-biological 50 ‘nitrate’ 32 rivers and then oceans denitrification ~150 Nitrate leaching plants, animals, microorganisms 1.3 x tonnes, soil, peat and litter 30 x tonnes

N is dominated by biological processes erosion soil solution stable inorganic labile inorganic microbes plants stable organic labile organic leaching (NO 3 - ) crop atmosphere N 2 fixation soil

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 NH 4 organic N NH 4 ammonifying organisms NO 3 nitrifying organisms NO 2 /NO/N 2 dentrifying organisms atmosphere nitrogen fixation soil solution

N availability NO 3 - and NH 4 + 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 N 2 (fertilizer applications)

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 leachingerosion crop atmosphere

P cycling Plants can use soluble inorganic P P is scarce and insoluble P is present in soil solution as H 2 PO 4 - and HPO 4 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 stable inorganic labile inorganic microbes plants stable organic labile organic leachingerosion crop atmosphere

Main forms of P in different pools organic P P in organisms (biomass) organic P sugar phosphates nucleic acids phytate (~ 80%) inorganic P soil solution (H 2 PO 4 - /HPO 4 2- ) 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 (Al 3+ ) 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 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, NH 4 + uptake<replacement  accumulation at root surface SO 4 2-

Replacement of nutrients in soil solution replacement in solution NP 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 NO 3 - > 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 (m 2 s -1 ) ionwet soil -10 kPa dry soil kPa NO 3 - (low buffering capacity) K+K H 2 PO 4 - (high buffering capacity) 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 surfaceNP diffusionrapidslow 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 diameter (µm) > <2 contribution to total porosity (%) Mg m 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 The ecology of soil fungi. Chapman Hall. Killham, K Soil Ecology. Cambridge University Press, Cambridge, UK. Oades, J. M The role of biology in the formation, stabilization and degradation of soil structure. Geoderma. 56: Paul, E.A. and Clark F.E Soil microbiology and biochemistry.2nd ed. Academic Press, San Diego, USA. Tinker, P.B. and Nye, P.H Solute Movement in the Rhizosphere. Oxford University Press, Oxford. Comerford, N. B Soil phosphorus bioavailabiilty. pp in Phosphorus in Plant Biology Vol 19, J. P. Lynch, J. Deikman J. Eds. American Society of Plant Physiologists, Rockville, Maryland, USA.