MANAGING SOIL ACIDITY IN THE SHORT AND LONG TERMS. Keith Helyar NSW Agriculture, Wagga Wagga Agricultural Institute, Australia

Transcription

1 MANAGING SOIL ACIDITY IN THE SHORT AND LONG TERMS Keith Helyar NSW Agriculture, Wagga Wagga Agricultural Institute, Australia

2 The objectives of research on farming systems Improved productivity and profitability the short-term imperative. lime, tolerance, enterprises that are currently profitable. to beat the cost-price squeeze. A sustainable production system the long-term necessity. the demand for sustainability constrains the scope of profitable systems that can be used.

3 Acid production processes Understanding the processes Effects of the acid reactions and transport The implications for management

4 Understanding acidification processes: the biological cycling of C and N C CYCLE Association/dissociation of added organic acids RCOOH = RCOO - + H + Oxidation of organic anions RCOO - + H + = RCOOH = CO 2 + H 2 O Synthesis organic acids and dissociation C 6 H 12 O 6 = RCOOH = RCOO - + H +

5 N CYCLE Ammonification RNH 2 + H + = NH 4 + Nitrification NH 4 + = NO H + Denitrification NO H + = N 2, N 2 O Excretion of H + or OH -, (HCO 3- ) H + excreted if cation uptake exceeds anion uptake OH -, (HCO 3- ) excreted if anion uptake exceeds cation uptake

6 Plant processes that indirectly affect soil acidity The reduction of nitrate (plants, microorganisms) NO (RCOOH) + 4H 2 = NH H 2 O + RCOO - Ammonium metabolism NH C 6 H 12 O 6 = RNH 2 + H + Production of organic acids in plants (TCA cycle) (CH 2O) n O2 CH3(CH 2O) n3 COOH H2O CO2

7 Review of C and N cycle processes

8 Leaching, H + addition from carbonic acid and weathering reactions If CO 2 soil air > CO 2 atmosphere and the ph is in the range 5.5 to 7.4 then net H + addition from CO 2(g) occurs CO H 2 H O H CO 2(g soilair) CO HCO HCO CO H H ph > 5.5

9 Consumption of acid in weathering reactions 2 CaCO 3 2H Ca CO2(g) H2O Illite 2 3 K0.6Mg0.25Al2.3Si 3.5O10(OH) 2 8H 2H2O 0.6K 0.25Mg 2.3Al 3.5H Kaolinite 3 Al2Si 2O5(OH) 4 6H 2Al 2H 4SiO 4 H2O 3 Al(OH) 3 3H Al 3H 2O 4 SiO 0 4

10 Adsorption/desorption H + at ph dependant CEC sites Organic matter RCOO - K (ex) H RCOOH K Clay mineral and oxide edges ( Fe) - Al O K (-Mn) (ex) H ( Fe) Al (-Mn) OH K (sol)

11 k moles leached/ha 210mm Review of leaching and weathering effects Acid or alkaline ions leached in an Al silicate- Al(OH) 3 /CaCO 3 /H 2 O/CO 2(g) system Al 3+ HCO 3-, OH - ph for low rate of weathering due to leaching Soil ph Ca H+ from N & C cycles

12 ph Ca (0-10 cm) Equilibration of ph in a layer for different production systems (Heenan et al.) WW N0 WW N100 WLWL WCWC Year

13 Profile processes Most activity: Uptake of nutrients and return of residues Topsoil Leaching Subsoil ammonification, nitrification, residues, most organic matter, etc. exponential trends Subsoil: water and nitrate uptake, uptake leached nutrients, accumulation leached elements.

14 Depth (cm) Exponential functions C cycle processes (% of organic anions oxidised etc./cm) OAadd & oxidised OAacc in soil OM Lime surface Lime mixed 0-10 cm

15 Depth (cm) Exponential functions for N cycle processes (% of nitrification etc./cm) Ammonification Nitrification 60 Denitrification 80 NH3g volatilisation 100

16 Depth (cm) Exponential functions for ion uptake (% of nitrate uptake etc./cm) Percent Roots NO3 uptake 40 NH4 uptake H2PO4- uptake SO42- uptake Uptk other Nut. Prop RL

17 Mean depth of layer (cm) Net acid addition all acids and lime (H + +ive, OH - -ive, kmol/ha.cm/year)

18 Soil depth (cm) 0 Steady-state ph profile lowest ph 5.0 Steady state ph

19 Effects of management on the ph profile NO 3 absorption profile i) Same as nitrification ii) Deeper than root system Half way between i) and ii) NO 3 leached 0 5 (5) 0.1 kg CaCO 3 equiv. /ha/year Lime required kg/ha/year (187) (194) 84 Depth min. ph (70 90) ph 0 10 cm (6.9) cm (6.5) cm (5.0) cm (5.6) cm (6.2) 5.0

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21 Yellow solodic / Sodosol

22 Solodic soil MASTER expt (Li et al., 2000) Depth ph ECEC %Al %Mg %Na %Ca

23 Soil ph Ca at 0-10 cm depth Soil ph Ca at 0-cm depth a) Annual systems AP+ AP- AP/C+ AP/C-

24 Soil depth (cm) MASTER ph profiles 1999, phalaris/clover Last lime -0.5 yrs Last lime -5.5 yrs PP-

25 %Al (exchangeable cations) Soil ph Ca at cm depth Subsoil acidity amelioration a) Soil ph Ca and %Al at cm depth ph = x, r 2 = %Al ph

26 %Al (exchangeable cations) Limits of plant tolerance to Al b) %Al in relation to plant tolerance Higher %Al reduce yield of Highly tolerant Tolerant 10 Sensitive 5 Highly sensitive

27 Components of 450 kg lime requirement (kg CaCO 3 /ha/year) preliminary estimates N cycle 14 C cycle 78 Move Sub Total 242 Missing 208 Undissolved lime, clay formation, movement below - 20 cm, underestimate NO 3 leaching

28 Effect of type of vegetation on soil development Eucalyptus vegetation - low nutrient recycling capacity Pastures or trees with high nutrient recycling capacity Topsoil Subsoil Net acid addition Leaching Net alkali addition Leaching restores balance exaggerates problem Result Solodic soil, acid surface, heavy clay subsoil Gradational well structured soil Net acid addition Leaching Net acid addition

29 Mg exch (cmol [+]/kg) Exchangeable Mg at 0-10 cm PP- PP+ AP- AP+

30 Have eucalypt woodlands been responsible for the widespread occurrence of duplex soils in Australia (excess acid addition to topsoil, excess alkali to the subsoil)?

31 Soil depth (cm) MASTER ph profiles 1999, phalaris/clover Last lime -0.5 yrs Last lime -5.5 yrs PP-

32 Conclusions Base management on an understanding of the processes involved Key points: is a measure of acid addition from N nitrate leaching cycle processes cultural practices, crops and pastures to minimise leaching of nitrate acid sub-surface layers develop if: nitrate is leached well below the nitrification zone before it is absorbed by the plant - continuous plant absorption, nitrification inhibition. most plants grown have low cation contents - change the species mix or use extra lime.

33 Further: The weathering rate of soil minerals is lowest if the soil solution ph is in the 5.0 to 6.0 range. Lime to at least this ph in the surface 10 cm A higher ph in the surface may be needed to neutralise sub-surface acidity The acidity index tables for fertiliser types need to be revised to account for modern understanding of acidification processes.

34 For your environment: You are the experts who understand your production systems. Study the nutrient recycling and acidification processes that are occurring. Solve the short term economic problem of stopping acidification. Design production systems so slow degradation processes, that get discounted out of contention in economic analyses, are not occurring. Encourage processes that slowly improve the soil - make good soils out of bad soils.

35 Acidity index values (kg CaCO 3 /kg N applied) Fertiliser No NO3 leached 100% NO3 leached Ammonium sulphate Biol. Fixed N, NH3 gas, Aqua ammonia, Ammonium nitrate, Urea Na, Ca and K nitrate Diammonium phosphate Monoammonium phosphate

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38 Example of solodic soil at the MASTER experiment, Book Book (Li et al., 2000) Depth Exchangeable cations (cmol [+]/kg) ECECB (cm) ph Al Mn Mg Na Ca K ECEC %Al %Mg %Na %Ca The %Al is the percentage exchangeable Al of ECEC. ECECB is the percentage exchangeable cations of mono- and divalent ECEC, excluding exchangeable Al.

39 Kraznozem/Oxisol