Soil ph: Review of Concepts

Transcription

1 Soils and Water, Spring 008 Soil ph: Review of Concepts Acid: substance that can donate a proton Base: substance that can accept a proton HA H A HA and A - are called conjugate acid-base pairs. The strength of acid-base pairs is evaluated through dissociation constants. [ HO ][ A ] HA HO HO A ; KHA = HA Dissociation Constant for Weak Acids Lecture 6, Soil ph and Salinity 1

2 Soils and Water, Spring 008 Definition of ph General definition: H O H OH = [ H ][ OH ] 14 = log[ H ] log[ OH ] 14 = ph poh ph = log[ H ] Where p refers to the negative logarithm. NOTE that a low ph implies a greater concentration of H than a high ph. Figure 9.11 Relationships existing in mineral soils between ph and the availability of plant nutrients. The relationship with activity of certain microorganisms is also indicated. The width of the bands indicates the relative microbial activity or nutrient availability. The jagged lines between the P band and the bands for Ca, Al, and Fe represent the effect of these metals in restraining the availability of P. When the correlations are considered as a whole, a ph range of about 5.5 to perhaps 7.0 seems to be best to promote the availability of plant nutrients. In short, if the soil ph is suitably adjusted for phosphorus, the other plant nutrients, if present in adequate amounts, will be satisfactorily available in most cases. Conceptual Model of Soil H Budget Atmospheric pool: H acid substances Output of acid gases: N, NH, NO X, CH 4,... Wet Deposition: H, NH Soil surface 4 Cation uptake H Sources Oxidation reactions Mineralization of cations Reverse weathering Dissociation of CO and organic acids Dry Deposition: CO, NH,NO X,SO 4 -. Reduction reactions Mineralization of anions Anion uptake SEE EXAMPLES IN TABLE 9.1 H Sinks Weathering of oxygen-rich metals Association of CO and organic acids Living dead biomass Export of H in the percolation water Lecture 6, Soil ph and Salinity

3 Soils and Water, Spring 008 The Role of CO in Soils Respiration (roots and microorganisms): Organic molecule from photosynthesis CH O O CO H O CO H O H CO H CO H HCO Oxygen is needed to carry out this reaction (aerobic systems). In the absence of O other reactions take place. The CO -H O System CO HO HCO HCO HCO H HCO CO H CO Ca CaCO log K a = 1.46 log K a = 6.5 log K a = 10. CO H O Ca CaCO H Lecture 6, Soil ph and Salinity

4 Soils and Water, Spring 008 The ph of the CO -H O System 50 ph = 6. ph = 10. Source: Bohn, McNeal, and O Connor Soil Chemistry Henry s Law CO (sol.) = k P[ CO(gas) ] High concentration of CO (gas): flooded soil Atmospheric concentration of CO (gas): dry soil surface Source: Hill and Petrucci (1999). General Chemistry The Soil System The soil is a mix of acid-base pairs with different affinity for the H. Consider addition of a strong acid to a calcareous clayey soil: ph 7 CaCO( s ) H A Ca HCO A Clay _ Ca H Clay _ H Ca ph vs. [H]: Non-Buffered Soil is a buffered system 4 ph moles of H added moles of H added Lecture 6, Soil ph and Salinity 4

5 Soils and Water, Spring 008 Figure 9.5 Buffering of soils against changes in ph when acid (HSO4) or base (CaCO) is added. A moderately buffered soil (B) and a strongly buffered soil (C) are compared to unbuffered water (A). Most soils are strongly buffered at low ph by the hydrolysis and precipitation of aluminum compounds and at high ph by the precipitation and dissolution of calcium carbonate. Most of the buffering at intermediate ph levels (ph 4.5 to 7.5) is provided by cation exchange and protonation or deprotonation (gain or loss of H ions) of ph-dependent exchange sites on clay and humus colloids. The well-buffered soil (C) would have a higher amount of organic matter and/or highly charged clay than the moderately buffered soil (B). [Curves based on data from Magdoff and Bartlett (1985) and Lumbanraja and Evangelou (1991)] Aluminum as an Acidic Cation ph = 4.7 ~ 6.5 Source: Bohn, McNeal, and O Connor Soil Chemistry Figure 9.6 Equilibrium relationship among residual, salt-replaceable (ex-changeable), and soil solution (active) acidity in a soil with organic and mineral colloids. Note that the adsorbed (ex-changeable) and residual (bound) ions are much more numerous than those in the soil solution, even when only a small portion of the ions associated with the colloids is shown. Most of the bound aluminum is in the form of Al(OH)xy ions that are held tightly on the surfaces of the clay or complexed with the humus; relatively few Al(OH)xy ions are exchangeable. Remember that the aluminum ions, by hydrolysis, also supply H ions in the soil solution. It is obvious that neutralizing only the hydrogen and aluminum ions in the soil solution will be of little consequence. They will be quickly re-placed by ions associated with the colloid. The soil, therefore, demonstrates high buffering capacity. Lecture 6, Soil ph and Salinity 5

6 Soils and Water, Spring 008 Relative Size of Acidity Pools Consider a soil containing 10% of montmorillonite clay Assume a formula for montmorillonite clay (1,118 g ): KAl 7 Si 11 O 0 ( OH ) 6 One kilogram of soil contains 100 g of montmorillonite clay, or 0.6 moles of Al and 0.54 moles of H. Number of mol c contained in 1 kg of soil: (0.6x)0.54x1 =.4 mol c /kg or 4 cmol c /kg Using information in Table 8.5: soils in a cool, humid region have a C.E.C. of about 0 cmolc/kg, of which ~ 0% are exchangeable H and Al. Number of exchangeable cmol c contained in 1 kg of soil 0 cmol c /kg x 0. = 6 cmol c /kg Relative Size of Acidity Pools According to Table 8.5 a soil with ~0% exchangeable H and Al has a ph of about 5. Number of mol c of H in a solution with ph = 5: mol c of H = mol of H = 10-5 or mol c /L Expressing the result in cmol c /L: mol c /L = cmol c /L Acidity Type Residual Exchangeable Active cmol c of H and Al 4 6 ~0.001 Acidifying Processes Leaching of basic cations (Ca, Mg, K ): Soil _ Ca H Soil _ H Ca H A M: Ca ; Mg ; K ; Na Negatively charged surface M A: NO ;SO4 ;Cl ;HPO4 HA: Acids produced by microorganisms (carbonic and sulfuric acids and organic acids), human activities (fertilizers, acid rain). M A Lecture 6, Soil ph and Salinity 6

7 Soils and Water, Spring 008 Acidifying Processes Respiration (roots and microorganisms): Organic molecule from photosynthesis CH O O CO H O CO H O H CO H CO H HCO Oxygen is needed to carry out this reaction (aerobic systems). In the absence of O other reactions take place. pe-ph Diagram for Soils pe = - log (e - ) pe = E h (mvolt) Source: Sposito, G The Chemistry of Soils. Sequence of Redox Reactions Source: Sposito, G The Chemistry of Soils. Lecture 6, Soil ph and Salinity 7

8 Soils and Water, Spring 008 Flooded Soils Source: Sposito, G The Chemistry of Soils. Root Uptake of Cations (M) M H Roots also uptake anions, but in less amounts. Uptake of anions result in the release of OH - and/or HCO - (soil ph rises). The Influence of N Fertilizer NO NH 4 Source: Heckman, JR and JE Strick Teaching Plant-Soil Relationships with Color Images of Rhizosphere ph. J. Nat. Resour. Life Sci. Educ. 5: Lecture 6, Soil ph and Salinity 8

9 Soils and Water, Spring 008 Map of Soil ph Near Roots Source: Gregory, P.J. and P. Hinsinger New approaches to studying chemical and physical changes in the rhizosphere. Plant and Soil 1-9. Conceptual Model of Soil H Budget Atmospheric pool: H acid substances Output of acid gases: N, NH, NO X, CH 4,... Wet Deposition: H, NH Soil surface 4 Cation uptake H Sources Oxidation reactions Mineralization of cations Reverse weathering Dissociation of CO and organic acids Dry Deposition: CO, NH,NO X,SO 4 -. Reduction reactions Mineralization of anions Anion uptake SEE EXAMPLES IN TABLE 9.1 H Sinks Weathering of oxygen-rich metals Association of CO and organic acids Living dead biomass Export of H in the percolation water Chemical Composition of Rainfall Ion NH 4 NO - ph Continental Rain 1 Ca 0.1- Mg mg/l K Na SO Cl ± ± ± ± ± ±0.1.11± ±0.01 New Jersey 4.7± kg/ha ± ± ±0.15.1±0.90.5± ±.71.8± ± Source: Berner EK and RA Berner Global Environment. Prentice Hall, NADP, Washington Crossing, NJ; average ± standard deviation (period ). Lecture 6, Soil ph and Salinity 9

10 Soils and Water, Spring 008 Global Acid Deposition Source: Berner EK and RA Berner Global Environment. Prentice Hall Chemical Composition of Precipitation and Soil Water Sears, S. O Inorganic and Isotopic Geochemistry of the Unsaturated Zone in a Carbonate Terrane. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA. Chemical Composition: Rain and Rivers Source: Berner EK and RA Berner Global Environment. Prentice Hall Lecture 6, Soil ph and Salinity 10

11 Soils and Water, Spring 008 Lowering Soil ph Add oxidizable compounds: Sulfur, biologically oxidizes to sulfuric acid Aluminum sulfate, which splits H O to produce H. Organic matter. Raising Soil ph Add Limestone: carbonates of Ca, Mg Burned/Quick lime (oxides of Ca, Mg ) Hydrated lime (hydroxides of Ca, Mg ) Lecture 6, Soil ph and Salinity 11