ENVIRONMENTAL AND ECOLOGICAL CHEMISTRY Vol. II - Acidity and Alkalinity of Soils - R.H. Merry

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1 ENVIRONMENTAL AND ECOLOGICAL CHEMISTRY Vol. II Acidity nd Alklinity of Soils R.H. Merry ACIDITY AND ALKALINITY OF SOILS R.H. Merry CSIRO Lnd nd Wter, Adelide, Austrli Keywords: cid rin, cid sulfte soils, crbon cycle, environmentl indictors, liming, nitrogen cycle, ph, ph buffering, ph mesurement, soil cidity, soil lklinity, soil minerlogy, soil qulity, sulfur cycle, wethering Contents 1. Introduction 2. World Distribution of Acidic nd Alkline Soils. Soil ph nd ph Buffering Cpcity.1. Soil ph nd the composition of the soil solution.2. Neutrlizing Cpcities, ph Buffer Cpcity nd Lime Requirement.. Mesurement nd Methodology for Soil ph.. ph s n Indictor of Soil Condition. Soil Properties nd Processes Controlling Acidity nd Alklinity.1. Wethering nd the Hydrologicl Cycle.2. Formtion of Alkline Soils.. The Soil Nitrogen Cycle.. The Soil Crbon Cycle.5. The Soil Sulfur Cycle Acid Sulfte Soils 5. Plnt Soil Systems 5.1. Nturl Plnt Systems 5.2. Agriculturl Systems nd Irrigtion 6. The Importnce of Scle nd Vribility in Soil Acidifiction nd Alkliniztion Processes 6.1. Vribility 6.2. Soil Microsites 6.. Soil Profiles 6.. Toposequences 6.5. Ctchments 6.6. Regions 7. The Future Need for Mngement of Soil Acidity nd Alklinity Glossry Bibliogrphy Biogrphicl Sketch Summry Soils tht re significntly cidic or lkline through ll or prt of the profile cover much of the Erth s lnd surfce. The properties of soils tht relte to their cidity nd lklinity, indicted by ph mesurement, re fundmentl to understnding the nture nd behvior of mny soils. Processes of cidifiction nd lkliniztion re importnt in the mngement nd productivity of griculture nd in environmentl mngement.

2 ENVIRONMENTAL AND ECOLOGICAL CHEMISTRY Vol. II Acidity nd Alklinity of Soils R.H. Merry The ph of soil smple reflects the dissocition of wek nd strong cids nd bses tht re ssocited with the solutions, slts nd minerl nd orgnic compounds tht mke up soil mterils. Chnge in ph mostly results from nitrogen, crbon nd sulfur cycle effects tht result from nturl biologicl processes in soils, nd rtes of chnge my be ccelerted by humn ctivities through griculture nd industry. Exmples of humn ctivities tht cn ccelerte or lter cidifiction or lkliniztion processes include fertilizer nd lime ppliction, product removl, irrigtion nd chnges to wter tbles, chnges to evpotrnspirtion conditions, or cid rin. Soil ph vries in the lndscpe t different scles, from microsite to profile, toposequence, ctchment nd region. This continuum mens tht effective mngement of soil cidity nd lklinity should be considered both t the field scle, for crop production for exmple, nd t the broder, ctchment scle to control wter qulity nd lnd degrdtion. 1. Introduction Acidity nd lklinity of soils, s indicted by the mesurement of soil ph, re importnt chemicl properties tht hve role in mny of the wys tht soils behve (see Soil Chemistry). ph is probbly the most common nlyticl mesurement mde on soils. The vilbility of mny plnt nutrients nd the cpcity of soil mterils to bsorb nd hold mterils re often medited by soil s ph. It is usully the extremes of cidity or lklinity tht cuse environmentl problems, difficulties with growing plnts, or with mnging physicl problems. In turn, soil s ph reflects its chemicl nd minerlogicl components nd their reltionships to the solutions tht reside in soil pores. The composition of soils lso controls their cpcity to resist chnges in soil ph environmentl perturbtions often cused by humn ctions such s cid rin (see Acid Deposition), irrigtion (see Wstewter Tretment nd Reuse for Irrigtion), griculture (see Orgnic Pollution from Agrochemicls) nd wste disposl (see Chemistry of Hzrdous nd Solid Wstes nd Lechtes). 2. World Distribution of Acidic nd Alkline Soils Soils tht re significntly cidic or lkline cover much of the world s lnd re. Generlly there re geogrphic, geologicl, biologicl nd climtologicl resons why soils tend to be either cidic or lkline, nd their nturl stte cn be gretly modified by humn ctions. Soils profiles vry in their properties nd my be either cidic or lkline throughout, or my be either cidic or lkline within different lyers, though very generlly the ner surfce lyers tend to be more cidic thn subsurfce lyers. Producing mp of the world s cidic or lkline soils is difficult for these resons, nd for resons of sptil vribility. In res where rinfll is low, soil leching is usully insufficient to completely remove slts from soil profiles nd it is common in these regions for soils to contin crbonte or bicrbonte ions tht re concentrted by evpotrnspirtion processes. With incresing ridity, these lkline mterils nd ssocited slts of sodium nd gypsum

3 ENVIRONMENTAL AND ECOLOGICAL CHEMISTRY Vol. II Acidity nd Alklinity of Soils R.H. Merry ccumulte closer to the soil surfce. Lrge res in the semirid nd rid climtic zones hve ccumulted clcium crbontes s rubble lyers or hrdened sheets tht my ct s chemicl or physicl brrier to penetrtion by plnt roots. The world s rid regions mount to bout 50 million squre kilometers, or bout 0 percent of its lnd re, nd their soils re usully lkline, though some will be cidic s result of pst climtes tht were more humid. There re dditionl res tht hve soils tht re lkline, such s the regions where loessil mterils (windblown soil dust sourced from more rid regions) contining clcium crbonte hve been deposited, or where soils hve developed on clcreous prent mterils. The ltter re usully emerged clcreous mrine or lcustrine sediments, but they my lso contin significnt sodium bicrbonte nd crbonte in regions of low rinfll. At more locl scle, lkline soils frequently develop in vlley floors where crbontes nd other slts ccumulte due to the concentrtion of lkline through flow or ground wters. Soils tht hve cidified significntly, with ph vlues less thn 5 or 5.5, tend to form in two climtic zones. The most importnt of these is perhps the tropicl belt of high nnul rinfll where intense wethering of soil minerls nd other processes hve left cidified soils. This region is importnt becuse of its incresing popultion density nd requirement for food production. There is lso lrge region t higher ltitudes, principlly in the northern hemisphere, where soils re cidic becuse of high effective rinfll nd the ccumultion of cidic orgnic mtter. It is estimted tht cidic soils cover bout 0 per cent of the globl lnd surfce. The remining soils on the erth s lnd mss re regrded s predominntly neutrl with respect soil ph, tht is, their ph vlues re mostly between bout 6 nd 8. It is possible tht these soils re in n environmentl equilibrium where cidifiction processes nd lkline inputs re bout equl, but it is likely tht they re subject to net cidifiction if used for griculture.. Soil ph nd ph Buffering Cpcity.1. Soil ph nd the composition of the soil solution The cidity or lklinity of soil solution is mesured using the ph scle. The ion ctivity product of wter ( pk w ) t 25 o C is 1.0 nd t ph of 7 (neutrlity), the ctivities of H nd OH re both 10 7 moles per liter (mol L 1 ). The ph (Eq. (1)) of soil solution in equilibrium with soil mterils is the negtive logrithm of the hydrogen ion ctivity: 10 ph = log (H ) (1) (H ), the hydrogen ion ctivity, equls γ [H ] where γ is the ctivity coefficient nd [H ] is the concentrtion of protons in the soil solution in moles per liter. Becuse the scle is logrithmic, ph decrese of one ph unit indictes tenfold increse in hydrogen ion ctivity nd ph increse of one unit indictes tenfold increse on hydroxyl ion ctivity.

4 ENVIRONMENTAL AND ECOLOGICAL CHEMISTRY Vol. II Acidity nd Alklinity of Soils R.H. Merry Most soil ph vlues re in rnge between bout 5 (10 5 mol L 1 hydrogen ion ctivity) nd 8.5 ( 10 9 mol L 1 hydrogen ion ctivity). Acid sulfte soil conditions my result in soil ph vlues s low s bout 2 nd lkline sodic soils my hve ph vlues bove 10. Understnding soil cidity nd lklinity requires n understnding of the ssocited chemistry. Acidic nd bsic compounds in soils re defined by Brønsted nd Lowry s being proton donors nd cceptors, respectively. Strong cids nd bses dissocite completely in solution (wter in the cse of soils). Most soil cids nd bses re wek nd re commonly dissocited bout 1 percent. Tble 1 lists dissocition constnts ( pk ) for some of the cids found in soil systems. When ph equls pk, the ctivities of n cid nd its conjugte bse re equl, tht is, the cid is hlf dissocited. The dissocition of these cids nd bses only buffers the ph of the soil solution significntly over rnge of bout 1 ph unit on either side of its pk. Becuse most soils re mixture of minerl nd orgnic components tht re cids nd bses, titrtions of soil mterils with cid or lkli produce buffer curves tht reflect the net contribution from these components nd my not pper similr to the buffer curves expected from, for exmple, simple polyprotic cids. In surfce lyers of soils where orgnic mtter is usully ccumulted, wekly dissocited orgnic cids buffer soil ph. Where soil ph is bove bout ph 6.5, the concentrtions of bicrbonte nd crbonte nions increse nd it is the presence of slts nd minerls of these nions, prticulrly sodium bicrbonte, tht results in the very high ph vlues of lkline sodic soils. Acid sulfte soils result from the oxidtion of sulfides to form sulfuric cid nd, if the cid neutrlizing cpcity of the soil mteril is low, ph vlues below cn result. Formul of Acid pk HCO 2 6. HCO 10. NH 9.2 HSO 2.0 HS HS 12.9 HBO 9.2 HSiO HCOOH (Formic).8 CHCOOH (Acetic).8 Tble 1: Dissocition constnts ( pk ) for cids found in soil solutions The ions present in soil solutions differ in mount nd form depending on whether the soil ph is cidic or lkline, reflecting the chrge properties, equilibrium stte, nd minerl nd orgnic mterils tht mke up the soil. Most soil systems re not closed,

5 ENVIRONMENTAL AND ECOLOGICAL CHEMISTRY Vol. II Acidity nd Alklinity of Soils R.H. Merry tht is, they re subject to solutions nd gses entering nd leving the profile nd re therefore lso in sttes of chnging chemicl equilibri. The specific form of ions present in solution re criticl, especilly s lklinity or cidity increses, s species tht re toxic to mny biologicl processes, soil orgnisms nd plnts increse in concentrtion (see Trce Elements, Specition of Hevy Metls nd Rdioisotopes). The ions nd complexes commonly found in soil solutions include: lkline soil solutions, ph bout Ctions: C, N, Mg, K Anions: 2 2 Cl, SO, HCO, NO, HPO, HBO, orgnic lignds cidic soil, ph bout Ctions: K, C, Al (orgnic lignds), Mg, N, Mn nd Fe Anions: SO , Cl, H2CO, orgnic lignds, F In lkline soils, the ionic forms N, K, Cl nd NO predominte with incresing presence of bicrbonte nd other complex forms of sulfte nd bicrbonte. The incresing presence of bicrbonte nd borte, nd possibly the luminte ion, often due to evportive concentrtion nd limited leching, cn result in very high soil ph nd plnt toxicities. Trce metl elements re present in such low concentrtions tht Fe, Mn nd Zn become deficient In cidic soils, the ionic forms K, C, Mg, N, Mn, Fe, SO nd Cl predominte, with complex forms of luminum ( Al ) with orgnic lignds, F nd OH. As ph decreses, incresingly is the very toxic Al relesed. The min ionic forms listed occur t close to 100 percent of the element present in solution nd re therefore redily vilble to plnts nd, significntly, my lso be redily leched. Plnt nutrient deficiency for elements such K, C nd Mg cn be common t low soil ph, usully becuse leching loss hs impoverished the soil..2. Neutrlizing Cpcities, ph Buffer Cpcity nd Lime Requirement The bility of soil mterils to bsorb the effects of cidic dditions is often termed its cid neutrlizing cpcity (ANC; or, inversely, its lkli or bse neutrlizing cpcity), tht is, the bility of soil components to ccept protons or hydroxyls. Although there my be some neutrlizing cpcity in the soil solution, it is the greter mss of solid, minerl mteril tht contributes most to this cpcity, especilly below ph 5 nd bove ph 8. For exmple, cid neutrlizing cpcity my be provided by the dissolution of crbonte minerls in lkline soils, the bse sturtion of the soil s exchnge complex, or the decomposition of less wetherble luminosilicte minerls in cidified soils. The dsorption rections tht involve the soil solution nd ion exchnge hve fst rection rtes compred to dissolution rections, but their cpcity to ct s proton sinks my be orders of mgnitude less. The processes tht result in soil lkliniztion or cidifiction cn be defined s those tht produce n increse or decrese, respectively, in ANC. ANC vries with soil ph but cn be mesured in

6 ENVIRONMENTAL AND ECOLOGICAL CHEMISTRY Vol. II Acidity nd Alklinity of Soils R.H. Merry generl wy for the solid components using Eqution 2, where the brckets indicte molr quntities: ANC (solid) = 2(CO) 2(MgO) 2(K O) 2(N O) 2(FeO) (NH ) 2(SO ) 2(P O ) (HCl) 2(N O ) (2) The ANC of the soil solution generlly simplifies to eqution, where A indictes orgnic nions: 2 ANC (solution) = (HCO ) 2(CO ) (OH ) (A ) (H ) () The cpcity of soils to resist ph chnge when chllenged by cid or lkline inputs is termed its ph buffer cpcity. It is usully expressed s the mount of protons ( H, cidic) or hydroxyl ( OH, lkline) ions needed to chnge specific mss of soil by one ph unit. More specificlly, in griculture, the lkli, usully clcium or mgnesium crbontes, needed to increse the ph of n cidic soil to desired ph by liming is termed its lime requirement. Fifty kilogrms of clcium crbonte ( CCO ) is equivlent to, nd ble to neutrlize, 1 kmol of H. In soil systems where chnges occur, nnul cidifiction rtes re often expressed s the equivlent kmol H h 1 y 1 or sometimes in kilogrms of CCO h 1 y 1. The ph buffering cpcity of soil is provided by its mixture of minerl nd orgnic constituents nd is modified by their surfce properties, the electricl chrge on which lso chnges with soil ph. Alkline soils (ph > 7) tht contin crbonte minerls my hve considerble cpcity to neutrlize the effects of cidic inputs. For soils with ph in the neutrl to modertely cidic rnge (ph 5 to 7), it is usully ion exchnge rections ssocited with clys nd orgnic mtter tht control ph. Within this pproximte rnge, the ph buffer curve of most soils is usully close to liner. Below ph vlues of bout 5, stronger ph buffering in mny soils is due to decomposition of clys nd other remining luminosilictes, if they re present. The relese of toxic luminum, mngnese nd other ions t soil ph vlues less thn 5 hs detrimentl consequences for the growth of mny nondpted plnts nd ccelerted relese of nutrient elements such s clcium, mgnesium nd potssium results in incresed leching loss nd potentil nutrient deficiency. TO ACCESS ALL THE 18 PAGES OF THIS CHAPTER, Visit:

7 ENVIRONMENTAL AND ECOLOGICAL CHEMISTRY Vol. II Acidity nd Alklinity of Soils R.H. Merry Bibliogrphy Dent D.L. (1986). Acid Sulphte Soils: Bseline for Reserch nd Development, 200 pp. Wgeningen, The Netherlnds: Interntionl Institute for Lnd Reclmtion nd Improvement (IILRI). [This volume comprehensively covers the properties, environments nd mngement of cid sulfte soils]. Evngelou V.P. (1998). Environmentl Soil nd Wter Chemistry: Principles nd Applictions, 56 pp. New YorkChichesterWeinheimBrisbneSingporeToronto: John Wiley nd Sons, Inc. [This textbook provides brodrnging nd detiled coverge of the soil nd wter chemistry tht pplies to soil cidity nd lklinity, nd lso to wider fields of soil science]. Kennedy I.R. (1992). Acid Soil nd Acid Rin, Second Edition, 25 pp. Tunton, Somerset, UK: Reserch Studies Press Ltd. [This book comprehensively presents the chemistry of ll spects of soil cidity]. FAO/UNESCO (197). Irrigtion, Dringe nd Slinity: n Interntionl Source Book, 510 pp. London, UK: FAO/UNESCO/Hutchinson nd Co Ltd. [Chpter 5 of this book, "Chemistry of sline nd lkline soils of rid zones", provides extensive nd detiled informtion on the processes of development nd mngement of soil lklinity under dry conditions]. Ll R., Kimble J.M., Eswrn H. Stewrt B.A. (eds.). (2000). Globl Climte Chnge nd Pedogenic Crbonte, 05 pp. Boc Rton, Florid, USA: CRC Press. [This book contins mteril relevnt to the distribution of clcreous soils nd the pedogenic processes tht occur in them]. Rengel Z. (ed.). (200). Hndbook of Soil Acidity, 96 pp. New York: Mrcel Dekker Inc. [This book discusses mny spects of soil cidity in griculture nd the environment, including the world distribution of cidic soils]. Rowell D.L. (1988). Soil cidity nd lklinity. Russell s Soil Conditions nd Plnt Growth, 11 th Edition (ed. A. Wild), Hrlow, Essex, UK: Longmn Scientific nd Technicl. [This presents brod, but detiled coverge of soil cidity nd lklinity, griculturl systems nd plnt growth]. Sumner M.E. (199). Mesurement of soil ph: problems nd solutions. Communictions in Soil Science nd Plnt Anlysis 25(7&8), [This pper covers the mesurement of soil ph in the lbortory]. Biogrphicl Sketch Richrd Merry is n honorry reserch fellow t CSIRO Lnd nd Wter, Adelide, South Austrli. He is grdute of the Universities of Tsmni nd Adelide. Over severl decdes his reserch hs been strongly fieldbsed in the disciplines of pedology, soil chemistry nd gronomy. Specilist reserch topics include: soil cidifiction in crop nd psture soils, nd in lndscpes; cid sulfte soils in costl nd inlnd environments nd wetlnds; toxic metls distributions (derived from smelters, horticulture nd urbn sources) in soils nd lndscpes; soil nlysis using mid infrred spectroscopy; sodicity nd slinity; soil fertility.