Problem soils and soil acidity, P K MANI


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Problem soils and soil acidity, P K MANI

  1. 1. Problem soils
  2. 2. 2 major groups 1) Acid soils 2) Salt affected soils (Saline and Alkali soils…….. ………………………Other type of problemtaic soil is 3) Calcareous soil Acid soils occur in those areas where rainfall is higher, i.e precipitation > evapo-transpiration Salt affected soils occur in arid and semi arid regions where, precipitation <evapo-transpiration Calcareous soils occur in semi-arid region which contains parent material like CaCO3 (pedogenic) In India Acid soil covers 49.0 million ha, whereas, salt affected soil covers 8.0 m.ha
  3. 3. Soil acidity is defined as proton (H+) yielding capacity of soil during its transition from a given soil pH to a reference pH. (Jackson,1958) Acid soils have been defined by the soil having pH less than 5.5 in 1:1 soil-water suspension (USDA). Properties of acid soil Low CEC Intermediate texture (Sandy loam to Loam) Low organic matter content (except hilly region and forest soils) Low P content but N is variable High amount of Fe and Al in soil soln. Soil Acidity and Poverty
  4. 4. Descriptive terms pH range Buffering mechanism Extremely acid <4.5 Iron range (pH 2.4–3.8) Very strongly acid 4.5–5.0 Aluminum/iron range (pH3.0–4.8) Strongly acid 5.1–5.5 Aluminum range (pH 3.0–4.8) Moderately acid 5.6–6.0 Cation exchange (pH 4.2–5.0) Slightly acid to neutral 6.1–7.3 Silicate buffers (all pH values typically >5) Slightly alkaline 7.4–7.8 Carbonate (pH 6.5–8.3)
  5. 5. Pedogenic causes of formation of Acid soil i) Laterisation of different degrees: In Tropical region: high rainfall coupled with high temperature causes laterisation which in turn leads to intense weathering and leaching of bases like Ca, Mg. (Orissa, M.P., Bihar, w.B.- Assansol, Medinipur, Bankura, Purulia) ii) Podzolisation in areas with sub-temperate to temperate climate, where organic matter content is high. (Low Temp. and high Rainfall) (H.P, terrai region, Kashmir, Assam (some parts) iii) Marshy, Peaty conditions with significant amount of under decomposed and partly decomposed O.M. iv) Intense leaching in light alluvial soil in high rainfall areas having partly decomposed O.M. (Occurs , N. Bengal, Coochbehar, N and S Dinajpur, Malda (some parts) V) Coastal Region: Acid sulphate soils: Inundation of sea water in low land areas (Kerala, Sundarban)
  6. 6. Laterization is a pedogenic process ; found in tropical and subtropical environments. High temperatures and heavy precipitation result in the rapid weathering of rocks and minerals. Movements of large amounts of water through the soil cause eluviation and leaching to occur.  Laterisation is the process of desilication, i.e., removal of silica and accumulation of sesquioxides ) Iron oxides give tropical soils their unique reddish coloring. Heavy leaching also causes these soils to have an acidic pH because of the net loss of base cations.
  7. 7. Podzolization is associated with humid cold mid-latitude climates and coniferous vegetation. Decomposition of coniferous litter and heavy summer precipitation create a soil solution that is strongly acidic. This acidic soil solution enhances the processes of eluviation and leaching causing the removal of soluble base cations and aluminum and iron compounds from the A horizon. This process creates a sub-layer in the A horizon that is white to gray in color and composed of silica sand.
  8. 8. Podzolisation: Iron and aluminium are leached down while silica is present in abundance as evidenced by the characteristic ash-coloured greyish E-horizon in Podzols. Laterisation: Most of the silica leaches out of soils while iron and aluminium get accumulates as oxides.
  9. 9. PEATY AND MARSHY SOIL – – – – Occur in Humid region. Formed by accumulation of organic matter. Black in colour. Highly acidic and heavy. • Areas: – Kottayam & Alleppey in Kerala, Coastal Orissa, Sundarbans of W.B
  10. 10. Sources of acidity in Soil 1.Aluminosilicate minerals  The most common source of H is the reaction of Al ions with water. The equation for this reaction in very acid soils (pH<4.0) is: The species of aluminum ions present vary with pH  The forms of aluminum are mostly exchangeable Al3+ under very acidic conditions (pH<4.5) to aluminum-hydroxyl ions at higher pH (4.5–6.5) .
  11. 11. Situation 1: In strongly acid condition: Here the exchangeable Al+3 and exch. H+ are mostly responsible for production of excess H+ in soil soln. Clay H+ + H Al3+ Al(OH)2+ Al(OH)2+ Exchangeable complex Soil soln. Al3+ Al(OH)2+ Al(OH)2+ Soil soln. Al3+ +H2O ⇋ Al(OH)2++ H+ Al(OH)2+ +H2O ⇋ Al(OH)2++ H+ Al(OH)2+ +H2O ⇋ Al(OH)3 + H+ Al(OH)3 +H2O ⇋ Al(OH)4- + H+ The Al3+ hydrolysis with the formation of insoluble Al(OH)3 and H+ and Al(OH)3 is pptd. As a result more Al+3 will comeout from the exch.complex (clay) to the soil soln to maintain an equilibrium and these process continued. Due to Al +3 hydrolysis more H+ are formed in soil soln which leads to more acidity to the soil. The species of aluminum ions generates hydrogen ions through a series of hydrolysis reactions shown above (Lindsay, 1979):
  12. 12. Situation 2: In moderately acid condition:pH ..... 5.5 Adsorbed H+ does not play any important role Clay Al(OH)2+ Al(OH)2+ Exchangeable complex Al(OH)2+ Al(OH)2+ Soil soln. Al(OH)2+ +H2O ⇋ Al(OH)2++ H+ Al(OH)2+ +H2O ⇋ Al(OH)3 + H+ Situation 3: In slightly acid condition:pH ..... 6.0 All ions like Al+3, Al(OH)2+ + Al(OH)2+ and also H+ disappers from the soil soln. Dissociation of H from –COOH, phenolic-OH, and –NH2 group is responsible for acidity. H+ dissociates at the broken bond at the edges and corners of the crystal Si-O-H Al-O-H
  13. 13. Lewis acid: A lewis acid is one which accepts a pair of electrons and a Lewis base is one which donates a pair of electrons Base Acid Adduct NH3 + 6H2O + H+ Al3+ ↔ ═ NH4+ [Al(H2O)6]3+ It is known that large charge to radius ratio (i.e. ionic potential, z2/r,) for cations results in an increase in hydration energy. Closely related to hydration is the process of hydrolysis. K+ +nH2O [K(H2O)n]+ hydration In case of hydrolysis, the acidity ( i.e. ionic potential) is so great as to rupture of H-O bond with ionisation of hydrate to yield H3O+ ions ( proton). H2O Al3+ + (H2O)6 ═ [Al(H2O)6]3+ Lewis acid [Al(H2O)5OH]2+ + H3O+ L base Cations that hydrolyse are either small (Be+2) and /or highly charged (Al3+, Fe3+, Sn+4) . A bare H+ should have the highest charge to radius ratio and
  14. 14. reveals that cations having a very large charge to radius ratio tend to polarize water sufficiently strongly to promote hydrolysis. This can be thought of as hydration taken to the extreme, where the "ionic potential," z2/r, of the ion is large enough to rupture 0 - H bonds.
  15. 15. Generally speaking, the greater the charge density of an ion, the more heavily hydrated it will be.  Anions are hydrated less than cations because of lesser charge density. Cations are heavily hydrated because of their higher charge density, and the process can be demonstrated as follows: Commonly, two processes take place when a metal salt is added to water: 1. Hydration (H2O molecules adsorb onto the ions) 2. Hydrolysis (degree to which adsorbed H2O dissociates to satisfy ion electronegativity).
  16. 16. the overall reactions of the metal ions with water into several plausible steps. 1. The metal ion is solvated by water molecules (we can also call this hydration); typically metal ions are surrounded by about six water molecules in what is known as the primary hydration sphere. Most of the heat liberated in the above reaction is due to the enthalpy of hydration. These hydrated cations are also called aqua complexes (or aqua acids). 2. The hydrated ion can now undergo step-wise hydrolysis in which it delivers one or more H+ to the bulk solvent. This is illustrated in the graphic below for the aluminum ion, though the same scheme applies for any element, differing only by degree. The 1st step in this diagram shows how a strong attraction of the cation for an oxygen of one of the directly attached water molecules results in the splitting-off of one H+: Since these are Brønsted equilibria, we can write a standard hydrolysis constant for any step in this reaction: metal hydroxide usually starts to precipitate when the pH of the solution is approximately equal to the pKa of the metal ion.
  17. 17. Structure of the free aqueous aluminum [Al(H2O)3+6 ] ion. Nordstrom and May (1996),
  18. 18. 2. Aluminium and iron polymers  The exchnaheable Al3+ ions displaced by cations from clay minerals are hydrolysed to monomeric and polymeric hydroxyaluminium complexes. The high charge to radius ratio i.e., high acidity of Al3+ ion enables it to rupture O-H bond in [Al(H2O)6 ]+3 Hydrate,causing ionisation of hydrate thereby yielding protons. The step wise hydrolysis of monomeric Al3+ forms, occurring progressively at higher pH. This rn will be facilitated by the presence of proton acceptor in soil soln. pH: <4.0-4.7 pH: 4.7-6.5 pH: 6.5-8.0 pH: >8.0
  19. 19. Sources of acidity in Soil 2. Rainfall / leaching: Soils become acidic due to leaching of the basic cations such as Ca2+, Mg2+, K+, and Na+ down the soil profile by excess rains. This situation is common in high rainfall areas, where precipitation exceeds evaporation, and leads to leaching. CO2 + H2O  H2CO3 H2CO3 H+ + HCO3The released hydrogen ions replace the calcium ions held by soil colloids, causing the soil to become acid. The displaced calcium (Ca++ ) ions combine with the bicarbonate ions to form calcium bicarbonate, which, being soluble, is leached from the soil. The net effect is increased soil acidity. Ca+2 + 2HCO3- Ca(HCO3-)2
  20. 20. 3. Fertilizers. Use of adequate amounts of nitrogen fertilizer is fundamental for higher yield of crops under all ecosystems. Urea and ammonium sulfate are dominant nitrogen carriers used for crop production around the world. The acidification of soils by using the ammonium form of nitrogen fertilizers can be explained by the following equation: Ca, Mg Ca (NO3)2, Mg (NO3)2 Highly soluble,Ca, Mg depleting from in soil above equation is known as the The oxidation of NH4+ nitrification and hetrotrophic and autrotrophic bacterias can carry it out. The most important autrotrophic genera of bacteria are Nitrosomonas and Nitrobacter
  21. 21. Reaction of Nitrogenous fertilizer containing N in ammonium form : + -2  (NH 4 )2SO 4 NH4 + SO4 + - then hydrolyzed into NH4 + OH -2 + SO4 + 2H  NH4 OH (weak base)  H2 SO4 (strong acid) As NH4 OH is a weak base, though the are produced in equivalent amount, NH 4 OH will slowly dissociate than y + H2 SO4 producing less OH than H producing acidity in soil. + The NH4 will be adsorbed on the soil colloids replacing the exchangeable bases like Na, Ca, Mg etc NH4 Ca -[soil + (NH 4 )2SO 4 [soil + CaSO 4 ( soluble, causing leaching of bases)  NH 4 the released bases will react with the corresponding anion , i.e., SO 4-, Cl-, and NO3- to form respective salts, e.g., CaSO4, CaCl2, Ca(NO3)2 etc., the Ca-salts, thus, produced may be lost from the surface soil by leaching. In this way, there will be some loss of exchangeable bases from the salts. Under well-drained and aerated condition, this adsorbed NH4+ will be liberated in soil solution and nitrified by microbes NH4+ + 1½ O2  NO2- + H2O+ 2H+ NO2- + ½ O2  NO3- + 18 kcal (Ntrobacter bacteria) + 66 kcal (Nitrosomonas bacteria)
  22. 22. 4. Use of legume crops continuously or in rotation can increase soil acidity The reason for generating acidity is associated with higher absorption of basic cations by these crops and the release of H+ ions by the root of legume crops to maintain ionic balance 5. Soil acidification is also caused by the release of protons (H+) during the transformation and cycling of carbon, nitrogen, and sulfur in the soil–plant–animal system 6. Soil acidification is caused by acid precipitation (acid rain), the result of industrial pollution 7. O.M. decomposition: organic acids ionized as follows : OH - R-COOH (H2O) R-COO- + H+ respiration: CO2 + H2O ----> H2CO3 = H+ + HCO3Humus contains Carboxylic and phenolic (OH) groups that behave as weak
  23. 23. Soil Acidity Two types: Active acidity (acidity produced due to H+ and Al3+ ions held in soil soln) Exchangeable acidity Reserve acidity/ Potential acidity Non-exchangeable acidity Exchangeble acidity : Acidity which is produced due to presence of exchangeable Al3+ and H+ from exchangeable complex. Non-exchangeable acidity: acidity produced due to dissociation of H+ from Al-OH, Si-OH at the broken edges or external surface of the clay minerals, and also H+ dissociate from -COOH , -OH phenolic and –NH2 group
  24. 24. Source of negative charge: Unsatisfied valences originated due to broken bond at the edges and corners of crystal Dissociation of the H+ from the OH of the Octahedral layer at definite pH.  Isomorphous repalcement by a action of similar size with lower valency. Source of positive charge: The protonation or addition of H+ to hydroxyl groups on edge of these minerals The exchange of the OH group for other anions such as Phosphate 〉 Al − OH + H 2 PO 4 − ⇔〉Al − H 2 PO 4 + OH − The negative OH- being replaced from positively charged Al+3 ions in the crystal
  25. 25. Creation of Clay Colloid Charge Isomorphous substitution equal shape/size • The replacement of one ion for another of similar size within the crystalline structure of the clay takes eons – doesn’t change rapidly
  26. 26. Permanent charge Octahedral sheet neutral Net negative charge
  27. 27. Edge Effects are pH Dependent Negative charges arises due to unsatisfied valences at the broken edges of the silica and alumina sheets At high pH, the hydrogen of these hydroxyls dissociates slightly and the colloidal surface is left with a negative charge carried by the oxygen Diagram of a broken edge of a Kaolinite crystal, showing O2 as the source of negative charge. At high pH values the H ions tend to be held loosely and can be exchanged for other cations
  28. 28. Adsorption of cations by humus colloids. The charge on humus colloids is pH dependent. Under strongly acid conditions H is tightly bound and not easily replacebale by other cations. The colloid therefore exhibits a low negative charge. With the addition of bases and the consequent rise in alkalinity, first the H from the carboxyl groups and then the H from the phenolic groups ionizes and is replaced by Ca,Mg and other
  29. 29. Negative charges on humus ENORMOUS external surface area! (but no internal surface – all edges) Central unit of a humus colloid (mostly C and H)
  30. 30. Deprotonation of hydroxyl group
  31. 31. Influence of pH on the CEC ( a measure of the negative charge) of montmorillonite and humus. Note that below pH 6.0 the charge for the clay mineral is relatively constant. This charge is considered permanent and is due to ionic substition in the crystal unit. Above pH 6.0 the charge on the mineral colloid increases because of ioniosation of hydrogen from exposed O-H groups at crystal edges.. In contrast to clay, essentially all of the charges on the organic colloid are considered pH dependent.
  32. 32. Exchange affinity Held more strongly Held more weakly H+ ≥ Al3+ > Ca2+ > Mg2+ > NH4+ = K+ > Na+ This is referred to as the “Lyotropic series” Strength of adsorption proportional to valence ÷ hydrated radius
  33. 33. Active acidity is very very low than Reserve Acidity Total acidity = Reserve acidity + Active acidity Active acidity is measured by pH of the soil.. Total acidity measurement: Soil suspension titrated with strong alkali (0.1N NaOH, 0.1N KOH, 0.1 M Ca(OH)2 at pH 7.0 . The amount of alkali reqd. measures the total acidity. Q.1 : 5 g soil, alkali consumed 10 ml 0.1N NaOH, to increase pH 5.0 to 7.0 1 ml 1 (N) alkali ≡ 1 meq of H+ 10 ml 0.1 N alkali ≡ 10* 0.1 *1 = 1 meq of H+ 5 g soil contain 1 meq H+ 100 g soil contain 20 meq H+ (total acidity) pH..5.0.(5 g soil, 10 cc water)... 10-5 g eq H+ / litre; 1 litre suspension contains 10-5 g eq H+ ; 10 cc contains 10-7 g eq H+ 5 g soil contains 10-7 g eq H+ ; 100 g soil contains = 20 x 10-7 g eq= 20x10-4 meq (Active acidity) Reserve acidity= 20 meq- 20x10-4 meq How to measure the exchange acidity: Unbuffered salt(KCl, NaCl) soln mixed with soil and shake. K+ will replace H+, Al+3 on the soil echange complex surface. Then the remaining soln containing H+, Al+3 is titrated by NaOH. The quantity of NaOH indicates the magbitude of exchangeable acidity.
  34. 34. Any resistance to change in pH of the soil or soln. by an external agency (alkali) is known as buffering capacity. β= db/dpH –Direct correlated with CEC of a soil, a high CEC is associated with a large number of exchange sites Mont > Illite > Kaolinite Soils having higher Humus shows higher bc –High buffered soils are organic soils, and 2:1clay soils. –Low buffered soils are low organic matter soils and 1:1 clay soils.
  35. 35. X ppm= x mg/kg = x. /(eq.wt) meq/kg = For Ca……x/20 meq/kg=x/20 m mol/kg=x/200 cmol/kg
  36. 36. Factors which influence the relationship between pH and PBS 1. Nature of the Colloidal material of the soil Soil A (dominated by Humus) vs Soil B (dominated by Silicate clay minerals) Binding energy of H+ with Humus < binding energy of H+ with silicate clay pH will be lower in soil A (release of H+ is easy, losoely held) pH will be higher in soil B 2. Nature of clay minerals: Soil A BS% Humus Silicate clays Dominant Soil B 50% 50% 1% 1% 30% 30% Montmorillonite Kaolinite Mont. Release H+ quickly pH( soil A) < pH ( soil B ) minerals acidic 3. Nature of adsorbed cation ( Ca, Na ) Soil A Soil B ( exch. Base 5 meq and CEC 10 meq) (Ca+2 3.0, Mg+2 1.0, Na+ 0.5, K+ 0.5meq.) (Ca+21.0, Mg+2 1.0, Na+ 2.0, K+1.0 meq.) pH( soil B) > pH ( soil A )
  37. 37. Problems of acid soil Adverse effects of Acid soils on plant growth may be due to following reason 1. Toxicity of elements (Al, Mn, Fe) 2. Deficiency of bases (Ca+2, Mg+2) 3. Imbalance of P,S and Mo 4. Poor microbial activity 1.a) Toxic effects of Al :- Al inhibits the root growth of the plants, interferes with the various physiological process of the plants like cell-division, respiration and DNA synthesis; restricts the uptake of Ca+2, P and H2O b) Mn–toxicity: In soils having pH below 5.0, excess Mn accumulates in all the tissue of the plant, the normal metabolism of the plant is seriously affected. c) Fe-toxicity: iron concentration in soil increases with the decrease in pH, increase in O.M. content and the intensity of soil redn. Under waterlogged condition of rice cultivation the soil undergoes reduction which reduces Fe+3 iron to Fe+2 iron which is more soluble and sometimes the concn. of Fe+2 in waterlogged rice soils becomes high and hence toxic to rice plants. This creates what is known as physiological diseases of rice(browning disease).
  38. 38. 2. Deficiency of bases: The amount of exchangeable Ca, Mg is lower in acid soils. Percentage base saturation is also low as the most exchangeable sites are occupied by Al and H. Ca and Mg are secondary essential elements as far as plant nutrients are concerned. Plant (Leguminous plant) require high amount of Ca and Mg. Due to lack of Ca and Mg yield will be hampered. But Rice, wheat do not require Ca, Mg, so not seriously affected. Ca, Mg improve the structure of the soils and so their deficiency will give rise to poor structure of the soil and thus they inhibit proper aeration. Microbial activities are also decreased due to the insufficiency of bases. So, mineralisation will be adversely affected . 3. Imbalance of Nutrient elements: a) Phosphorus: i) P in soil is precipitated due to formation of AlPO4 and FePO4. In acid soil Al and Fe concn is high. So availability of P in acid soil is very low. ii) Plant generally take up P from soil in the form of H2PO4- and HPO4-2. Chemical adsorption on the surface of the colloidal material and soil dominated with kaolinite clay mineral adsorbs more P. Therefore, P is not released from the surface and P availability will be low.
  39. 39. b) Sulphur: most of the S in soils present in organic form. These organic S are mineralised by some soil microbes. But in acid condition, they can’t function well to mineralise the S. these micro org. can’t grow well in low pH. Unless these S are changed to inorganic form, plant can’t absorbs S as nutrients . c) Molybdenum (Mo): generally micro nutrients are more soluble in acid soil but Mo is the exception. Mo is less soluble in low pH and thus becomes less available to the plants. In acidic condition it produces insoluble molybdates. Lack of Mo reduces N-fixation. 4. Poor microbial activity: most of the microbes prefer neutral pH. So in acid condition, their activity will be affected.
  40. 40. Effect of pH on the availability of nutrients important in plant growth and of microorganisms. As the band for a particular nutrient or microbe widens, the availability of the nutrient or activity of the microbes is greater. For example, with K the greatest availability is from pH ~6–9. From Brady (1984),
  41. 41. BIOLOGICAL CONSTRAINTS Fungi prefer acidic soil reaction; Bacteria and actinomycetes perform better in intermediate pH ranges. Activity of Nitrosomonas and Nitrobacter reduced significantly. Azotobacter population is less in acid soils. Activity of BGA and Rhizobia is also less. Ca and Mo deficiency adversely affects nodulation process.
  42. 42. luminum_and_test_interpretation.htm
  43. 43. How to overcome the deleterious effect i) Agronomic approach –grow acid tolerant crops ii) Chemical approach- use liming material to increase the soil pH LIME REQUIREMENT(LR)The lime requirement of an acid soil is the amount of a liming material that must be added to raise the soil pH to a desired level.( usually in the range of 6.0 to 7.0) The lime requirement ranges from 3.5 to 15 t/ha. Common Liming materials: Calcium Carbonate (CaCO3) , Dolomite (CaCO3,MgCO3), Burnt lime (CaO), Slaked lime (Ca(OH)2 Marl, Oyster shell, Basic slag (CaSiO3), Paper Mill Sludge,
  44. 44. Reactions of lime in acid soil : 1. CO2 + H2O  H2CO3 2. H2CO3 + CaCO3 (insoluble)  Ca(HCO3)2 Soluble 3. Ca(HCO3)2  Ca+2 + 2 HCO3H 4. (Clay) + Ca+2 Ca----(Clay) + 2H+ H 5. H+ + HCO36. H2CO3 H2CO3+ CaCO3 (insoluble)  Ca(HCO3)2 Calcium Carbonate Equivalent (CCE): Defined as the acid neutralizing capacity of an agricultural liming material expressed as a weight percentage of Calcium Carbonate (CaCO3).
  45. 45. The molecular constitution is the determining factor in the neutralising value of chemically pure liming materials; CaCO3+ 2HCl →CaCl2 + H2O + CO2 : MgCO3 + 2HCl →MgCl2 + H2O + CO2 MW : 100 MW : 84 84g MgCO3 =100g CaCO3 100 g MgCO3 = 119 g CaCO3 Name of Liming material Mol wt CCE (Neutralizing value ) (%) CaO Ca(OH)2 56 74 179 136 CaMg(CO3)2 92 109 CaSiO3 116 86 How much quantity of lime is required
  46. 46. The titration curve of soil A, with a low buffer capacity, and soil B, with a high buffer capacity. The lime requirements of these soils are indicated (in units of centimoles of base per kilogram of soil) assuming that they are to be limed to pH 6.0.
  47. 47. Factors on which the rate of reaction of liming material depends: Soil moisture : The greater the amount of moisture the more rapid is the rate of Rn. (Higher the moisture, soil will develop gradually more anaerobic condition giving rise more CO2 which forms H2CO3. This acid is solubilising agent of CaCO3) A+B  C+D  Temperature : Limestone reacts more rapidly at higher temp. this effect is probably related to diffusion rates of end products A+B C+D Fineness of the material: A more finely ground limestone exposes more surface area than a coursely ground lime stone. (40 mesh vs 10 mesh) To get maximum benefits from liming or for improving crop yields, liming materials should be applied in advance of crop sowing thoroughly mixed into the soil to enhance its reaction with soil exchange acidity. Fineness is measured by the proportion of processed agricultural lime which passes through a sieve with an opening of a particular size. A 60-mesh sieve, which is the standard for comparisons of lime fineness and efficiency rating of 100%, is assigned
  48. 48. Beneficial effects of Liming: 1. Liming raises soil pH, base saturation, and Ca and Mg contents, and reduces aluminum concentration in acid soil solid i) Ca and Mg react with H+ on the exchange complex and H+ is replaced by Ca2+ and Mg2+ on the exchange sites (negatively charged particles of clay or organic matter), forming HCO3-. ii) HCO - reacts with H+ to form CO and H O to increase pH.
  49. 49. 2.Reducing phosphorus immobilization Increase in availability of P in the pH range of 5.0 to 6.5 was associated with release of P ions from Al and Fe oxides, which were responsible for P fixation (Fageria, 1989). At higher pH (>6.5), the reduction of extractable P was associated with precipitation of P as Ca phosphate (Naidu et al., 1990). These increases in extractable P or liberation of this element in the pH range of 5.0–6.5 and reduction in the higher pH range (>6.5) can be explained by the following equations: The liming acid soils result in the release of P for plant uptake; this effect is often referred to as ‘‘P spring effect’’ of lime (Bolan et al., 2003).
  50. 50. The base forming cations are the sources of hydroxyl ions. H and Al ions are replaced on the exchange complex by Ca+2 ions when an acid soil is limed, thereby lowering the concn. of H + and Al3+ and raising the concn. of OH- ion and hence soil pH
  51. 51. 3. Improving soil structure Calcium in liming materials helps in the formation of soil aggregates, hence improving soil structure. The lime-induced improvement in aggregate stability manifested through the effect of liming on dispersion and flocculation of soil particles (Bolan et al., 2003). Liming is often recommended for the successful colonization of earthworm in pasture soils. The lime-induced increase in earthworm activity may influence soil (Springett and Syers, 1984) structure and macroporosity through the release of polysaccharide and the burrowing activity of earthworm
  52. 52. 4.Improving nutrient use efficiency The improvement in efficiency of these nutrients was associated with decreasing soil acidity, improving their availability, and enhanced root system Disadvantages of Overliming Overliming effects are more pronounced on coarse-textured soils than on fine-textured soils. Fine-textured soils have high buffer capacity; hence, changes in chemical properties due to liming are not as pronounced as for coarse-textured soils. Overliming can create deficiencies of micronutrients like Fe, Mn, Zn, Cu, and B. These nutrients are usually adsorbed onto sesquioxide soil surfaces
  53. 53. Why gypsum is not considered as liming material? Gypsum is not considered as liming materials because on its application to an acid soil it dissociates into Ca+2 and SO4-2 ions: CaSO 4 ↔ Ca +2 + SO 4 -2 The accompanying anion is sulphate and it reacts with soil moisture produces mineral acid (H2SO4) which also increases soil acidity instead of reducing soil acidity SO 4 -2 + H 2 O →H 2 SO 4 Basic slag can also be used as liming material when it is applied to an acid soil the following chemical reaction takes place. H Soil Ca + 2CaSiO3 + 3H2O = Soil Al +2H2SiO3 + Al(OH)3 Ca The metasilicic acid is weakly dissociated, much less so that the clay adsorbed H+ ions and the pH of the soil raised
  54. 54. Q. If sandy loam soil having CEC, 20 cmol(p+)kg-1 and 30% base saturation at pH 4.5. Calculate the theoretical amount of lime (CaCO3) required per ha of land (0-15 cm depth) for raising BS to 60%.( wt of soil 1 ha furrow slice(15 cm )=2.2x106 kg Ans. PBS = [meq of basic cations (S) / Total exch cations (T)] x 100 BS i =30%, BSf= 60% , (S i ) =30*20/100 =6 meq /100g soil (S f )=60*20/100= 12 meq / 100g soil So meq of basic cation will be required to raise the base saturation at 60% = 12-6 =6 meq 1 meq of CaCO3 weighs 50 mg (Eq. wt of CaCO3 = 100/2 = 50) 6 meq of CaCO3 weighs 50 x 6 mg= 300 mg 100 g soil requires 300mg = 300x 10-6 kg CaCO3 1 kg soil requires = 300* 10-5 CaCO3 2.2 *106 kg requires = 300 x 2.2x 106 x 10-5 = 6600kg= 6.6 t
  55. 55. Q2. To titarte 100 g of soil of initial pH 4.5 to a specified pH of 6.5 with N/10 NaOH, 50 ml of N/10 NaOH was required. Calculate the amount of CaCO3 to be applied in 1 ha 30 cm layer of soil. Assuming, all the CaCO 3 present in the material undergoes dissociation. Ans: 1 ml 1N NaOH ≡ 1 meq of bases 50 ml N/10 NaOH = 5 meq of bases 5 meq of NaOH ≡ 5 meq of CaCO3 = 5 x 50 mg CaCO3 = 250 mg CaCO3 100 g soil requires 250 mg CaCO3 1 kg soil requires 250*10 mg=2500mg=2.5 g CaCO3 4.5 x 106 kg soil requires= 11.25 t/ha
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  57. 57. Some important crop species, pasture species, and plantation crops tolerant to soil acidity in the tropics
  58. 58. Practices to be considered during liming  Lime application should be done split doses.  Smaller doses with frequent application.  Lime applied in furrows at the time of sowing.  Applied in alternate year till the soil pH is brought to normal range. • pH • Texture • Organic matter content • Types of clay present
  59. 59. Ready rekoner Soil pH Lime Requirement in Kg/ha Sandy loam Loam Clay loam 5.0 1262 1892 2944 5.2 1093 1639 2551 5.4 925 1387 2159 5.6 757 1135 1766 5.8 589 883 1374 6.0 421 630 981 Source : http/
  60. 60. Acid sulfate soils Dredging waterways, draining swamps, spoil piles, mine tailings
  61. 61. What are acid sulfate soils? Soils formed from the  weathering of sulfide-bearing parent materials, which results in extremely low pH (commonly < 3.0) and precipitation of sulfate salts.
  62. 62. Potential acid sulfate soil Potential acid sulfate soil contains  sulfidic soil material that contains pyrite  but has not oxidized to an extent that the soil-pH dropped to a value below 3.5
  63. 63. Formation of pyrite Fe2O3 + 4SO42- + 8CH2O + 1/2O2 = 2FeS2 + 8HCO3- + 4H2O  Iron must be present  Sulfur must be present  Anaerobic condition must prevail to reduce SO42- & Fe3+  Organic matter as energy source for the microbes The process increases pH
  64. 64. Location of pyrite in the landscape In delta regions and lagunes where sea water is meeting fresh water. Inland wetland areas which are enriched with ferro iron and sulfate from higher parts of the landscape Soil material with high content of pyrite is called sulfidic soil materials
  65. 65. Fluvisols and gleysols
  66. 66. Histosols
  67. 67. Oxidation of pyrite If the soil is drained pyrite will be oxidized: 4FeS2 + 15O2 + H2O -> 2 Fe2(SO4)3 + 2H2SO4 pH drops significantly and not only ferro iron but also ferri iron will be mobile. Soils which become very acid due to oxidation of pyrite are classified as actual acid sulfate soils
  68. 68. Agriculture problems actual acid sufate soils  Low soil pH  Aluminium toxicity  Salinity (from sea water)  Phosphorous deficiency (precipitation of aluminiumphosphates)  H2S toxicity if flooded  N-deficiency due to slow microbial activity  Engeneering problems as soil acidity attacks steel and concrete structures
  69. 69. Oxidation of pyrite might form a sulfuric horizon • Definition of sulfuric horizon • A sulfuric horizon must: • have a soil-pH < 3.5 (in 1:1 water suspension); and • have – yellow/orange jarosite [KFe3(SO4)2(OH)6] or yellowish-brown schwertmannite [Fe16O16(SO4)3(OH)10.10H2O] mottles; or – concretions and/or mottles with a Munsell hue of 2.5Y or more and a chroma of 6 or more; or – underlying sulfidic soil materials; or – 0.05 percent (by weight) or more of water-soluble sulphate; and • have a thickness of 15 cm or more.
  70. 70. Root system affected by acidity Poor maize crop in an acid field Source : http// …..
  71. 71. • Cation exchange- the interchange between a cation in solution and another cation on a soil surface • Cation exchange capacity (CEC)- the total sum of exchangeable cations that a soil can adsorb.
  72. 72. pH independent charge (permanent) Isomorphic substitution: substitution of one element for another in ionic crystals without changing the structure of the crystal a. Substitution of Al+++ for Si++++ in tetrahedral b. Mg++, Fe++, Fe+++ for Al+++ in octahedral Leaves a net negative charge (permanent) pH dependent charge: positive charge developed at low pH and excess negative charge formed at high pH Gain or loss of H+ from functional groups on the surface of soil solids. a. Hydroxy (-OH) b. Carboxyl (-COOH) c. Phenolic (-C6H4OH)
  73. 73. pH dependent charge (cont.) Only 5-10% of the negative charge on 2:1 layer silicates is pH dependent whereas 50% or more of the charge developed on 1:1 minerals can be pH dependent. 1. Kaolinite: 1:1 At high pH, these H+ are weakly held and may be exchanged. At low pH, H+ are held very tightly and are not exchanged. Deprotonation or dissociation of H+ from OH- groups at the broken edges of clay particles is the prime source of negative charge in the 1:1 clay minerals. High pH values favor this deprotonation of exposed hydroxyl groups. This creates some confusion since high pH is seldom associated with weathered soils. Fifty percent or more of the charge developed on 1:1 clay minerals can be pH dependent. In a weathered soil, hydrous oxides (Fe and Al oxides) are a more important source of pH dependent charge. Si (as silicic acid H4SiO4) is weathered and leached. 2:1’s become 1:1’s become  Fe-Al oxides Where does the negative charge come from in an acid soil? 2. Organic Matter Most soils have a net negative charge due to negative charges on layer silicates and organic matter, however, some highly weathered soils dominated by allophane and
  74. 74. The Colloidal Fraction: Seat of Soil Chemical and Physical Activity Some of the many types: • Layer silicate clays • Iron and Aluminum oxide clays • Organic soil colloids: humus Colloids are small particles in soil that act like banks: managing the exchange of nutrient currency in the soil Different soils, like checking accounts, have different capacities to hold nutrient currency: cations and anions
  75. 75. Problem Soils in South and Southeast Asia (Adapted from Ponnamperuma and Bandyopadhya 1980) Ion Hydrated radii in nm* Al3+ 0.90 Ca2+ 0.60 K+ 0.30 Cl- 0.30 H+ 0.90 *From Lindsay (1979)
  76. 76. Surface charge on soil colloids is developed in two ways: isomorphic substitution (permanent charge) and deprotonation of surface functional groups (pH dependent charge). pH dependent charge occurs on the edges of layer silicates, on variable charge minerals such as oxides of Fe and Al, and organic matter. It is called pH dependent charge because it increases in magnitude as the pH of the aqueous soil environment increases. Most of the pH dependent charge associated with agricultural soils is due to the deprotonation of organic functional groups. As the pH of the soil environment increases weak acid functional groups such as the carboxylic acid donate a proton and generate negative charge: COOH + OH- = COO- + H2O One practical way to increase the CEC of agricultural soils is to increase the organic matter content through tillage practices and increase the pH by adding lime.
  77. 77. Isomorphic substitution is the replacement of one atom by another of similar size in a crystal lattice without disrupting or changing the crystal structure of the mineral. If you remember from the clay and primary mineral tutorials, cations are coordinated to oxygen or hydroxyl anions in mineral structures. The negative charge of the anions is balanced by the positive charge of the cations that are coordinated to it. Net negative charge is developed when a cation of similar size and less positive charge substitutes for one of higher positive charge. Isomorphic substitution can also take place between cations of the same charge or a cation of higher positive charge. In the case of isomorphic substitution between cations of the same charge no charge is developed. In the case of isomorphic substitution between a cation of higher positive charge with one of lower positive charge a net positive charge is developed. The important thing to remember is that isomorphic substitution only occurs between cations of similar ionic radii. In the tutorials below we will be strictly dealing with permanent charge developed through isomorphic susbtsitution.
  78. 78. In clogged environments oxygen is present at lower concentrations and is quickly exhausted by the respiration of soil micro-organisms and plant roots (Prade et al., 1990). With the exhaustion of the oxygen, NO3 -, Mn4+, Fe3+, and SO42- type elements can act as electron receivers for microbial respiration and hence be found in reduced form in flooded rice fields. Oxygen and nitrates are used in the early stages of the flooding. Reduction of manganese (Mn4+) also occurs quickly, since soil manganese content isusually low (Ponnamperuma, 1977). It is some days after flooding, when the redox potential of the environment becomes lower than 180-150 mV, that the reduction of the ferric iron Fe3+ starts (Patrick and Reddy, 1978). Under the reductive effect in this clogged environment the ferric ion Fe3+ is reduced into ferrous ion (Fe2+), moreover the bacteria present in the environment begin anaerobic respiration that releases high quantities of ferrous ions (Fe2+); in the solution
  79. 79. Potentiometric titration curve of H- and Albentonite (a smectitic clay). Coleman and Harward (1953). One of the most useful ways to characterize soil acidity is to use potentiometric and conductometric titration analyses. With potentiometric titration analysis, one is investigating the relationship between pH and the amount of base added (Fig. 9.10). The first inflection point or break in the titration curve is ascribed to the end of the neutralization of exchangeable H+ and the beginning of the titration of exchangeable Al3+.
  80. 80. CEC and pH high Na+ binds loosely, exchanges readily CEC H+ binds tightly, doesn’t exchange low 3 Soil pH 8