Lime

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Lime What is lime? (Or liming material) Lime is a material that is used in acid soils to raise the pH and eliminate the adverse effects on plant growth and make the soil condition favorable for plant growth. In strict chemical term, lime is a calcium oxide (CaO). But, in practical term, a material containing the carbonates, oxides and/or hydroxides of calcium and magnesium used to neutralize soil acidity is known as lime. # liming in the addition of lime materials in….. • When & why liming materials are used? Lime can be applied at any time between the harvest of one crop & the planting of the next. The major constituents are the availability of the lime & convenience of spreading. Lime is usually broadcasted on the soil surface before tillage operations so that the soil & lime are mixed to increase soil & lime contact. Strongly acid soils are not productive for most crops. To increase the productivity of acid soils, the addition of lime is essential for the following reasons: i. The addition of lime rises the soil pH ii. By the addition of lime, the problems of acid soil, i.e. Al, Mn toxicity, & Ca, Mg deficiency etc are mostly overcome iii. Lime monitors the physiological balance of plant nutrients in the soil. iv. Lime increases the activities of N-fixing bacteria which increases the availability of nitrogen. v. Beneficial soil bacteria are encouraged by adequate supplies of lime in the soil. vi. Lime reduces the loss of nitrogen from soils. vii. A good liming program improves the physical condition of the soil by decreasing its bulk density, increasing its infiltration capacity & increasing its rate of percolation of water. viii. Liming improves soil structure by increasing microbial activities.

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Liming influences root distribution of plants & plant can distribute their roots in necessary zones to collect necessary nutrients. x. Lime reduces the uptake of heavy metals such as Cd, Pb, and Ni etc. xi. Lime increases the availability of nitrogen, phosphorus & sulfur by hastening the decomposition of organic matter. xii. Lime adds essential calcium to acid soils for greater plant growth. xiii. Lime makes P more available. In acid soils, Fe & Al phosphates are relatively insoluble. Liming reduces the solubility of the Fe & Al & therefore, less P is held in these slowly soluble & relatively unavailable forms. xiv. Lime makes K more efficient in plant nutrition. When K is plentiful, all plants adsorb more K than they need. Lime reduces the excessive uptake of K as plants uptake more Ca than K. xv. Lime furnishes Ca & mg (if the lime is dolomite) for plant nutrition. xvi. There is less soil erosion following an adequate liming program. • What is liming material? Mention the agricultural liming materials generally used in soil. Liming materials: The materials that are used in acid soils to raise the pH & eliminate the adverse effects on plant growth and make the soil condition favorable for plant growth are known as liming materials. More than 90% of the agricultural lime used is calcium carbonate (CaCO3), some are calcium & magnesium carbonate and a much smaller quantity is calcium oxide or calcium hydroxide. To a chemist, lime is calcium oxide. But, to a farmer, an agronomist, and a soil scientist, lime means calcium carbonate equivalent. Name of the liming materials: Soil acidity is commonly decreased by adding carbonates, oxides or hydroxides of Ca & Mg, compounds that are referred to as agricultural limes. The common agricultural liming materials are stated below: ix.

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Calcium oxide: Calcium oxide (CaO) is the only material to which the term lime may be correctly applied. Commercially it is also known as unslacked lime, burned lime or quick-lime, or often simply as the ‘oxide’. CaO is a white powder, shipped in paper bags because of its caustic properties. It is manufactured by roasting/heating limestone (CaCO3) in a furnace, driving of the CO2 CaCO3(Calcite) + Heat CaO + CO2 CaMg(CO3)2 (Dolomite) + Heat CaO + MgO +2CO2 CaO produced in this method varies with its chemical guarantee. The obtained CaO is about 95% pure, but purity ranges from 85-98%, depending on the source of liming material. CaO is the most effective of all liming materials, with a neutralizing value or calcium carbonate equivalent (CCE) of 179%, compared to pure CaCO3. Oxide of lime is considerably more costly than limestone. It is also considerably more caustic then limestone and consequently is difficult to handle, but it reacts much more rapidly with the soil than does limestone. Complete mixing of CaO with the soil may be difficult, because immediately after application, adsorbed water causes the material to form flakes or granules. These granules may harden due to CaCO3 formation on their surfaces, which can remain in the soil for long periods of time. Calcium Hydroxide: Calcium hydroxide [CaOH)2] is commonly referred to as slacked lime, hydrated lime or builder’s lime, because it is produced by adding water to burned lime. The reaction is – CaO + MgO + 2H2O Ca(OH)2 + Mg(OH)2 It is a white powder and is more caustic than burned lime. Like the oxide, it also requires bagging, preferably in waterproof bags. It is used where a rapid rate of reaction is desired and/or where a high soil pH is necessary. Like burned lime, hydrated lime is quite expensive compared to limestone, and its use is confined largely to home gardens and specially crops. Representative samples of hydrated lime are generally about 95% calcium & magnesium hydroxide. It has neutralizing value(CCE) of 136.

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Calcium & calcium-magnesium carbonates: Calcium carbonate(CaCO3) or calcite and calciummagnesium carbonate[CaMg(CO3)2] or dolomite are common liming materials. These materials occur in varying proportions in limestone. When little or no dolomite is present, the limestone is referred to as calcitic. As the magnesium increases, this grades into dolomitic limestone. Finally, if the stone is almost entirely composed of calcium-magnesium carbonate and impurities, the term dolomite is used. Most of the crushed limestone on the market today is calcite and/or dolomite. Limestone is most often mined by open-pit methods. The quality of crystalline limestone depends on the degree of impurities they contain, such as clay. The neutralizing values range from 65 to a little more than 100%. The neutrality value of pure CaCO3 has been theoretically established at 100%, while pure dolomite has a neutralized value of 109%. As a general rule, however, the CEC of most agricultural limestone is between 90 and 98% because of impurities. The average total carbonate level of the representative crushed limestone is about 94%. The carbonates are preferable to CaO or Ca (OH)2 because of their slow reaction. They are also less expensive than Ca(OH)2 or CaO. Marl: Marls are soft, unconsolidated deposits of CaCO3, frequently mixed with earth and usually quite moist. Marl deposits are generally thin, recovered by dragline or power shovel after the overburden has been removed. The fresh material is stockpiled and allowed to dry before being applied to the land. Marls are almost always low in Mg, and their neutralized value lies between 70 and 90%, depending on their clay content. Chalk: Chalk(CaCO3) is resulted from soft limestone deposited long ago in oceans. Marl(CaCO3), from the bottom of small ponds in areas where the soils are high in lime. The lime is accumulated by precipitation from drainage waters high in lime. Some marls contain many shell remains from marine animals.

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Slags: Blast furnace slag(CaSiO3) is a by-product of the manufacture of pig iron. In the reduction of Fe, the CaCO3 in the charge losses its CO2 and forms CaO, which combines with the molten Si to produce a slag that is either air-cooled or quenched with water. Silicic acid, formed when slag is added to acid soils, is quickly dissociated; thus, the pH of the soil is raised. The neutralized value of blast furnace slags ranges from about 75% to 90%, and they usually contain appreciable amounts of Mg. Basic slag is a by-product of the open-hearth method of making steel from pig-iron, which in turn, is produced from high-P-Fe ores. The impurities in the Fe, including Si & P, are removed with lime. In addition to its P content, basic slag has a neutralized value of about 6070%. It is generally applied for its P content rather than as liming material, but because of its neutralized value it is a good material to use on low-P, acid soils. Electric-furnace slags are produced from the electricfurnace reduction of phosphate rock in the preparation of elemental P and in the manufacture of pig iron & steel. The slag is formed when the Si and CaO fuse, producing Ca silicate. The electric-furnace slag contains 0.9 to 2.3% P2O5, and the neutralized value ranges from 65 to 80%. Miscellaneous liming materials: Other materials that are used as liming agents in localized areas close to their source includei. Ground oyster shell ii. Wood ashes iii. Bone meal (Ca3(PO4)2) iv. By-product lime resulting from peppermills, sugar beat plants, tameries and water-softening plants v. Fly ash from coal-burning power generating plants vi. Sludge from water treatment plants vii. Cottrell lime or flue dust from cement manufacturing viii. Sugar lime ix. Carbide lime x. Pulp mill lime xi. Acetylene lime xii. Packing house lime, and so on. These miscellaneous liming materials contain varying amounts of Ca & Mg.

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OR, The common liming materials are: 1. Calcic limestone (CaCO3), which is ground limestone. 2. Dolomitic limestone [CaMg(CO3)2] from high in Mg. 3. Quick lime (CaO), which is burned lime 4. Hydrated (slacked) lime [Ca(OH)2], from quicklime that has changed to the hydroxide form as a result of reactions with water. 5. Marl(CaCO3), from the bottom of small ponds in areas where the soils are high in lime. The lime has accumulated by precipitation from drainage waters high in lime. Some marls contain many shell remains from marine animals. 6. Chalk(CaCO3), resulting from soft limestone. 7. Blast-furnace slag(CaSiO3 & CaSiO4), a by-product of the iron industry. Some slags contain phosphorus & a mixture of CaO and Ca(OH)2. This product is called ‘basic slag’ and is used primarily for its phosphorus content. 8. Miscellaneous sources, such as ground oyster shell, wood ashes and by-product lime resulting from peppermills, sugar beat plants, tanneries, and watersoftening plants. [ Gypsum (CaSo4) is sometimes added to soil to supply calcium, but it has no influence on soil pH & therefore is not considered to be a liming material. Reaction of lime in the soil: When liming materials are added to a soil the calcium & magnesium compounds react with CO2 and with the acid colloidal complex. Reaction with Carbon-dioxide: When applied to an acid soil, all liming material whether the oxide, hydroxide or carbonate reacts with CO2 and H2O to yield the bicarbonate form. The CO2 partial pressure in the soil, usually several hundred times greater than that in atmospheric air, is generally high enough to drive such reaction to the right. For example: CO2 + H2O + 2CO2 Ca(HCO3)2 Ca(OH)2 + 2CO2 Ca(HCO3)2 CaCO3 + H2O +CO2

Ca(HCO3)2

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Reaction with soil colloids: All liming materials will react with acid soils, the calcium & magnesium replacing hydrogen & aluminium on the colloidal complex. The adsorption with respect to calcium may be indicated as follows, assuming hydrogen ions are replaced. H+ Micelle +Ca (OH)2 Ca²+ Micelle +2H2O H+ H+

Micelle

+Ca (HCO3)2

Ca²+

Micelle

+2H2O

H+ (in solution)

H+ Micelle H+

+CaCO3

Ca²+ H2O +CO2

(solid phase)

As these reactions proceed, CO2 is freely involved. In addition, the adsorption of the calcium and magnesium ions raises the percentage base saturation of the colloidal complex, and the pH of the soil solution increases correspondingly. Similarly, Al³+, adsorbed on colloidal surface, are also replaced by Ca²+ and thereby precipitated. 2Al-soil + 3CaCO3 + 6HOH H+ Clay or humus

+ 2Ca²+ + 2HCO3ˉ

3Ca-soil +2Al (OH)3 +3H2CO3 Ca²+ Clay or

c humus

Al³+ Clay or humus

+ Al (OH)3 + 2CO2 Ca²+

H+ + 2CaCO3 + H2O

CaMg (CO3)2 +2H2O +2CO2

Clay or humus

Ca++ + Al (OH)3 + 2CO2

Ca + 2HCO3ˉ+Mg +2HCO3ˉ

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(Dolomitic limestone) The Ca & Mg bicarbonates are much more soluble than are the carbonates, so the bicarbonates formed is quite reactive with the exchangeable & residual acidity in acid soils.[In both cases the Ca++ & Mg++ replace H+ & Al³+ on the colloidal complex]. *Explain the mechanism by which liming materials increase the pH of soils. Liming reactions begin with the neutralization of H+ in the soil solution by either OHˉor HCO3ˉoriginating from the liming materials. For example, CaCO3 behaves as follows: CaCO3 + H2O Ca++ + HCO3ˉ+OHˉ The rate of the reaction is directly related to the rate at which the OHˉions are removed from soil solution. As long as sufficient H+ ions are in the soil solution, Ca++ & HCO3ˉwill continue to go into solution. When the H+ ion concentration is lowered, formation of the Ca++ and HCO3ˉions is reduced. The continued removal of H+ from the soil solution will ultimately result in the precipitation of Al³+ and Fe³+ as Al (OH)3 and Fe(OH)3 and their replacement on the CEC with Ca²+ and/or Mg²+. The overall reaction for neutralization of Al-derived soil acidity can be written as follows: K+ Al³+ Ca²+ Ca++ Ca++ Clay Mg++ + 3CaCO3 +3 H2O Clay Mg²+ + 2Al (OH)3 K+ Ca²+ +3CO2 Al³+ Ca²+ As this reaction proceeds, the adsorption of the Ca++ & Mg++ ions raises the percentage base saturation of the colloidal complex & the pH of the soil solution increases correspondingly.

• State the effects of over liming on soil properties and plant growth:

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Over liming is the addition of lime until the soil pH is above that required for optimum plant growth. Under such condition many crops that ordinarily respond to lime are detrimentally affected. The detrimental effects of over liming are as follows: 1. Fe, Mn, Cu, Zn, Mo & other micronutrient availability may be reduced due to over liming & the deficiency of these elements occurs. 2. If liming materials provide large amounts of Ca & Mg, they may react with soluble & available phosphate (PO4³ˉ) & form Ca3(PO4)2 and Mg3(PO4)2 which are insoluble. So, at very high pH, plant suffers from P-deficiency. 3. Uptake & utilization of K & B may be reduced. 4. Exerts adverse effects on microbial activity. 5. pH increases and more hydroxyl (OHˉ) groups are formed, which affect the growth and structure of plant roots. 6. Buffer capacity of the soil is hampered. 7. The adsorption of P by plants and especially its metabolic use may be restricted. 8. yield production & soil structure are hampered. Over liming injury may be reduced by the application of large amounts of manure, green-manure crops, compost, phosphorus fertilizers, boron or a mixture of minor elements. Over liming injury however is not very common. • Why CaCO3 is used as lime instead of CaO? 1. If CaO is added to the soil, it reacts quickly with soil water & produce heat, which brings out drastic changes. Subsequently, microbial activity is affected & soil organic matter is destroyed. 2. Besides, complete mixing of CaO with the soil may be difficult, because immediately after application adsorbed water causes the material to form flakes or granules. 3. Moreover, CaO is considerably more costly & more caustic than CaCO3 & consequently is difficult to handle. On the other hand, when CaCO3 is added to soil, it reacts slowly & not generates so much heat. Therefore, microbial activity & soil organic matter are not affected. In contrast to CaO, CaCO3 is cheap & less caustic. For the above reasons, CaCO3 is used as lime instead of CaO. • Liming favors the microbial activity-explain.

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Microbial activity in soils can be favored by liming because: 1. Liming stimulates heterotrophic organisms due to the provision of suitable pH. Below the pH 5.5, only some fungal functions are observed in soil. But, above pH 6.0 huge bacteria, some fungi & some algae perform vary well. This helps their soil properties to maintain optimum level. 2. Liming enhances humus formation. Microorganisms, by both decomposition & synthesis, take part in humus formation. The formed humus then has large cation exchange property to retain nutrients. 3. Microorganisms improve the biological nitrogen fixation through providing optimum pH by liming. Bacteria, blue-green algae & some fungi fix atmospheric N2 by both symbiotically & nonsymbiotically. Among symbiotic fixer the Rhizobium genera has nine species. Only one of these namely R. japonium prefer low pH (<5) & from nodule within 6 days with legume species. Except this one, other 8 species except pH above 6 for adequate nodulation. Within the non-symbiotic N-fixer, the general pH preference is 6-7. Blue-green algae prefer pH above 7 to fix N2 freely. 4. Liming increase the nitrification process. A large variety of microorganisms are involved in proteolysis & ammonification process whose end product is NH4+. Most of the organisms responsible for the conversion of NH4+ to NO3ˉ, require large amount of Ca. therefore, nitrification is enhanced by liming to a pH of 5.5 to 6.5. The chemoautotrophic Nitrosomonas sp. Convert NH4+ to NO2ˉ and grow well in Ph above 6.0. Only two species of Nitrosomonas are acid tolerant; convert NH4+ even of pH value 4.0. But, other majority of Nitrosomonas is acid sensitive. Protein A Peptone A Amino acid NH4+ B NOC2+H+ C NO3ˉ In all agricultural pH pH>6 pH<6 Where, A= proteolytic activity B= Ammonification C= Nitrification 5. Some pathogenic activity is retarded due to liming (e.g. potato scab). * Liming of soil is not always favorable for phosphorus availabilityexplain:

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liming is the addition of liming materials in acid soils to raise the pH & eliminate the adverse effects on plant growth and make the soil condition favorable for plant growth.. more than 90% of the agricultural lime used is CaCO3, some are Ca & Mg carbonate and a much smaller quantity is CaO or Ca (OH)2. From pH 3 to 7, the effectiveness of hydrated iron to precipitate soluble phosphate, remains very high, but it decreases rapidly from pH 7 to 7.5. Hydrated Al is, however, highly effective in precipitating phosphate from pH 3.5 to 9.0. Approximately, 90% of the phosphate will be fixed by Al at pH 6.5, and 70% at pH 9.0. this indicates that even with alkaline & calcareous soils, Al is a serious fixer of soluble phosphate. Less than 10% of the phosphate precipitated by Fe& Al at pH 4.0 would be solubilized by increasing the pH to 6.0. Lime adds Ca & Mg to acid soils. At very high pH PO4³ˉ ions react with Ca & Mg and form insoluble Ca3(PO)4 & Mg3(PO4)2. Thus, phosphate availability may decrease because of the formation of complex & insoluble Ca & Mg compounds. [ Thus, it would appear that the direct effect on phosphate availability produced by liming acid soils is probably less than the indirect effect produced by creating more effective conditions for increased production of plant residues and improved micro organic activity.] • Importance of liming in agriculture: Strongly acid soils are not productive for most crops. To increase the pH of acid soil, the addition of lime is essential. Lime is seldom needed in low-rainfall areas where leaching is minimal. Crop responses from the application of lime are usually attributed to decreased toxicity of Al³+, although the plant nutrient value of the Ca or Mg also is important. The importance of liming in agriculture can be discussed in two ways1. Direct benefits & 2. Indirect benefits. Direct benefits: Al toxicity is probably the most important growth limiting factor in many acid soils, particularly when pH<5.0 to 5.5. Excess Al interferes with cell division in plant roots; inhibits nodule initiation; fixes P in less available forms in soil & in or on plant

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roots, decreases root respiration; interferes with enzymes governing the deposition of polysaccharides in cell wall rigidity by cross-limiting with pectin; and interferes with the uptake, transport and use of nutrients and water by plants. When lime is added to acid soils, the activity of Al³+ is reduced by precipitation as Al (OH)3. The lime treatment raises soil pH while greatly reducing the level of extractable Al. not only is Al³+ in the soil solution also restricts the plant uptake of Ca and Mg. At pH 4.5 or less, another benefit is the removal of H+ toxicity, which damages root membranes and also is determined to the growth of many beneficial bacteria. The greatest single direct benefit of liming many acid soils is the reduction in the acidity or solubility of Al and Mn. Indirect benefits: 1. Effect on P availability: At low pH values and on soils high in Al and Fe, P precipitates as insoluble Fe/Al-P compounds. Liming acid soils will precipitate Fe & Al as Fe (OH)3 and Al(OH)3, thus increasing plant-available P. Alternatively, liming soils to pH 6.8 to 7.0 can reduce P availability because of the precipitation of Ca or Mg phosphates. [A liming program should be planned so that the pH can be kept between 5.5 & 6.8 to 7.0 if maximum benefit is to be derived from the applied P.] 2. Micronutrient availability: With the exception of Mo, the availability of the micronutrients increases with decreased pH. This can be detrimental because of the toxic nature of many micronutrients even at relatively low solution concentrations. The addition of adequate lime reduces the solution concentration of many micronutrients, and soil pH values of 5.6 to 6.0 are usually sufficient to minimize toxicity while maintaining adequate availability of micronutrients. Mo nutrition of crops is improved by liming, and deficiencies are infrequent in those soils limed to pH values in excess of 7.0. Because of the effect on availability of other micronutrients, liming to this value or above is not normally recommended for most crops in humid areas. 3. Nitrification: Most of the organisms responsible for the conversion of NH4+ to NO3ˉrequire large amounts of Ca. therefore, nitrification is enhanced by liming to a pH of 5.5 to 6.5. Decomposition of plant residues and breakdown of soil organic matter are also faster in this ph range than in more acidic soils.

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4. N-fixation: Symbiotic & non-symbiotic N2 fixation is favored by adequate liming. Activity of some Rhizobia species is greatly restricted by soil pH levels below 6.0, thus liming will increase the growth of legumes because of increased N2 fixation. With the non-symbiotic N2-fixing organisms, N2fixation increases in adequately limed soils, which increases the degradation of crop residues. 5. Soil physical condition: The structure of fine-textured soils may be improved by liming, as a result of an increase in the organic matter content and of the flocculation of Ca-saturated clay. Favorable effects of lime on soil structure include reduced soil crusting, better emergence of small-seeded crops, and lower power requirements for tillage operations. Ca also improves the physical conditions of sodic soils. Increased electrolyte concentration due to CaCO3 dissolution is responsible for preventing clay dispersion and decreases in hydraulic conductivity of such soils. 6. Disease: Correction of soil acidity by liming may have a significant role in the control of certain plant pathogens. Club root is a disease of cole crops that produces yields and causes the infected roots to enlarge and become distorted. Lime does not directly affect the club root organism, but at soil pH greater than 7.0, germination of club root spores is inhibited. [On the other hand, liming will increase the incidence of diseases such as scab in root crops.] Chemical guarantee of lime: The chemical composition liming materials affects the rate of reaction of these compounds with soils. The chemical composition of limestone determines their long-term effects on soil pH. The effects of liming are so important that they have been recognized in laws governing the scale of liming materials. These laws require guarantees as to the chemical composition of limes, the composition usually being listed in terms of one or more of the following: 1. Content of elemental Ca & Mg 2. Conventional oxide content (percentages of CaO & MgO) 3. CaO equivalent (neutralizing ability of all compounds expressed in terms of CaO).

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4. Total carbonates (used for limestone, the sum of Calcite & dolomite) 5. Calcium carbonates equivalent or total neutralizing power (neutralizing ability of all compounds expressed in terms of calcium carbonate). These points are described in below: (the calculation of the neutralizing value of liming materials) 1. Content of elemental Ca & mg: The composition of liming materials is sometimes expressed in terms of the Ca and mg content of the pure mineral. For example, pure CaCO3 contains 40% Ca. the calcification would be % Ca= Atomic wt. of ca ×100=40×100=40% Ca equivalent molecular wt of CaCO3 100 Similarly, pure MgCO3 contains 28.6% Mg, calculated by the ration of molecular weights: 24 gm/m Mg ×100= 28.6% 84g/m MgCO3 To convert % Ca to CCE (calcium carbonate equivalent); multiply by 100/40 or 2.5 [100 g CaCO3 contains 40g Ca, so 1 gm Ca is contained by 100/40= 2.5 gm CaCO3] and to convert % Mg to MgCO3, multiply by 84/24 or 3.5[ 84g MgCO3 contains 24g Mg, so, 1gm Mg is contained by 84/24=3.5 gm MgCO3.] 2. Ca & Mg oxide content: The quality of limestone is also expressed by its Ca or Mg oxide equivalent. For example, Pure CaCO3 contains 40% Ca. CaO has a molecular weight of 56, which means that 16g of o is combined with 40g of Ca. therefore, if the Ca in CaCO3 were expressed as the oxide, it would contain (56/100)×100, or 56% CaO equivalent. CCE of pure CaO CaCO3/CaO =100/56=1.786 100g CaCO3 contain 56g CaO 1g … ….. 56/100g CaO %= (56/100) ×100=56% Thus, to convert % Ca to % CaO, multiply the Ca by 56/40 or1.4; and to convert % CaCO3 to % CaO, multiply the % CaCO3 by 56/100. Or 0.56. [Similar figures may be derived for the Mg-containing limestone.] CaCO3 equivalent of CaO = molecular wt. of CaCO3 = 100/56 Molecular wt. of CaO

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The CaCO3 equivalent of CaO can be obtained by multiplying the amount of pure CaO by 1.786 or 100/56. Then, 100 Kg of pure CaO has a CaCO3 equivalent of 100×1.786= 178.6. If the burned lime (CaO) is only 95% pure, 100 Kg would supply only 95 Kg of CaO so that 95% burned lime would have a CaCO3 equivalency of 95×1.786= 169.6. 3. CaO equivalent: If pure CaCO3 were converted to its CaO equivalent, the calculation would be: % CaO equivalent = 56/100 ×100 =56% CaO eq. 56g CaO neutralize the same amount of acid as does 100g of CaCO3. 4. Total carbonates: The quality of limestone also can be related to the total CO3²ˉand is the sum of the %CO3 contained in a given liming material. For example, assume that a limestone contains 78% CaCO3 & 12% MgCO3. The total CO3 content would be 90%. 5. CaCO3 equivalent(total neutralizing power): The value of a liming material depends on the quantity of acid that a unit weight will neutralize, which, in turn is related to the molecular composition and purity. Pure CaCO3 is the standard against which other liming materials are measured, and is neutralizing value is considered to be 100%. The CCE is defined as the acid neutralizing capacity of agricultural liming material expressed as a weight percentage of CaCO3. The process of calculating CCE is % CaCO3 equivalent= molecular weight of CaCO3 ×100 Molecular weight of substance Let us consider the following reactions: CaCO3 + 2H+ Ca++ + H2O +CO2 MgCO3+2H+ Mg²+ + H2O +CO2 In the above reactions, each liming material (CaCO3, MgCO3 & CaO) neutralizes the same acidity (2H+) in each case. Hence their molecular weights are equivalent to each other. 100g CaCO3= 84g MgCO3=56g CaO Now, CaCO3 equivalent of chemically pure CaO: %CCE=Molecular wt. of CaCO3 × 100 Molecular wt. of CaO = 100/56 × 100 =178.6 i.e., 178.6% CaCO3 equivalent.

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Factors determining the selection of a liming program: Intended group: Plants differ widely in their sensitivity to soil acidity and thus to added lime. The type of crop to be grown is the most important factor to consider in developing a lime program. Soil texture and organic matter content: In a coarse-textured, low organic matter soil, the lime requirement will be less than for a fine-textured or high organic matter soil. •

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Time and frequency of liming applications: For relations that include leguminous crops, lime should be applied 3 to 6 month before the lime of seeding; this is particularly important on very acid soils. Lime may not have adequate time to react with the soil if applied just before seeding. The caustic forms of lime [CaO & Ca (OH)2] should be spread well before planting to prevent injury to germinating seeds. The frequency of application generally depends on the texture of the soil, N source & rate, crop removal, precipitation patterns, and lime rate. On sandy soils, frequent light applications are preferable, whereas in fine textured soils, larger amounts may be applied less often. Finely divided lime reacts more quickly, but its effect is maintained over a shorter period than that of coarse materials. The most satisfactory means of determining re-liming needs is by soil tests. Samples should be taken every 3 years. Depth of tillage: Lime recommendations are made on the basis of a 6 inch furrow slice. When land is tilled to a depth of 10 inch, the lime recommendations should be increased by 50%. Lime requirement: The lime requirement is the amount of lime that must be applied to acid soil to change soil pH from its present value to any desired value. This value is usually the range of pH 6.0-7.0, since this is an easily attainable value within the optimum growth range of most crops, plants. The amount of liming material required to bring about a desired pH change is determined by several factors, includinga. the change in pH required b. the buffer capacity of the soil c. the chemical composition of the liming materials The determination of lime requirement can be made by using two methods: A. Buffer curve method B. Incubation with liming material (lab exp.) CaCO3 is the most common agricultural lime, but its incubation in the lab is too much time consuming to obtain relatively quick determination in lab, Ca (OH)2 is generally used for incubation experiment & converts to amount in CaCO3. Materials: i. Soil (suppose pH 3.6); ii. Apparatus: a) 8 Beaker (50 ml); b) pH meter; c) Stirring rod; d) electric balance; e) Paraffin; iii. Reagents: a) Ca(OH)2; b) Standard buffer solution of pH 4 & 7; c) Distilled water.

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Procedure: 1. 20 gm of soil sample is taken in each of the 8 beakers of 50 ml. 2. Then 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14 gm of Ca(OH)2 is added to the beakers one after another. One beaker is considered as blank Ca(OH)2 is not added. 3. Soil & Ca(OH)2 is mixed well by stirring rod. Thereafter, 6ml of water is added to each beaker and the suspension is mixed thoroughly with glass rod. 4. Then the top of the beakers are well packed with Para films and left untouched for 1 month. 5. After, 1 month, the pH values are measured with the help of the pH meter and a curve is drawn with the obtained values against the amount of Ca(OH)2 used. Calculation: Suppose, the soil has to make of pH 7.0, then the amount of Ca(OH)2 is got 0.06 ppm/0.06g. So, 20gm soil require 0.06gm Ca(OH)2 100gm … ….

3gm Ca(OH)2

Now, 1Kg soil require 3gm Ca(OH)2 6 2×10 soil require 6 tons Ca (OH)2 /ha soil CaCO3 =100 & Ca(OH)2=74 74 tons Ca(OH)2 = 1000 ton CaCO3 6 tons Ca(OH)2 = 8.108 tons CaCO3/ ha soil