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350 LIMING
LIMING E J Kamprath and T J Smyth, North Carolina State University, Raleigh, NC, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction Approximately 40% of the arable land in the world is naturally acid. The naturally acid soils have developed where rainfall exceeds evapotranspiration. Acid soils have also developed with intensive agriculture. The acidity of soils can be divided into that associated with the soil solution, exchangeable, and nonexchangeable forms. The poor growth of plants on acid soils is due to toxicities of Al, Mn, and H, and deficiencies of Ca, Mg, and Mo. Liming of acid soils increases plant growth due to neutralization of toxic elements, supplying Ca and/or Mg and increasing availability of Mo and P. Determination of the lime requirement for a soil can be based on the amount needed to adjust pH to a desired level, neutralization of exchangeable Al, or the amount estimated by the buffer pH method. Selection of a liming material has to take into account the neutralization value of the lime, fineness of the material, and the need for Mg.
Development of Acid Soils In nature acid soils are found where rainfall exceeds evapotranspiration. Several processes contribute to soil acidification under these conditions. The excess water moving through the soil carries Ca2þ and Mg2þ which balance the negative charge of soluble anions like NO 3 and Cl . Leaching of Ca and Mg in conjunction with soluble anions decreases the percentage of exchange sites occupied by basic cations. The sites vacated by Ca and Mg are initially replaced with Hþ supplied by decomposition of organic matter and residues. The Hþ-clays then decompose to form Al-clays. With intensive agriculture Hþ is also produced by the conversion of ammonium to nitrate: þ NHþ 4 þ 2O2 ! NO3 þ H2 O þ 2H
½1
The application of 100 kg N ha1 in the ammonium form can require a maximum of 360 kg CaCO3 ha1 to neutralize the acidity produced by the conversion þ of NHþ 4 to NO3 . Plants also release H from roots when their uptake of basic cations exceeds that of anions. Since legumes fix atmospheric N and have relatively high concentrations of Ca, cation uptake exceeds anion uptake. To maintain internal plant electroneutrality legume roots excrete Hþ which acidifies
the rhizosphere. Neutralization of acidity produced in one growing season of N-fixing legumes can correspond to 30 – 400 kg CaCO3 ha1. Another source of Hþ is acid precipitation produced upon burning fossil fuels containing sulfur compounds which give rise to SO2. The SO2 reacts with water in the atmosphere to produce sulfuric acid.
Nature of Soil Acidity Soil Solution Acidity
The solutions of acid soils contain two ions that are detrimental to plant growth, hydrogen and aluminum. The concentration of Al3þ in the soil solution increases with the proportion of cation exchange sites which are occupied with Al (i.e., the percentage Al saturation of the cation exchange capacity (CEC)). There is a marked increase in soil solution Al3þ concentration when the saturation is greater than 50%. The soil solution Hþ concentration is controlled by the hydrolysis of Al3þ, as shown by eqn [2]. Al3þ þ H2 O ! AlOH2þ þ Hþ
½2
Between pH values of 5.6 and 5.8 the soil solution Al3þ concentration is nil in mineral soils since the Al saturation is essentially zero. Organic soils with appreciable mineral matter have very low concentrations of soil solution Al3þ at pH values of 4.8–5.0, because Al is strongly bound to the carboxyl groups. High concentrations of soil solution Al3þ only occur in organic soils at pH values of 4.5 or lower. Exchangeable Acidity
Early concepts were that exchangeable Hþ was the source of soil acidity. Titration studies showed that acid clays behaved as weakly ionized acids, which led to the conclusion that acid soils contained exchangeable Hþ. Soils were assumed to be 100% basesaturated at pH values of 7 and the difference between the CEC at pH 7 and the sum of basic cations was the amount of exchangeable Hþ. Later studies revealed that the weakly ionized acid characteristic of clays was due to exchangeable Al and the hydrolysis of Al3þ was the source of soil solution Hþ. When saturated with Hþ the clays decomposed, releasing Al, which occupied the exchange sites. Exchangeable acidity is defined as that which is extracted with a neutral unbuffered salt solution such as KCl. Neutral unbuffered salt solutions must be used to avoid changing the soil pH and altering the
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Table 1 The exchangeable and nonexchangeable acidity of acid soils Acidity
Soil
pH
Organic matter (g kg1)
Typic Kandiudult Typic Paleaquult Aeric Paleaquult Typic Haplustox
4.5 4.7 4.5 4.5
20 27 40 –
Exchange Al (cmol kg1)
Al saturation (%)
Titratablea (cmol kg1)
Nonexchangeable cmol kg1
0.91 1.04 2.33 1.55
82 78 73 68
1.70 1.72 3.77 3.85
0.57 0.68 1.44 2.70
a Acidity titrated to pH 6. Data from Evans CE and Kamprath EJ (1970) Lime response as related to percent Al saturation, solution Al, and organic matter content. Soil Science Society of America Proceedings 34: 895–896. Kamprath EJ (1970) Exchangeable aluminum as a criterion for liming leached mineral soils. Soil Science Society of America Proceedings 34: 252–254. Gonzalez-Erico E, Kamprath EJ, Naderman GC, and Soares WV (1979) Effect of depth of lime incorporation on the growth of corn on an Oxisol of Central Brazil. Soil Science Society of America Journal 43: 1155–1158.
exchangeable acidity. In mineral soils the exchangeable acidity is Al3þ. At pH values of 5 and lower, over half of the active exchange sites are countered by Al3þ (Table 1). As previously discussed, exchangeable Al is essentially zero in mineral soils at pH 5.6 and the active cation exchange sites are 100% base-saturated.
Table 2 Soybean tap and lateral root length after 12 days of exposure to solutions with different pH and Ca concentrations Solution
pH
4.0
Nonexchangeable Acidity
Soils also contain H which is not extractable with neutral unbuffered salt solutions, but is titratable. The nonexchangeable H is associated with carboxyl groups of organic matter, hydroxy Al, and hydrated oxides of Fe and Al. The nonexchangeable acidity of the three Ultisols in Table 1 was primarily due to organic matter. However, with the Oxisol, the nonexchangeable acidity was associated with the hydrated oxides of Fe and Al. Once the exchangeable Al is neutralized the acid buffering capacity of the soil is a function of the nonexchangeable acidity. Neutralization of the nonexchangeable acidity has little effect per se on plant growth, but is the source of the pH-dependent CEC. With increasing neutralization of the nonexchangeable acidity, there is an increase in soil pH, CEC, and the retention of the basic cations Ca, Mg, and K.
Acid Soil Constraints to Plant Growth Hydrogen Toxicity
Poor plant growth in acid soils is usually associated with low soil pH, but the direct effects of Hþ are often confounded with changes in solubility of various elements affecting plant growth. Experiments in hydroponics, therefore, are often used to assess the direct effects of Hþ. Roots growing in solutions at pH 4.6 or lower have visual symptoms of injury, namely stunted growth, brownish color, and little lateral root development. Increasing solution Ca concentration alleviates root injury due to Hþ (Table 2). At low pH and
4.6 5.5
Root length Ca (mmol l 1)
Tap (cm per plant)
Lateral (cm per plant)
0.2 2.0 0.2 2.0 0.2 2.0
1.1 3.2 10.2 22.2 27.8 44.6
0.0 1.0 3.4 36.3 40.1 85.6
Data from Sanzonowicz C, Smyth TJ, and Israel DW (1998) Calcium alleviation of hydrogen and aluminum inhibition of soybean root extension from limed soil into acid subsurface solutions. Journal of Plant Nutrition 21: 785–804.
Ca concentrations root membranes are damaged, leading to loss of organic substrates and absorbed cations. Under soil conditions with pH below 4.6, root injury due to Hþ can be expected along with reduced plant uptake and possible deficiencies of Ca and Mg. Aluminum Toxicity
Aluminum toxicity is a major cause of poor plant growth in acid soils. Current evidence indicates that Al interferes with various root growth processes: disruption of regulatory signals in root cap cells and interference with cell division in root apices, enzyme activities, DNA replication, and P availability at membranes. The immediate and visible evidence of Al toxicity is a reduction in root length which, in turn, limits plant access to soil water and nutrients. Consequently, plants growing in soils with toxic levels of Al are underdeveloped, sensitive to drought stress, and often present symptoms of multiple nutrient deficiencies. Concentrations of soil solution Al that reduce crop growth are in the micromolar range. As illustrated in
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Figure 1 Relations between Al saturation of cation exchange sites and concentration of Al in the soil solution of Oxisols and Ultisols. Filled circles, Haplustox; open circles, Umbraquult; inverted triangles, Paleudult. Data source: Gonzalez-Erico E, Kamprath EJ, Naderman GC, and Soares WV (1979) Effect of depth of lime incorporation on the growth of corn on an Oxisol of Central Brazil. Soil Science Society of America Journal 43: 1155 –1158; and Jallah JK (1994) Assessment of Some Chemical Constraints to Root Growth in Four Acid North Carolina Soils. PhD thesis. Raleigh, NC: North Carolina State University.
Figure 1, the concentration of Al in soil solutions is related to the percentage of Al saturation of the cation exchange sites, with a marked increase in the Al concentration when the Al saturation exceeds 50%. Most field assessments of crop response to toxic levels of soil solution Al have been based on percentage of Al saturation as a proxy variable, due to its greater ease of measurement. Root length of corn, for example, is markedly reduced at percentage of Al saturation levels corresponding to high concentrations of Al in the soil solution (Figure 2). Tolerance to Al varies widely among species and even varieties of the same species (Table 3). Yields for most varieties of cotton, mung bean, pearl millet, and wheat are reduced by low levels of percentage of Al saturation, whereas cassava and cowpea tolerate high levels of soil Al. However, considerable differences in Al tolerance/susceptibility have been observed among varieties within a given species. For example, critical percentage of Al saturation levels among field trials with different varieties range from 0 to 40 with corn, 0 to 44 with soybean, and 0 to 60 with upland rice. Manganese Toxicity
Acid soils with high contents of Fe and Al oxides often contain large amounts of soluble Mn. Plant growth in such soils is reduced when large quantities of the soluble soil Mn accumulate in their tissues. Although Mn is an essential nutrient, excessive plant uptake is toxic to plants. Liming these soils to pH 5.5 or greater decreases the solubility of soil Mn, which
Figure 2 Effect of Al saturation on relative root length of fieldgrown corn in Oxisols and Ultisols. Filled circles, Umbraquult; open circles, Paleudult; inverted triangles, Haplustox. Data source: Gonzalez-Erico E, Kamprath EJ, Naderman GC, and Soares WV (1979) Effect of depth of lime incorporation on the growth of corn on an Oxisol of Central Brazil. Soil Science Society of America Journal 43: 1155 –1158; and Jallah JK (1994) Assessment of Some Chemical Constraints to Root Growth in Four Acid North Carolina Soils. PhD thesis. Raleigh, NC: North Carolina State
University.
Table 3 Threshold values of percentage Al saturation of the cation exchange capacity above which crop yield for various species is normally reduced by Al in mineral soils Crop
Critical % Al saturation
Cassava Corn Cotton Cowpea Mung bean Peanut Pearl millet Phaseolus bean Potato Sorghum Soybean Upland rice Wheat
75 30 0 60 0 40 0 15 20 15 15 40 0
Data from Osmond DL, Smyth TJ, Reid WS et al. (2002) Nutrient Management Support System, NuMaSS (version 2.0). Raleigh, NC: Soil Management Collaborative Research Support System, North Carolina State University.
reduces the uptake of excess Mn and increases plant growth. In many instances, improved plant growth upon liming soils with percentage of Al saturation below the critical level is associated with the alleviation of Mn toxicities. Calcium and Mg Deficiencies
Acid soils with high percentage of Al saturation have very low amounts of Ca and Mg on both the exchange sites and in the soil solution. Soils with very
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Figure 3 Root elongation of wheat as a function of Ca levels and sources in Ca-deficient Oxisols. Squares, lime; circles, CaCl2; triangles, Ca(H2PO4)2. Data source: Ritchey KD, Silva JE, and Sousa DMG (1983) Relac¸a˜o entre teor de calcio no solo e desenvolvimento de raizes avaliado por um metodo biologico. Revista Brasileira Ciencia Solo 7: 269–275.
low CEC have a limited capacity to retain Ca and Mg and supply of this nutrient may not be sufficient for normal plant growth. The immediate supply of Ca for continued growth of root tips is dependent on the soil solution concentration, because this nutrient is relatively immobile in plants. Root length of wheat seedlings, for example, was increased by over 50% when 0.1 cmol Ca kg1 was added to an Oxisol which initially contained 0.02 cmol Ca kg1 (Figure 3). Although it is often difficult to separate the extent to which a plant growth response to liming is due to correction of a Ca deficiency versus alleviation of an Al toxicity, the improved root growth of wheat was similar among various sources of Ca. Plant growth responses to additions of Mg are often associated with soil conditions where exchangeable Mg2þ and/or the proportion of cation exchange sites occupied by Mg2þ is very low.
Response to Liming A major response of plants to liming is due to the neutralization of H, Al, and Mn, which if present in relatively high concentrations in the soil solution are toxic to plant growth. Toxic concentrations of H generally do not occur in soils except in certain organic soils when pH values are less than 4.5. In acid mineral soils with pH values less than 5.5 liming removes the negative effect of H on absorption of Ca, Mg, and K. The response of plants to liming is mainly due to the neutralization of exchangeable Al and its replacement on the exchange sites with Ca and/or Mg. Liming of acid mineral soils to pH 5.6 reduces
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Figure 4 Relationship between soil pH and exchangeable Al for Coastal Plain soil. Data source: unpublished data of Soil Science Department North Carolina State University.
Figure 5 Relative yield of soybean (circles), corn (triangles), and cotton (squares) as related to Al saturation of the effective cation exchange capacity. Data source: E.J. Kamprath (unpublished data); and McCart GD and Kamprath EJ (1965) Supplying calcium and magnesium for cotton on sandy, low cation exchange soils. Agronomy Journal 57: 404 – 406.
exchangeable Al to essentially zero (Figure 4). This essentially eliminates Al from the soil solution and removes it as a factor limiting plant growth. As previously discussed, the concentration of Al in the soil solution is a function of the Al saturation of the active cation exchange sites. Since the Al saturation of a soil is much easier to determine than soil solution Al, it is an excellent criterion for determining the need to lime a given soil. Since crops and genotypes vary in their tolerance to Al, knowledge about the Al saturation of a given soil provides information whether a particular crop or genotype will require liming. An example of this is the differential Al tolerance of corn as compared to soybean and cotton in a greenhouse study (Figure 5). Corn showed little effect to Al until
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the Al saturation was greater than 50%, where there is a marked increase in soil solution Al. Relative grain yields of corn on three soils decreased sharply when Al saturation was greater than 40% (Figure 6), a value similar to that found for dry-matter production in the greenhouse. Organic soils and soils high in organic matter have a lower critical pH than most mineral soils. Maximum plant growth on acid organic soils is generally obtained at pH 5. On these soils the exchangeable Al is held on carboxyl sites and at pH 5 the Al is held quite strongly so there is very little Al in the soil solution. Liming of these soils to pH 5 provides adequate Ca to overcome the H effects on plant growth. Response to liming can also be obtained on acid soils with low Al saturation but that have high amounts of Mn. Soil with high contents of sesquioxides often contain appreciable amounts of Mn minerals and in these soils Mn toxicity is a problem when pH values
Figure 6 Relative grain yields of corn on three soils (filled circles, Goldsboro; open circles, Wharton; inverted triangles, Morrill) as related to exchangeable Al saturation of the effective cation exchange capacity. Data source: Alley MM (1981) Short-term chemical and crop yield responses to limestone applications. Agronomy Journal 73: 687– 689; and Fox RH (1979) Soil pH, aluminum saturation and corn grain yield. Soil Science 127: 330 – 334.
drop to 5 and less. Liming to pH 5.6 and above eliminates Mn toxicity on these soils by converting the soluble Mn2þ to the relatively insoluble oxides, Mn2O2 and MnO2. Response to liming due to supplying of Ca per se is not often the case. In most instances Ca saturations of 20–25% will supply adequate Ca for plant growth except where toxicities to Al, H, or Mn limit plant growth. There is a good relationship between Ca or Ca þ Mg saturation and plant growth, because these are the mirror image of Al saturation. On soils with very low CECs a minimum of 1 cmol Ca kg1 soil is needed for optimum growth of most plants. When toxicities are not a factor the percentage of Ca saturation of the CEC is a good indicator of Ca availability. Calcium concentrations of a given plant species are similar when grown on soils with the same Ca saturation even though the soils contain a wide range of exchangeable Ca (Table 4). This is because the Ca concentration of the soil solution is determined by the percentage of Ca saturation rather than the amount of exchangeable Ca. Calcium is transported to the root by mass flow of water and thus the amount arriving at the root surface is a function of the Ca concentration. Magnesium deficiencies on sandy soils are generally associated with pH values of less than 5. The concentration of Al3þ and Hþ at pH 5 in mineral soils is sufficient to inhibit Mg uptake by plants. Liming to neutralize Al and H is necessary for optimum uptake of Mg where soil reserves are adequate. On acid sandy soils response to liming is also related to the supply of Mg with the application of dolomitic lime. Availability of soil Mg is related more to the Mg saturation of the CEC rather than the amount of exchangeable Mg. Various studies have shown that Mg saturation needs to be in the range of 5–10% for optimum plant growth. Soybean and other legumes often show a response to increase in pH beyond that at which Al is neutralized. Raising the pH of mineral soils to 6 has
Table 4 Effect of Ca saturation of the effective cation exchange capacity (ECEC) on Ca availability Soil Ca
Soil
pH
ECEC saturation (%)
Exchangeable (cmolc kg1)
Solution (cmolc l 1)
Soybean Ca (%)
Norfolk
4.9 5.8 4.8 5.3 4.2 4.7
4 72 26 77 31 70
0.06 1.29 3.00 8.31 7.08 31.75
0.75 4.00 2.92 5.00 3.75 5.12
0.39 0.86 0.61 0.99 0.47 0.98
Portsmouth Organic
Data from Evans CE and Kamprath EJ (1970) Lime response as related to percent Al saturation, solution Al, and organic matter content. Soil Science Society of America Proceedings 34: 895–896. Evans CE (1968) Ion Exchange Relationships of Aluminum and Calcium in Soils as Influenced by Organic Matter. PhD thesis. Raleigh, NC: North Carolina State University.
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increased yield because of increased solubility of soil Mo, which is required by bacteria for N2 fixation. Increasing the OH concentration with liming brings about the replacement of MoO2 absorbed 4 by the Fe and Al hydrated oxides. Raising the pH to 6.0–6.2 makes sufficient Mo available for optimum N2 fixation and optimum yields of legumes. Thus the higher critical pH for legumes as compared to nonlegumes on mineral soils is related to the Mo requirement for N2 fixation and the effect of pH on Mo availability. Availability of soil P in acid soils has been increased by liming. In acid soils Al at the root surface precipitates P and decreases the amount of P transported to plant tops. Removal of Al from the soil solution by liming prevents this from happening. Neutralization of exchangeable Al also results in more root growth and exploration of a greater soil volume. Phosphorus gets to roots by diffusion and when root growth in a given volume is increased, the uptake of P is increased. The increased root growth in a given volume of soil when Al is neutralized by liming has a number of general beneficial effects on plant growth. Nutrient and soil water availability are increased. Microbial activity is enhanced; this affects N2 fixation, decomposition of organic matter and plant residues, making available organically bound nutrients.
Cation Exchange Properties Affecting Liming of Acid Soils The CEC of a soil is largely determined by the mineral composition of the clay-sized fraction. The source of the negative charge in clay minerals can be either permanent or variable. Permanent charge results from isomorphous substitution among cations with differing valences within the crystalline clay structure, and is a permanent characteristic of the mineral. Smectite, vermiculite, and illite are commonly occurring clay minerals with permanent charge. Variable charge results from the protonation and deprotonation of OH groups on the surfaces of Al and Fe oxides, crystalline or amorphous, and exposed edges of layer-silicate minerals. Kaolinite, hydroxy-interlayered vermiculite, goethite, gibbsite, quartz, and allophane are some of the clay minerals with variable charge. Carboxyl and OH groups in soil organic matter also have a variable-charge behavior, wherein the amount of negative charge increases with soil pH. Most acid soils have a mixed composition of both permanent- and variable-charge clay minerals. However, the dominant type of clay mineral charge can be estimated through the sum of basic cations and
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exchangeable acidity (unbuffered salt extraction) relative to the amount of soil clay. Soils dominated by minerals with permanent charge have CEC values >12 cmol kg1 of clay, whereas soils dominated by variable-charge clays have CEC values <12 cmol kg1 of clay. When lime is applied to soils with variable-charge clay minerals, part of the potential base reaction is consumed in the neutralization of Hþ released from OH groups. Consequently, the lime required to neutralize a targeted amount of exchangeable acidity is increased.
Lime Requirement Determinations Soil pH
Soil pH was one of the first methods for determining the need for lime. At pH 7 soils were considered to be 100% base-saturated and had zero base saturation at pH 4. The relationship between soil pH and base saturation was considered to be linear between pH 4 and 7. Measurement of the soil pH provided an estimate of the base saturation. To determine the amount of lime to raise the soil to the desired soil pH requires information about the CEC of the soil which can be estimated based on soil texture and organic matter content. As an example, the amount of lime to bring the pH to 6 (90% base saturation) for a soil which had an initial pH of 5 (50% base saturation) and a CEC of 10 cmol kg1 soil would be 10 cmol kg1 soil 40% ¼ 4 cmol lime kg1 soil, which is equal to 4000 kg lime ha1. One of the limitations of this approach is in estimating the CEC of the soil. If the estimate of the CEC is off, this will result in recommendations which are either too much or too little of the required amount of lime. Titration of Soil with a Base
The lime requirement of soils can be determined by titrating acid soils to the desired pH with a base such as KOH or Ca(OH)2. Titration of mineral soils to pH 5.6–5.8 will give the amount of lime required to neutralize exchangeable Al. The amount of lime on an equivalent basis is more than the exchangeable Al because, as the pH increases, nonexchangeable acidity ionizes and reacts with the lime. Titration to pH values above 5.6–5.8 results in the reactions of the base with nonexchangeable acidity which is H associated with hydroxy Al, carboxyl groups of organic matter, and hydrated oxides of Fe and Al. Reaction of lime with exchangeable and nonexchangeable acidity is shown by the following equations:
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1. neutralization of exchangeable acidity: 2Al-soil þ 3CaCO3 þ 6H2 O ! 4Ca-soil þ 2AlðOHÞ3 þ 3H2 CO3
3H2 CO3 ! 3H2 O þ 3CO2"
½3
2. neutralization of nonexchangeable acidity:
Exchangeable Al
2ðR-C-OOHÞ þ CaCO3 ! CaðR-C-OOÞ2 þ 2H2 CO3
2H2 CO3 ! 2H2 O þ 2CO2"
The lime recommendation by this method is to adjust the pH to 6.5. The Mehlich buffer was developed to predict the amount of lime needed to neutralize exchangeable Al. For mineral soils lime rates based on the acidity extracted with the Mehlich buffer raise the pH in the range of 5.8–6.0, which neutralizes the exchangeable Al.
½4
This method is rather time-consuming and does not lend itself to use where large number of samples need to be analyzed. Buffer pH Method
Many soil-testing laboratories use the soil-buffer pH method to determine the lime requirement. This method is essentially a titration of an acid, the soil, with a base, the buffer solution. The molarity of the buffer solution is known and the amount of base required to neutralize the acidity of the soil can be determined by the change in the pH of the buffer solutions. With these methods a given volume of the buffer solution is added to a given weight or volume of soil and the pH is measured. The decrease in pH of the buffer solution is related to the amount of acidity in the soil. Four buffer pH methods are being used in the USA to determine lime requirements. The amount of lime needed for each 0.1 decrease of buffer pH to bring the soil to the indicated pH is given in Table 5 for each of the methods. The Woodruff buffer was developed for determining the amount of lime to adjust the pH of Mollisols to a range of 6.5–7.0. The SMP buffer is used for soils with a large amount of three-layer clays and high-organic-matter content, such as Alfisols. The Adams and Evans buffer was developed for soils with low CEC and kaolinitic clay mineralogy.
Liming mineral soils on an equivalent basis to the amount of exchangeable Al targets achievement of pH 5.6–5.8 and a soil solution Al3þ concentration approaching zero. As previously discussed, lime will neutralize exchangeable Al and also react with protons on variable-charge mineral surfaces. Consequently, the lime equivalence factor to achieve 0% Al saturation of the soil CEC is greater than 1.0 and is usually in the order of 1.5–3. Yields for many crops are not reduced until soil Al saturation values exceed 40% or even 60% (Table 3 and Figure 5). For these acid-tolerant crops and, especially, in regions where lime materials are expensive, lime requirements can be adjusted to achieve a targeted percentage of Al saturation instead of neutralizing all of the exchangeable Al. An example of this approach is the following: CaCO3 equivalent ðt ha1 Þ ¼ LF½Al TAS ðAl þ Ca þ MgÞ=100
½5
where LF ¼ the product of 1.5 equivalents of CaCO3/ equivalent of exchangeable Al and the conversion to a 20-cm layer for a hectare of soil, Al, Ca, and Mg ¼ exchangeable cations extracted with a neutral unbuffered salt, such as KCl, and TAS ¼ the targeted percentage of Al saturation after liming. Liming soils to achieve a given percentage of Al saturation requires more information than when liming to neutralize all of the exchangeable Al. In addition to exchangeable Al, analytical data are needed for exchangeable Ca and Mg. Furthermore,
Table 5 Lime requirement as determined with buffer solutions Method
Buffer pH
Target pH
Intended use
SMP
6.8
High exchangeable Al Alfisols
Woodruff Adams and Evans Mehlich
7.0 8.0 6.6
6.8 6.4 6.0 6.5–7.0 6.5 6.0
Mollisols Low cation exchange capacity Ultisols Neutralize exchangeable Al, Ultisols
Lime/0.1 pH decrease in buffer (t CaCO3 ha1)
0.6 0.5 0.4 0.5 0.1 0.16
Data from van Lierop W (1990) Soil pH and lime requirement determination. In: Westerman RL (ed.) Soil Testing and Plant Analysis, 3rd edn, pp. 73–126. Madison, WI: Soil Science Society of America. CEC, cation exchange capacity.
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the percentage of Al saturation tolerated by the intended crop must also be known.
Factors to Consider in Lime Application Liming Materials
There are a number of different materials which can be used for liming. The most common ones are calcium carbonate (calcitic limestone) and calciummagnesium carbonate (dolomitic limestone). These are crystalline compounds which must be ground. Marls are soft, unconsolidated calcium carbonates which may contain some clay. Calcium oxide (burnt lime or quick lime) is a white powder and is caustic when it absorbs water. It is difficult to get uniform mixing with the soil because immediately after application absorption of water causes granules to form. Calcium hydroxide (hydrated lime) is a white powder and when wet is caustic. It is also difficult to apply. Basic slag (calcium silicate) is a by-product of the steel industry. Neutralizing Value
The neutralizing value (CaCO3 equivalent) of liming materials is related to the amount of acid that a unit weight of lime will neutralize. The standard for evaluating materials is pure calcium carbonate which is given a value of 100%. The neutralizing value for pure liming materials is given in Table 6. The purity of the liming material must also be taken into account when determining the neutralizing value. Most liming materials are not 100% pure. In many instances the purity of the material will have been determined so that the buyer is given this information. Lime recommendations are often based on 100% purity of the material so that the application rate will have to be increased when the purity is less than 100%. Fineness of Liming Materials
The most commonly used liming materials, calcitic and dolomitic limestone, are crystalline compounds. These materials must be ground to a fineness which will provide a large number of particles per unit
Figure 7 Relative efficiency of various mesh sizes of calcitic (circles) and dolomitic (squares) lime in neutralizing acidity. Data source: unpublished data of E.J. Kamprath.
volume of soil. The lime particles dissolve when coming into contact with acid soils and will release þ Ca2þ and HCO 3 : this results in neutralization of H 3þ and Al . The rate at which the pH of the soil mass is affected depends on how close the individual lime particles are to one another. Since the diffusion of Ca is relatively slow the particles need to be close together so that the zones of neutralization around each particle overlap in a relatively short time. Calcitic and dolomitic limestones are generally ground so that all of the particles pass a 20-mesh sieve and a certain percentage passes a 100-mesh sieve depending on the regulations of the state or province. Calcitic limestones are often softer than dolomitic limestones and therefore dissolve at a faster rate. The relative efficiency of different particle sizes of these two materials is given in Figure 7. Particles held on a 10-mesh sieve have little value for neutralizing acid soils. Incorporation of Lime in the Soil
Since Caþ2 and HCO 3 ions have a slow rate of diffusion, the lime must be well mixed with the soil to neutralize soil acidity throughout the zone which is to be amended. One approach is to apply half of the lime and incorporate it by disking, and then apply the other half and disk to incorporate. However this results in high costs of application and incorporation rather than doing it only once.
Table 6 Neutralizing value of pure liming materials Material
Neutralizing value (%)
Calcium carbonate, CaCO3 Calcium-magnesium carbonate, CaMg(CO3)2 Calcium oxide, CaO Calcium hydroxide, Ca(OH)2 Calcium silicate, CaSiO3
100 109 179 135 86
Conclusion Poor plant growth on acid soils is due to toxicities of Al, H, and Mn and deficiencies of Ca, Mg, and Mo. Oxisols, Ultisols, and some Alfisols in their native state have pH values less than 5 and exchangeable Al of the effective CEC greater than 50%. Plant response to liming is due to neutralization of the toxic elements
358 LIPMAN, JACOB G.
and the supply of Ca and Mg. The lime requirement for a soil can be based on the amount of lime required to: (1) adjust the pH to a desired value; (2) neutralize exchangeable Al; or (3) neutralize the acidity measured by buffer pH methods. Factors to consider in selecting a lime material are fineness, purity, and the need for Mg. See also: Acidity; Calcium and Magnesium in Soils; pH
Further Reading Adams F (ed.) (1984) Soil Acidity and Liming, 2nd edn. Madison, WI: American Society of Agronomy, Crop Science Society of America, Soil Science Society of America. Alley MM (1981) Short-term chemical and crop yield responses to limestone applications. Agronomy Journal 73: 687–689. Cochrane TT, Salinas JG, and Sanchez PA (1980) An equation for liming acid soil mineral soils to compensate crop aluminium tolerance. Tropical Agriculture 57: 133–140. Evans CE (1968) Ion Exchange Relationships of Aluminum and Calcium in Soils as Influenced by Organic Matter. PhD thesis. Raleigh, NC: North Carolina State University. Evans CE and Kamprath EJ (1970) Lime response as related to percent Al saturation, solution Al, and organic matter content. Soil Science Society of America Proceedings 34: 895–896.
Fox RH (1979) Soil pH, aluminum saturation and corn grain yield. Soil Science 127: 330–334. Gonzalez-Erico E, Kamprath EJ, Naderman GC, and Soares WV (1979) Effect of depth of lime incorporation on the growth of corn on an Oxisol of Central Brazil. Soil Science Society of America Journal 43: 1155–1158. Jallah JK (1994) Assessment of Some Chemical Constraints to Root Growth in Four Acid North Carolina Soils. PhD thesis. Raleigh, NC: North Carolina State University. Kamprath EJ (1970) Exchangeable aluminum as a criterion for liming leached mineral soils. Soil Science Society of America Proceedings 34: 252–254. McCart GD and Kamprath EJ (1965) Supplying calcium and magnesium for cotton on sandy, low cation exchange soils. Agronomy Journal 57: 404–406. Osmond DL, Smyth TJ, Reid WS et al. (2002) Nutrient Management Support System, NuMaSS. (version 2.0). Raleigh, NC: Soil Management Collaborative Research Support System, North Carolina State University. Ritchey KD, Silva JE, and Sousa DMG (1983) Relac¸ a˜ o entre teor de calcio no solo e desenvolvimento de raizes avaliado por um metodo biologico. Revista Brasileira Ciencia Solo 7: 269–275. Sanzonowicz C, Smyth TJ, and Israel DW (1998) Calcium alleviation of hydrogen and aluminum inhibition of soybean root extension from limed soil into acid subsurface solutions. Journal of Plant Nutrition 21: 785–804. van Lierop W (1990) Soil pH and lime requirement determination. In: Westerman RL (ed.) Soil Testing and Plant Analysis, 3rd edn, pp. 73–126. Madison, WI: Soil Science Society of America.
LIPMAN, JACOB G. J C F Tedrow, Formerly with Rutgers University, New Brunswick, NJ, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Jacob G. Lipman was one of the most prominent international leaders in soil science. In 1901 he formed the Department of Soil Chemistry and Bacteriology at Rutgers University, the first of its kind in the USA, and probably the world. He was the driving force in internationalizing soil science. He also initiated and edited Soil Science, the first international journal dealing with soil chemistry, soil fertility, soil physics, and soil microbiology. Lipman (Figure 1) played a major role in the development of agriculture in the USA and beyond during the 1901–1939 period. His main contributions were in the area of soil microbiology and soil fertility,
particularly in relation to increased crop production. During later years, he was also involved in worldwide agricultural problems. Not only did he advance science on several fronts but he was also a gifted administrator. He addressed the broader aspects of agriculture, particularly in increasing the world food supply, controlling soil erosion, and improving the general welfare of humanity.
The Early Years Jacob Goodale Lipman was born in Friedrichstat, Russia, on November 18, 1874. He received his early academic training under private tutors in Moscow and in the classical gymnasium in Orenberg. In 1888, with his parents, he settled at the Baron de Hirsch colony in Woodbine, New Jersey. After graduating from farm school there he entered Rutgers
LIMING E J Kamprath and T J Smyth, North Carolina State University, Raleigh, NC, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction Approximately 40% of the arable land in the world is naturally acid. The naturally acid soils have developed where rainfall exceeds evapotranspiration. Acid soils have also developed with intensive agriculture. The acidity of soils can be divided into that associated with the soil solution, exchangeable, and nonexchangeable forms. The poor growth of plants on acid soils is due to toxicities of Al, Mn, and H, and deficiencies of Ca, Mg, and Mo. Liming of acid soils increases plant growth due to neutralization of toxic elements, supplying Ca and/or Mg and increasing availability of Mo and P. Determination of the lime requirement for a soil can be based on the amount needed to adjust pH to a desired level, neutralization of exchangeable Al, or the amount estimated by the buffer pH method. Selection of a liming material has to take into account the neutralization value of the lime, fineness of the material, and the need for Mg.
Development of Acid Soils In nature acid soils are found where rainfall exceeds evapotranspiration. Several processes contribute to soil acidification under these conditions. The excess water moving through the soil carries Ca2þ and Mg2þ which balance the negative charge of soluble anions like NO 3 and Cl . Leaching of Ca and Mg in conjunction with soluble anions decreases the percentage of exchange sites occupied by basic cations. The sites vacated by Ca and Mg are initially replaced with Hþ supplied by decomposition of organic matter and residues. The Hþ-clays then decompose to form Al-clays. With intensive agriculture Hþ is also produced by the conversion of ammonium to nitrate: þ NHþ 4 þ 2O2 ! NO3 þ H2 O þ 2H
½1
The application of 100 kg N ha1 in the ammonium form can require a maximum of 360 kg CaCO3 ha1 to neutralize the acidity produced by the conversion þ of NHþ 4 to NO3 . Plants also release H from roots when their uptake of basic cations exceeds that of anions. Since legumes fix atmospheric N and have relatively high concentrations of Ca, cation uptake exceeds anion uptake. To maintain internal plant electroneutrality legume roots excrete Hþ which acidifies
the rhizosphere. Neutralization of acidity produced in one growing season of N-fixing legumes can correspond to 30 – 400 kg CaCO3 ha1. Another source of Hþ is acid precipitation produced upon burning fossil fuels containing sulfur compounds which give rise to SO2. The SO2 reacts with water in the atmosphere to produce sulfuric acid.
Nature of Soil Acidity Soil Solution Acidity
The solutions of acid soils contain two ions that are detrimental to plant growth, hydrogen and aluminum. The concentration of Al3þ in the soil solution increases with the proportion of cation exchange sites which are occupied with Al (i.e., the percentage Al saturation of the cation exchange capacity (CEC)). There is a marked increase in soil solution Al3þ concentration when the saturation is greater than 50%. The soil solution Hþ concentration is controlled by the hydrolysis of Al3þ, as shown by eqn [2]. Al3þ þ H2 O ! AlOH2þ þ Hþ
½2
Between pH values of 5.6 and 5.8 the soil solution Al3þ concentration is nil in mineral soils since the Al saturation is essentially zero. Organic soils with appreciable mineral matter have very low concentrations of soil solution Al3þ at pH values of 4.8–5.0, because Al is strongly bound to the carboxyl groups. High concentrations of soil solution Al3þ only occur in organic soils at pH values of 4.5 or lower. Exchangeable Acidity
Early concepts were that exchangeable Hþ was the source of soil acidity. Titration studies showed that acid clays behaved as weakly ionized acids, which led to the conclusion that acid soils contained exchangeable Hþ. Soils were assumed to be 100% basesaturated at pH values of 7 and the difference between the CEC at pH 7 and the sum of basic cations was the amount of exchangeable Hþ. Later studies revealed that the weakly ionized acid characteristic of clays was due to exchangeable Al and the hydrolysis of Al3þ was the source of soil solution Hþ. When saturated with Hþ the clays decomposed, releasing Al, which occupied the exchange sites. Exchangeable acidity is defined as that which is extracted with a neutral unbuffered salt solution such as KCl. Neutral unbuffered salt solutions must be used to avoid changing the soil pH and altering the
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351
Table 1 The exchangeable and nonexchangeable acidity of acid soils Acidity
Soil
pH
Organic matter (g kg1)
Typic Kandiudult Typic Paleaquult Aeric Paleaquult Typic Haplustox
4.5 4.7 4.5 4.5
20 27 40 –
Exchange Al (cmol kg1)
Al saturation (%)
Titratablea (cmol kg1)
Nonexchangeable cmol kg1
0.91 1.04 2.33 1.55
82 78 73 68
1.70 1.72 3.77 3.85
0.57 0.68 1.44 2.70
a Acidity titrated to pH 6. Data from Evans CE and Kamprath EJ (1970) Lime response as related to percent Al saturation, solution Al, and organic matter content. Soil Science Society of America Proceedings 34: 895–896. Kamprath EJ (1970) Exchangeable aluminum as a criterion for liming leached mineral soils. Soil Science Society of America Proceedings 34: 252–254. Gonzalez-Erico E, Kamprath EJ, Naderman GC, and Soares WV (1979) Effect of depth of lime incorporation on the growth of corn on an Oxisol of Central Brazil. Soil Science Society of America Journal 43: 1155–1158.
exchangeable acidity. In mineral soils the exchangeable acidity is Al3þ. At pH values of 5 and lower, over half of the active exchange sites are countered by Al3þ (Table 1). As previously discussed, exchangeable Al is essentially zero in mineral soils at pH 5.6 and the active cation exchange sites are 100% base-saturated.
Table 2 Soybean tap and lateral root length after 12 days of exposure to solutions with different pH and Ca concentrations Solution
pH
4.0
Nonexchangeable Acidity
Soils also contain H which is not extractable with neutral unbuffered salt solutions, but is titratable. The nonexchangeable H is associated with carboxyl groups of organic matter, hydroxy Al, and hydrated oxides of Fe and Al. The nonexchangeable acidity of the three Ultisols in Table 1 was primarily due to organic matter. However, with the Oxisol, the nonexchangeable acidity was associated with the hydrated oxides of Fe and Al. Once the exchangeable Al is neutralized the acid buffering capacity of the soil is a function of the nonexchangeable acidity. Neutralization of the nonexchangeable acidity has little effect per se on plant growth, but is the source of the pH-dependent CEC. With increasing neutralization of the nonexchangeable acidity, there is an increase in soil pH, CEC, and the retention of the basic cations Ca, Mg, and K.
Acid Soil Constraints to Plant Growth Hydrogen Toxicity
Poor plant growth in acid soils is usually associated with low soil pH, but the direct effects of Hþ are often confounded with changes in solubility of various elements affecting plant growth. Experiments in hydroponics, therefore, are often used to assess the direct effects of Hþ. Roots growing in solutions at pH 4.6 or lower have visual symptoms of injury, namely stunted growth, brownish color, and little lateral root development. Increasing solution Ca concentration alleviates root injury due to Hþ (Table 2). At low pH and
4.6 5.5
Root length Ca (mmol l 1)
Tap (cm per plant)
Lateral (cm per plant)
0.2 2.0 0.2 2.0 0.2 2.0
1.1 3.2 10.2 22.2 27.8 44.6
0.0 1.0 3.4 36.3 40.1 85.6
Data from Sanzonowicz C, Smyth TJ, and Israel DW (1998) Calcium alleviation of hydrogen and aluminum inhibition of soybean root extension from limed soil into acid subsurface solutions. Journal of Plant Nutrition 21: 785–804.
Ca concentrations root membranes are damaged, leading to loss of organic substrates and absorbed cations. Under soil conditions with pH below 4.6, root injury due to Hþ can be expected along with reduced plant uptake and possible deficiencies of Ca and Mg. Aluminum Toxicity
Aluminum toxicity is a major cause of poor plant growth in acid soils. Current evidence indicates that Al interferes with various root growth processes: disruption of regulatory signals in root cap cells and interference with cell division in root apices, enzyme activities, DNA replication, and P availability at membranes. The immediate and visible evidence of Al toxicity is a reduction in root length which, in turn, limits plant access to soil water and nutrients. Consequently, plants growing in soils with toxic levels of Al are underdeveloped, sensitive to drought stress, and often present symptoms of multiple nutrient deficiencies. Concentrations of soil solution Al that reduce crop growth are in the micromolar range. As illustrated in
352 LIMING
Figure 1 Relations between Al saturation of cation exchange sites and concentration of Al in the soil solution of Oxisols and Ultisols. Filled circles, Haplustox; open circles, Umbraquult; inverted triangles, Paleudult. Data source: Gonzalez-Erico E, Kamprath EJ, Naderman GC, and Soares WV (1979) Effect of depth of lime incorporation on the growth of corn on an Oxisol of Central Brazil. Soil Science Society of America Journal 43: 1155 –1158; and Jallah JK (1994) Assessment of Some Chemical Constraints to Root Growth in Four Acid North Carolina Soils. PhD thesis. Raleigh, NC: North Carolina State University.
Figure 1, the concentration of Al in soil solutions is related to the percentage of Al saturation of the cation exchange sites, with a marked increase in the Al concentration when the Al saturation exceeds 50%. Most field assessments of crop response to toxic levels of soil solution Al have been based on percentage of Al saturation as a proxy variable, due to its greater ease of measurement. Root length of corn, for example, is markedly reduced at percentage of Al saturation levels corresponding to high concentrations of Al in the soil solution (Figure 2). Tolerance to Al varies widely among species and even varieties of the same species (Table 3). Yields for most varieties of cotton, mung bean, pearl millet, and wheat are reduced by low levels of percentage of Al saturation, whereas cassava and cowpea tolerate high levels of soil Al. However, considerable differences in Al tolerance/susceptibility have been observed among varieties within a given species. For example, critical percentage of Al saturation levels among field trials with different varieties range from 0 to 40 with corn, 0 to 44 with soybean, and 0 to 60 with upland rice. Manganese Toxicity
Acid soils with high contents of Fe and Al oxides often contain large amounts of soluble Mn. Plant growth in such soils is reduced when large quantities of the soluble soil Mn accumulate in their tissues. Although Mn is an essential nutrient, excessive plant uptake is toxic to plants. Liming these soils to pH 5.5 or greater decreases the solubility of soil Mn, which
Figure 2 Effect of Al saturation on relative root length of fieldgrown corn in Oxisols and Ultisols. Filled circles, Umbraquult; open circles, Paleudult; inverted triangles, Haplustox. Data source: Gonzalez-Erico E, Kamprath EJ, Naderman GC, and Soares WV (1979) Effect of depth of lime incorporation on the growth of corn on an Oxisol of Central Brazil. Soil Science Society of America Journal 43: 1155 –1158; and Jallah JK (1994) Assessment of Some Chemical Constraints to Root Growth in Four Acid North Carolina Soils. PhD thesis. Raleigh, NC: North Carolina State
University.
Table 3 Threshold values of percentage Al saturation of the cation exchange capacity above which crop yield for various species is normally reduced by Al in mineral soils Crop
Critical % Al saturation
Cassava Corn Cotton Cowpea Mung bean Peanut Pearl millet Phaseolus bean Potato Sorghum Soybean Upland rice Wheat
75 30 0 60 0 40 0 15 20 15 15 40 0
Data from Osmond DL, Smyth TJ, Reid WS et al. (2002) Nutrient Management Support System, NuMaSS (version 2.0). Raleigh, NC: Soil Management Collaborative Research Support System, North Carolina State University.
reduces the uptake of excess Mn and increases plant growth. In many instances, improved plant growth upon liming soils with percentage of Al saturation below the critical level is associated with the alleviation of Mn toxicities. Calcium and Mg Deficiencies
Acid soils with high percentage of Al saturation have very low amounts of Ca and Mg on both the exchange sites and in the soil solution. Soils with very
LIMING
Figure 3 Root elongation of wheat as a function of Ca levels and sources in Ca-deficient Oxisols. Squares, lime; circles, CaCl2; triangles, Ca(H2PO4)2. Data source: Ritchey KD, Silva JE, and Sousa DMG (1983) Relac¸a˜o entre teor de calcio no solo e desenvolvimento de raizes avaliado por um metodo biologico. Revista Brasileira Ciencia Solo 7: 269–275.
low CEC have a limited capacity to retain Ca and Mg and supply of this nutrient may not be sufficient for normal plant growth. The immediate supply of Ca for continued growth of root tips is dependent on the soil solution concentration, because this nutrient is relatively immobile in plants. Root length of wheat seedlings, for example, was increased by over 50% when 0.1 cmol Ca kg1 was added to an Oxisol which initially contained 0.02 cmol Ca kg1 (Figure 3). Although it is often difficult to separate the extent to which a plant growth response to liming is due to correction of a Ca deficiency versus alleviation of an Al toxicity, the improved root growth of wheat was similar among various sources of Ca. Plant growth responses to additions of Mg are often associated with soil conditions where exchangeable Mg2þ and/or the proportion of cation exchange sites occupied by Mg2þ is very low.
Response to Liming A major response of plants to liming is due to the neutralization of H, Al, and Mn, which if present in relatively high concentrations in the soil solution are toxic to plant growth. Toxic concentrations of H generally do not occur in soils except in certain organic soils when pH values are less than 4.5. In acid mineral soils with pH values less than 5.5 liming removes the negative effect of H on absorption of Ca, Mg, and K. The response of plants to liming is mainly due to the neutralization of exchangeable Al and its replacement on the exchange sites with Ca and/or Mg. Liming of acid mineral soils to pH 5.6 reduces
353
Figure 4 Relationship between soil pH and exchangeable Al for Coastal Plain soil. Data source: unpublished data of Soil Science Department North Carolina State University.
Figure 5 Relative yield of soybean (circles), corn (triangles), and cotton (squares) as related to Al saturation of the effective cation exchange capacity. Data source: E.J. Kamprath (unpublished data); and McCart GD and Kamprath EJ (1965) Supplying calcium and magnesium for cotton on sandy, low cation exchange soils. Agronomy Journal 57: 404 – 406.
exchangeable Al to essentially zero (Figure 4). This essentially eliminates Al from the soil solution and removes it as a factor limiting plant growth. As previously discussed, the concentration of Al in the soil solution is a function of the Al saturation of the active cation exchange sites. Since the Al saturation of a soil is much easier to determine than soil solution Al, it is an excellent criterion for determining the need to lime a given soil. Since crops and genotypes vary in their tolerance to Al, knowledge about the Al saturation of a given soil provides information whether a particular crop or genotype will require liming. An example of this is the differential Al tolerance of corn as compared to soybean and cotton in a greenhouse study (Figure 5). Corn showed little effect to Al until
354 LIMING
the Al saturation was greater than 50%, where there is a marked increase in soil solution Al. Relative grain yields of corn on three soils decreased sharply when Al saturation was greater than 40% (Figure 6), a value similar to that found for dry-matter production in the greenhouse. Organic soils and soils high in organic matter have a lower critical pH than most mineral soils. Maximum plant growth on acid organic soils is generally obtained at pH 5. On these soils the exchangeable Al is held on carboxyl sites and at pH 5 the Al is held quite strongly so there is very little Al in the soil solution. Liming of these soils to pH 5 provides adequate Ca to overcome the H effects on plant growth. Response to liming can also be obtained on acid soils with low Al saturation but that have high amounts of Mn. Soil with high contents of sesquioxides often contain appreciable amounts of Mn minerals and in these soils Mn toxicity is a problem when pH values
Figure 6 Relative grain yields of corn on three soils (filled circles, Goldsboro; open circles, Wharton; inverted triangles, Morrill) as related to exchangeable Al saturation of the effective cation exchange capacity. Data source: Alley MM (1981) Short-term chemical and crop yield responses to limestone applications. Agronomy Journal 73: 687– 689; and Fox RH (1979) Soil pH, aluminum saturation and corn grain yield. Soil Science 127: 330 – 334.
drop to 5 and less. Liming to pH 5.6 and above eliminates Mn toxicity on these soils by converting the soluble Mn2þ to the relatively insoluble oxides, Mn2O2 and MnO2. Response to liming due to supplying of Ca per se is not often the case. In most instances Ca saturations of 20–25% will supply adequate Ca for plant growth except where toxicities to Al, H, or Mn limit plant growth. There is a good relationship between Ca or Ca þ Mg saturation and plant growth, because these are the mirror image of Al saturation. On soils with very low CECs a minimum of 1 cmol Ca kg1 soil is needed for optimum growth of most plants. When toxicities are not a factor the percentage of Ca saturation of the CEC is a good indicator of Ca availability. Calcium concentrations of a given plant species are similar when grown on soils with the same Ca saturation even though the soils contain a wide range of exchangeable Ca (Table 4). This is because the Ca concentration of the soil solution is determined by the percentage of Ca saturation rather than the amount of exchangeable Ca. Calcium is transported to the root by mass flow of water and thus the amount arriving at the root surface is a function of the Ca concentration. Magnesium deficiencies on sandy soils are generally associated with pH values of less than 5. The concentration of Al3þ and Hþ at pH 5 in mineral soils is sufficient to inhibit Mg uptake by plants. Liming to neutralize Al and H is necessary for optimum uptake of Mg where soil reserves are adequate. On acid sandy soils response to liming is also related to the supply of Mg with the application of dolomitic lime. Availability of soil Mg is related more to the Mg saturation of the CEC rather than the amount of exchangeable Mg. Various studies have shown that Mg saturation needs to be in the range of 5–10% for optimum plant growth. Soybean and other legumes often show a response to increase in pH beyond that at which Al is neutralized. Raising the pH of mineral soils to 6 has
Table 4 Effect of Ca saturation of the effective cation exchange capacity (ECEC) on Ca availability Soil Ca
Soil
pH
ECEC saturation (%)
Exchangeable (cmolc kg1)
Solution (cmolc l 1)
Soybean Ca (%)
Norfolk
4.9 5.8 4.8 5.3 4.2 4.7
4 72 26 77 31 70
0.06 1.29 3.00 8.31 7.08 31.75
0.75 4.00 2.92 5.00 3.75 5.12
0.39 0.86 0.61 0.99 0.47 0.98
Portsmouth Organic
Data from Evans CE and Kamprath EJ (1970) Lime response as related to percent Al saturation, solution Al, and organic matter content. Soil Science Society of America Proceedings 34: 895–896. Evans CE (1968) Ion Exchange Relationships of Aluminum and Calcium in Soils as Influenced by Organic Matter. PhD thesis. Raleigh, NC: North Carolina State University.
LIMING
increased yield because of increased solubility of soil Mo, which is required by bacteria for N2 fixation. Increasing the OH concentration with liming brings about the replacement of MoO2 absorbed 4 by the Fe and Al hydrated oxides. Raising the pH to 6.0–6.2 makes sufficient Mo available for optimum N2 fixation and optimum yields of legumes. Thus the higher critical pH for legumes as compared to nonlegumes on mineral soils is related to the Mo requirement for N2 fixation and the effect of pH on Mo availability. Availability of soil P in acid soils has been increased by liming. In acid soils Al at the root surface precipitates P and decreases the amount of P transported to plant tops. Removal of Al from the soil solution by liming prevents this from happening. Neutralization of exchangeable Al also results in more root growth and exploration of a greater soil volume. Phosphorus gets to roots by diffusion and when root growth in a given volume is increased, the uptake of P is increased. The increased root growth in a given volume of soil when Al is neutralized by liming has a number of general beneficial effects on plant growth. Nutrient and soil water availability are increased. Microbial activity is enhanced; this affects N2 fixation, decomposition of organic matter and plant residues, making available organically bound nutrients.
Cation Exchange Properties Affecting Liming of Acid Soils The CEC of a soil is largely determined by the mineral composition of the clay-sized fraction. The source of the negative charge in clay minerals can be either permanent or variable. Permanent charge results from isomorphous substitution among cations with differing valences within the crystalline clay structure, and is a permanent characteristic of the mineral. Smectite, vermiculite, and illite are commonly occurring clay minerals with permanent charge. Variable charge results from the protonation and deprotonation of OH groups on the surfaces of Al and Fe oxides, crystalline or amorphous, and exposed edges of layer-silicate minerals. Kaolinite, hydroxy-interlayered vermiculite, goethite, gibbsite, quartz, and allophane are some of the clay minerals with variable charge. Carboxyl and OH groups in soil organic matter also have a variable-charge behavior, wherein the amount of negative charge increases with soil pH. Most acid soils have a mixed composition of both permanent- and variable-charge clay minerals. However, the dominant type of clay mineral charge can be estimated through the sum of basic cations and
355
exchangeable acidity (unbuffered salt extraction) relative to the amount of soil clay. Soils dominated by minerals with permanent charge have CEC values >12 cmol kg1 of clay, whereas soils dominated by variable-charge clays have CEC values <12 cmol kg1 of clay. When lime is applied to soils with variable-charge clay minerals, part of the potential base reaction is consumed in the neutralization of Hþ released from OH groups. Consequently, the lime required to neutralize a targeted amount of exchangeable acidity is increased.
Lime Requirement Determinations Soil pH
Soil pH was one of the first methods for determining the need for lime. At pH 7 soils were considered to be 100% base-saturated and had zero base saturation at pH 4. The relationship between soil pH and base saturation was considered to be linear between pH 4 and 7. Measurement of the soil pH provided an estimate of the base saturation. To determine the amount of lime to raise the soil to the desired soil pH requires information about the CEC of the soil which can be estimated based on soil texture and organic matter content. As an example, the amount of lime to bring the pH to 6 (90% base saturation) for a soil which had an initial pH of 5 (50% base saturation) and a CEC of 10 cmol kg1 soil would be 10 cmol kg1 soil 40% ¼ 4 cmol lime kg1 soil, which is equal to 4000 kg lime ha1. One of the limitations of this approach is in estimating the CEC of the soil. If the estimate of the CEC is off, this will result in recommendations which are either too much or too little of the required amount of lime. Titration of Soil with a Base
The lime requirement of soils can be determined by titrating acid soils to the desired pH with a base such as KOH or Ca(OH)2. Titration of mineral soils to pH 5.6–5.8 will give the amount of lime required to neutralize exchangeable Al. The amount of lime on an equivalent basis is more than the exchangeable Al because, as the pH increases, nonexchangeable acidity ionizes and reacts with the lime. Titration to pH values above 5.6–5.8 results in the reactions of the base with nonexchangeable acidity which is H associated with hydroxy Al, carboxyl groups of organic matter, and hydrated oxides of Fe and Al. Reaction of lime with exchangeable and nonexchangeable acidity is shown by the following equations:
356 LIMING
1. neutralization of exchangeable acidity: 2Al-soil þ 3CaCO3 þ 6H2 O ! 4Ca-soil þ 2AlðOHÞ3 þ 3H2 CO3
3H2 CO3 ! 3H2 O þ 3CO2"
½3
2. neutralization of nonexchangeable acidity:
Exchangeable Al
2ðR-C-OOHÞ þ CaCO3 ! CaðR-C-OOÞ2 þ 2H2 CO3
2H2 CO3 ! 2H2 O þ 2CO2"
The lime recommendation by this method is to adjust the pH to 6.5. The Mehlich buffer was developed to predict the amount of lime needed to neutralize exchangeable Al. For mineral soils lime rates based on the acidity extracted with the Mehlich buffer raise the pH in the range of 5.8–6.0, which neutralizes the exchangeable Al.
½4
This method is rather time-consuming and does not lend itself to use where large number of samples need to be analyzed. Buffer pH Method
Many soil-testing laboratories use the soil-buffer pH method to determine the lime requirement. This method is essentially a titration of an acid, the soil, with a base, the buffer solution. The molarity of the buffer solution is known and the amount of base required to neutralize the acidity of the soil can be determined by the change in the pH of the buffer solutions. With these methods a given volume of the buffer solution is added to a given weight or volume of soil and the pH is measured. The decrease in pH of the buffer solution is related to the amount of acidity in the soil. Four buffer pH methods are being used in the USA to determine lime requirements. The amount of lime needed for each 0.1 decrease of buffer pH to bring the soil to the indicated pH is given in Table 5 for each of the methods. The Woodruff buffer was developed for determining the amount of lime to adjust the pH of Mollisols to a range of 6.5–7.0. The SMP buffer is used for soils with a large amount of three-layer clays and high-organic-matter content, such as Alfisols. The Adams and Evans buffer was developed for soils with low CEC and kaolinitic clay mineralogy.
Liming mineral soils on an equivalent basis to the amount of exchangeable Al targets achievement of pH 5.6–5.8 and a soil solution Al3þ concentration approaching zero. As previously discussed, lime will neutralize exchangeable Al and also react with protons on variable-charge mineral surfaces. Consequently, the lime equivalence factor to achieve 0% Al saturation of the soil CEC is greater than 1.0 and is usually in the order of 1.5–3. Yields for many crops are not reduced until soil Al saturation values exceed 40% or even 60% (Table 3 and Figure 5). For these acid-tolerant crops and, especially, in regions where lime materials are expensive, lime requirements can be adjusted to achieve a targeted percentage of Al saturation instead of neutralizing all of the exchangeable Al. An example of this approach is the following: CaCO3 equivalent ðt ha1 Þ ¼ LF½Al TAS ðAl þ Ca þ MgÞ=100
½5
where LF ¼ the product of 1.5 equivalents of CaCO3/ equivalent of exchangeable Al and the conversion to a 20-cm layer for a hectare of soil, Al, Ca, and Mg ¼ exchangeable cations extracted with a neutral unbuffered salt, such as KCl, and TAS ¼ the targeted percentage of Al saturation after liming. Liming soils to achieve a given percentage of Al saturation requires more information than when liming to neutralize all of the exchangeable Al. In addition to exchangeable Al, analytical data are needed for exchangeable Ca and Mg. Furthermore,
Table 5 Lime requirement as determined with buffer solutions Method
Buffer pH
Target pH
Intended use
SMP
6.8
High exchangeable Al Alfisols
Woodruff Adams and Evans Mehlich
7.0 8.0 6.6
6.8 6.4 6.0 6.5–7.0 6.5 6.0
Mollisols Low cation exchange capacity Ultisols Neutralize exchangeable Al, Ultisols
Lime/0.1 pH decrease in buffer (t CaCO3 ha1)
0.6 0.5 0.4 0.5 0.1 0.16
Data from van Lierop W (1990) Soil pH and lime requirement determination. In: Westerman RL (ed.) Soil Testing and Plant Analysis, 3rd edn, pp. 73–126. Madison, WI: Soil Science Society of America. CEC, cation exchange capacity.
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357
the percentage of Al saturation tolerated by the intended crop must also be known.
Factors to Consider in Lime Application Liming Materials
There are a number of different materials which can be used for liming. The most common ones are calcium carbonate (calcitic limestone) and calciummagnesium carbonate (dolomitic limestone). These are crystalline compounds which must be ground. Marls are soft, unconsolidated calcium carbonates which may contain some clay. Calcium oxide (burnt lime or quick lime) is a white powder and is caustic when it absorbs water. It is difficult to get uniform mixing with the soil because immediately after application absorption of water causes granules to form. Calcium hydroxide (hydrated lime) is a white powder and when wet is caustic. It is also difficult to apply. Basic slag (calcium silicate) is a by-product of the steel industry. Neutralizing Value
The neutralizing value (CaCO3 equivalent) of liming materials is related to the amount of acid that a unit weight of lime will neutralize. The standard for evaluating materials is pure calcium carbonate which is given a value of 100%. The neutralizing value for pure liming materials is given in Table 6. The purity of the liming material must also be taken into account when determining the neutralizing value. Most liming materials are not 100% pure. In many instances the purity of the material will have been determined so that the buyer is given this information. Lime recommendations are often based on 100% purity of the material so that the application rate will have to be increased when the purity is less than 100%. Fineness of Liming Materials
The most commonly used liming materials, calcitic and dolomitic limestone, are crystalline compounds. These materials must be ground to a fineness which will provide a large number of particles per unit
Figure 7 Relative efficiency of various mesh sizes of calcitic (circles) and dolomitic (squares) lime in neutralizing acidity. Data source: unpublished data of E.J. Kamprath.
volume of soil. The lime particles dissolve when coming into contact with acid soils and will release þ Ca2þ and HCO 3 : this results in neutralization of H 3þ and Al . The rate at which the pH of the soil mass is affected depends on how close the individual lime particles are to one another. Since the diffusion of Ca is relatively slow the particles need to be close together so that the zones of neutralization around each particle overlap in a relatively short time. Calcitic and dolomitic limestones are generally ground so that all of the particles pass a 20-mesh sieve and a certain percentage passes a 100-mesh sieve depending on the regulations of the state or province. Calcitic limestones are often softer than dolomitic limestones and therefore dissolve at a faster rate. The relative efficiency of different particle sizes of these two materials is given in Figure 7. Particles held on a 10-mesh sieve have little value for neutralizing acid soils. Incorporation of Lime in the Soil
Since Caþ2 and HCO 3 ions have a slow rate of diffusion, the lime must be well mixed with the soil to neutralize soil acidity throughout the zone which is to be amended. One approach is to apply half of the lime and incorporate it by disking, and then apply the other half and disk to incorporate. However this results in high costs of application and incorporation rather than doing it only once.
Table 6 Neutralizing value of pure liming materials Material
Neutralizing value (%)
Calcium carbonate, CaCO3 Calcium-magnesium carbonate, CaMg(CO3)2 Calcium oxide, CaO Calcium hydroxide, Ca(OH)2 Calcium silicate, CaSiO3
100 109 179 135 86
Conclusion Poor plant growth on acid soils is due to toxicities of Al, H, and Mn and deficiencies of Ca, Mg, and Mo. Oxisols, Ultisols, and some Alfisols in their native state have pH values less than 5 and exchangeable Al of the effective CEC greater than 50%. Plant response to liming is due to neutralization of the toxic elements
358 LIPMAN, JACOB G.
and the supply of Ca and Mg. The lime requirement for a soil can be based on the amount of lime required to: (1) adjust the pH to a desired value; (2) neutralize exchangeable Al; or (3) neutralize the acidity measured by buffer pH methods. Factors to consider in selecting a lime material are fineness, purity, and the need for Mg. See also: Acidity; Calcium and Magnesium in Soils; pH
Further Reading Adams F (ed.) (1984) Soil Acidity and Liming, 2nd edn. Madison, WI: American Society of Agronomy, Crop Science Society of America, Soil Science Society of America. Alley MM (1981) Short-term chemical and crop yield responses to limestone applications. Agronomy Journal 73: 687–689. Cochrane TT, Salinas JG, and Sanchez PA (1980) An equation for liming acid soil mineral soils to compensate crop aluminium tolerance. Tropical Agriculture 57: 133–140. Evans CE (1968) Ion Exchange Relationships of Aluminum and Calcium in Soils as Influenced by Organic Matter. PhD thesis. Raleigh, NC: North Carolina State University. Evans CE and Kamprath EJ (1970) Lime response as related to percent Al saturation, solution Al, and organic matter content. Soil Science Society of America Proceedings 34: 895–896.
Fox RH (1979) Soil pH, aluminum saturation and corn grain yield. Soil Science 127: 330–334. Gonzalez-Erico E, Kamprath EJ, Naderman GC, and Soares WV (1979) Effect of depth of lime incorporation on the growth of corn on an Oxisol of Central Brazil. Soil Science Society of America Journal 43: 1155–1158. Jallah JK (1994) Assessment of Some Chemical Constraints to Root Growth in Four Acid North Carolina Soils. PhD thesis. Raleigh, NC: North Carolina State University. Kamprath EJ (1970) Exchangeable aluminum as a criterion for liming leached mineral soils. Soil Science Society of America Proceedings 34: 252–254. McCart GD and Kamprath EJ (1965) Supplying calcium and magnesium for cotton on sandy, low cation exchange soils. Agronomy Journal 57: 404–406. Osmond DL, Smyth TJ, Reid WS et al. (2002) Nutrient Management Support System, NuMaSS. (version 2.0). Raleigh, NC: Soil Management Collaborative Research Support System, North Carolina State University. Ritchey KD, Silva JE, and Sousa DMG (1983) Relac¸ a˜ o entre teor de calcio no solo e desenvolvimento de raizes avaliado por um metodo biologico. Revista Brasileira Ciencia Solo 7: 269–275. Sanzonowicz C, Smyth TJ, and Israel DW (1998) Calcium alleviation of hydrogen and aluminum inhibition of soybean root extension from limed soil into acid subsurface solutions. Journal of Plant Nutrition 21: 785–804. van Lierop W (1990) Soil pH and lime requirement determination. In: Westerman RL (ed.) Soil Testing and Plant Analysis, 3rd edn, pp. 73–126. Madison, WI: Soil Science Society of America.
LIPMAN, JACOB G. J C F Tedrow, Formerly with Rutgers University, New Brunswick, NJ, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Jacob G. Lipman was one of the most prominent international leaders in soil science. In 1901 he formed the Department of Soil Chemistry and Bacteriology at Rutgers University, the first of its kind in the USA, and probably the world. He was the driving force in internationalizing soil science. He also initiated and edited Soil Science, the first international journal dealing with soil chemistry, soil fertility, soil physics, and soil microbiology. Lipman (Figure 1) played a major role in the development of agriculture in the USA and beyond during the 1901–1939 period. His main contributions were in the area of soil microbiology and soil fertility,
particularly in relation to increased crop production. During later years, he was also involved in worldwide agricultural problems. Not only did he advance science on several fronts but he was also a gifted administrator. He addressed the broader aspects of agriculture, particularly in increasing the world food supply, controlling soil erosion, and improving the general welfare of humanity.
The Early Years Jacob Goodale Lipman was born in Friedrichstat, Russia, on November 18, 1874. He received his early academic training under private tutors in Moscow and in the classical gymnasium in Orenberg. In 1888, with his parents, he settled at the Baron de Hirsch colony in Woodbine, New Jersey. After graduating from farm school there he entered Rutgers