Extract from:

Potentially beneficial effects from liming: chemical and physical

E.O.McLean

Soil Crop Sci. Soc. Fla. Proc. 31:189-196 (1971)

...Lime may affect crops differently on temperate and tropical soils. ... The difference is not in whether crops respond to lime under both conditions but in what factors affect response to lime under each condition. Hence we are addressing the question of how responses to lime differ under tropical and temperate conditions and why they do so, rather than the question of whether responses occur. ... Since the primary objective in liming is to alter the chemical properties of a soil, it is essential to examine those properties that are altered. Solution of the world food production problem may well depend on our success in finding ways of making tropical soils more productive under a tropical climate which favors year-round plant growth. Correcting soil acidity or making conditions favorable for crops to tolerate it are steps in this direction.

The soil as a continuum from temperate to tropical conditions.

Each soil is a product of weathering in which environmental factors have been imposed upon a parent material through a period of time. The influence of these factors is modified by topography and by biological activity. Although exceptions occur, a soil developed in the humid temperate zone is usually less weathered than one developed in the humid tropical zone, due to lower temperatures and less intensive biological activity.

The leaching action of CO2-charged water percolating through the profile of a base-saturated soil removes free salts very quickly and exchangeable basic cations more slowly. Eventually a well-drained humid region soil becomes quite acid, unless bases are replaced by man or nature. H+ initially adsorbed to the cation exchange sites eventually become sufficiently concentrated to attack the clay crystal releasing Si4+ and Al3+. The released Al ions partially neutralized as AlOH2+ or Al(OH)2+ polymerize in the interlayers of the clay fraction or become complexed with organic materials, while the Si4+ leaches to lower levels in the profile. As the soil becomes still more acid, more Al and Fe are released from the clay minerals, and Al3+ remains as the dominant exchangeable cation. Ultimately most of the crystalline clay minerals give way to amorphous hydrous oxides of Al and Fe. In this frame of reference, the acid soil can be considered as a continuum from the slightly leached and weathered Mollisol through the progressively more leached and weathered Alfisols and Ultisols to the highly leached and weathered Oxisols (Table 1).

Table I Acid soil continuum: soil attributes pertinent to liming effects.

Attribute Mollisols Alfisols Ultisols Oxisols
1. Permanent Charge Acidity H+ (some AlOH2+) AlOH2+ Al3+ Al3+ (Fe3+)
2. Type of clays Illite, Smectite Chloritized smectite & Illite Kaolinite, Hydrous oxides Hydrous oxides, Kaolinite
3. Crystallinity of clays Crystalline Crystalline Crystalline & amorphous Amorphous & crystalline
4. Ionic Exchange Capacity High cation High cation (AlOH blocked) Low cation, low anion High anion, low cation
5. Base (Ca + Mg) saturation Relatively high Medium to low Low Very low
6. Phosphate fixing tendency Low Medium High Very high
7. Potassium release tendency High Medium Low Low
8. Structural stability (aggregate binder) Excellent (Ca) Good - fair (Ca & Al) Fair - poor (Fe & Al) Good - excellent (Fe & Al)

Soil attributes in the acid soil continuum pertinent to liming effects.

1. Permanent charge form of acidity. Starting with a Mollisol, H ions accumulate until a threshold conc. of approx. 10-3 mol (pH 5) is reached at which Al release becomes pronounced. Incorporation of the H+ into the crystal raises the pH of the soil solution forming polymerizable or complexable OH-Al. Polymerized AlOH neutralizes the permanent charges of the clay. Also, the massive size of the polymers prevent their displacement by other cations. If instead of a continuous layer of hydroxy-Al, the polymers occur as islands rather than complete layers, then the effective (net) CEC of the soil decreases with acid leaching or increases with liming. The change in net CEC of the soil is a consequence of a constant negative charge on the surfaces of the clay crystals being partially neutralized by the AlOH islands which vary in positive charge as they adsorb additional OH ions from the lime reaction, or lose them by neutralization with H+. At this point in the continuum we may have a fairly typical Alfisol.

With more leaching and greater concentration of H ions, the hydroxyls associated with the Al ions are neutralized leaving Al3+ as the predominant form of exchangeable acidity. With this increased weathering, practically none of the 2:1 type clays having internal surfaces for polymerizing OH-Al remain in the Ultisols and Oxisols.

2 & 3. Type and crystallinity of clays. Gradual removal of K from the illite permits it to swell forming smectite (or vermiculite) and concomitantly exposes the interlayers on which polymerization can occur. Further weathering and removal of silica causes destruction of the 2:1 type clays and initiation of synthesis of the 1:1 type clay (kaolinite).

Still further weathering removes most of the silica leaving hydrous oxides of Fe and Al with some residual kaolinite as the predominant forms of clay. Naturally as the crystalline clay minerals are broken down and the amorphous hydroxides accumulate; the distinct, characteristic X-ray patters give way to diffuse lines of the non-crystalline substances (Table I).

4. Ionic exchange capacity. Total CEC of soils is roughly proportional to concentrations of organic matter (OM) and of clay, especially swelling clay. Hence the CEC of young soils increases: a) as K is weathered from micaceous clays permitting them to swell, and b) as plants proliferate producing OM in the soil. However, as OH-Al accumulates and begins to polymerize on internal surfaces of the clay or is complexed by the COOH groups of OM, the effective CEC becomes progressively lower. With the destruction or inactivation of the organic exchange sites, the effective CEC may be almost nil.

On the other hand, the gradual to marked increase in hydrous oxide clays accounts for the concomitant increase in anion exchange capacity (AEC). Since the negative charges on both OM and hydrous oxide clays are highly pH-dependent, and the effective permanent charge of the clays of tropical soils is relatively low, a much higher proportion of the total CEC is pH-dependent in tropical than in temperate soils.

5. Base saturation (Ca). Since the weathering process removes Ca and other basic cations from the soil causing it to become progressively more acid, the saturation of the CEC with bases decreases correspondingly. The inverse relationship between exchangeable Ca + Mg, or total bases, and exchangeable Al has been widely reported.

6. Phosphate fixing tendency. As weathering progresses releasing Al and Fe from relatively inaccessible positions in mineral crystals to accessible positions in solution, on exchange sites, or a constituents of exposed surfaces; more of each ion reacts with soluble phosphate forming relatively insoluble compounds. However, once the Al and Fe are released in large quantities and coat most of the exposed surfaces of individual particles and granules, lime may not favorably affect the availability of P they retain.

7. Potassium release tendency. Parent materials containing micaceous clays generally have sufficient K for release of appreciable to quite large quantities of non-exchangeable K during the weathering process. As smaller amounts of micaceous clays remain, correspondingly less of this type of K is present to be released. However, as the H ion concentration and the temperature increase under tropical conditions, relatively more K is weathered from primary minerals, and a more rapid turnover of that in the plant residues also helps replenish the soil supply.

8. Natural stability of structure. The structure of the surface horizon of the relatively less weathered soils is basically granular with Ca- and OM- stabilized aggregates. The stability of this structure is excellent except where it has been destroyed by excessive cultivation, or where erosion, leaching, residue removal, and/or lack of lime have exhausted the reactive Ca and OM from the soil. As more Al and Fe are solubilized by the weathering process, the aggregates become stabilized by Fe- and Al-complexes and oxide or hydrous oxide coatings. Soils intermediate in the weathering continuum tend to have relatively unstable structure, since they have neither the high Ca saturation nor the Fe or Al complexes or coatings to produce the most stable structures (Table I).

Chemical effects of liming acid soils.

Ground limestone is a salt which acts as a base added to an acid:

Consequently, lime causes the following events to occur in an acid soil:

Not all of these events following liming are beneficial to plants. Under certain conditions some may be detrimental. Most of them are highly situation dependent. If a crop depends on acid weathering for one or more of the elements it requires, liming may be quite detrimental to its growth. Also, an adequately fertilized corn crop under continuous culture may not benefit at all from increased N fixation and mineralization while one grown in a rotation under limited fertilization may benefit much from the effect of lime on these microbial-effected events. It is evident that the sequence of these occurrences as primary and secondary causes of plant response to lime will differ with individual soil situations.

Physical effects of liming acid soils.

Less seems to be known about the effects of lime on the physical properties of soils than on the chemical properties. There are several reasons for this, First, changes in physical properties of soils are more gradual than most chemical changes. Second, many physical properties of a soil which exist in situ simply do not exist when the soil is disturbed. Hence, fewer studies have been made, and when made, inconclusive results are often the reward for one's effort.

1. Calcium as a flocculent and aggregate stabilizer. Since Ca is an excellent flocculent for negatively charged colloids, one might expect that temperate regions soils would reflect the direct effects on aggregation of additions of Ca from liming. However, field and laboratory data do not confirm any direct effect of lime on soil structure. It is recognized, however, that liming promotes greater development of vegetation and production of OM, which usually causes a regeneration of structure. It seems safe to conclude, in the light of all existing information that liming promotes better soil structure through its indirect effects upon OM production. and microbial activity. This suggests that maintenance of fertility through applications of fertilizers and lime and the management of crop residues would be more important to maintenance of good soil structure than any direct effects attributed to liming. This is not to imply that these indirect effects are any less real or any less important than they would be, if liming were directly responsible for them.

2. Lime (or Ca) as a microbial stimulant. Through the stimulation of microbial activity by liming, organic substances such as gums and resins are produced which coat and stabilize mineral aggregates. Yet the fact that liming may promote excessive rates of microbial decomposition of soil OM has been mentioned as a reason for not liming tropical soils to neutrality. Any accelerated rate of decomposition of soil OM as a consequence of liming such soils would be expected to adversely affect the structure and percolation rate of water through the soil profile.

3. Detrimental effects of calcium to iron and aluminum stabilized structure. In general, the structure of Oxisols has reached a high level of stability which accounts for the high infiltration rates and consequent rapid leaching of bases from these soils. The Ultisols have less free hydrous oxides for stabilizing the structure. ... Aggregate analysis has indicated that Ca and Mg have a dispersing action which increases the number of small aggregates at the expense of larger ones. Presumably this is a case of Fe- or Al-stabilized aggregates being dispersed by Ca2+, a phenomenon not unlike that where Na+ disperses Ca-stabilized aggregates in a Mollisol. In most cases, these effects are relatively small until the pH of the soil exceeds 7. It has been noted that liming has a much different effect on grey soils than on red soils of the tropics. The permeability on a grey soil increased with level of liming even above pH 7.3, while the permeability on a red soil decreased with liming (pH 6.0 to 7.9). The decreased permeability of the red soils was explained as a consequence of the peptizing effect of OH- ions in soils of low CEC (high AEC) overshadowing the coagulating effect of Ca ions. ...

Chemical changes likely affecting or not affecting crop response to lime on tropical soils.

Assuming no source of weatherable bases from recent deposits, the intense weathering moves rapidly toward depletion of bases, destruction of the crystalline clays, removal of the silica and formation of the residual hydrous oxide clays. Hence the list of events following liming would be expected to take on an entirely different sequence of priorities when considering plant response to lime on tropical soils. For example, Nos. 1, 7, and 9 are probably of minor importance while 2, 3, 5, 6, 8 and 10 may individually or collectively affect crop response to lime under tropical conditions. Since toxic levels of soluble Al (and Mn) are often the most limiting factors for plant growth in such highly weathered soils (No 5), the response to lime appears to be primarily a consequence of inactivation of these substances. In such cases, only sufficient lime to inactivate them is required. This amount also seems to provide adequate Ca (No 2) to meet the crop needs. Also any more than that amount may so curtail acid weathering (No 6) or compound solubility (No 4) as to cause deficiencies of Mn, Z, Cu or B.

The decreased pH-dependent AEC resulting from liming tropical soils releases SO42- ions (No 8) which couple with Ca2+ ions and leach readily through the profile. Also, this high concentration of salts (No 10) displaces H ions causing lower soil pH, as does the adsorption by the AEC of OH ions produced by hydrolysis of bases. This seems to be the reason for the relatively low pH at which maximum yields of crops occur on tropical soils. The high salt content may well displace more exchangeable Al into solution making it more toxic to plants.

With low effective CEC and large numbers of + charges from AEC to repel cations, the addition of lime results in more Ca remaining in solution and more adsorbed to relatively weak pH-dependent charges, which cause marked shifts in the rations of concentrations of Ca compared to those of other nutrients. This is quite the opposite problem to that in the temperate region soil where it is important to have some Ca adsorbed to pH-dependent CEC to mobilize additional Ca.

Summary of various phenomena relating to crop responses to liming in the acid soil continuum.

It seems evident that differences in pH for optimum growth not only exist for different plants, but also may occur for a given plant from one soil condition to another. As a matter of fact, it has been shown that in Hawaiian soils alfalfa took up maximum amounts of P from the fertilizer source at progressively lower pH values when the soil was more weathered. These observations and those showing progressively more lime-induced trace element deficiency and lime-induced structure deterioration with degree of weathering suggest that the optimum pH range for most plants decreases in the acid soil continuum from the Mollisols through the Oxisols (Table II).

Table II Summary of various phenomena related to crop responses to liming in the acid soil continuum.

Liming effect

Mollisols Alfisols Ultisols Oxisols
1. Optimum pH range 6.2 - 6.8 6.0 - 6.6 5.6 - 6.2 5.0 - 5.6
2. Crop response, rel. magnit. fair to good excellent excellent good to excellent
3. Cause of favorable response:

    a. chemical

1. pH adjustment

2. mobilized Ca

3. N fixation and metabolism

1. ph adjustment

2. Al inactivation

3. Mobilized Ca

4. N fixation, metab.

1. Al (& Mn)

2. Ca addition

1. Al (& Mn) inactivation

2. Ca addition inactivation

3. slowed weathering

    b. physical

improved aggreg. (flocculation and OM coatings) slightly improved aggregation none when limed at optimum pH none when limed at optimum pH
4. Adverse effects of overliming:

    a. chemical

hardly any 1. Zn or Mn def.

2. Mo toxicity

1. Zn, Mn (Cu, B) Def.

2. Mo toxicity

3. excess Ca

1. Zn, Mn, Cu, B def.

2. Mo toxicity

3. excess Ca

    b. physical

hardly any none dispersed aggregates dispersed aggregates

Although the pH range 5.0 - 5.6 may seem low for Oxisols compared to 6.0 - 6.6 for Alfisols; the former is reasonably consistent with reported data. The causes of favorable response to lime and the adverse effects on the soil of overliming are listed in the table in decreasing order in which each is most likely to occur. In a given situation, if a higher priority event does not prove limiting or stimulating to the crop, events of a lower priority (even those not listed) may then become significant. For example, a crop on a very acid soil which for some reason is low in exchangeable Al may not respond to inactivation of Al, but may respond to pH adjustment or to additional Ca.

The physical causes of favorable lime response appear to boil down to some slight improvement in aggregation in the less weathered soils, which effect is largely nullified by accelerated microbial decomposition of OM in tropical soils. Adverse physical effects of overliming are mainly a consequence of dispersed aggregates resulting from the dispersing action of the Ca2+ ion compared to Fe3+ or Al3+ ions or of the peptizing effect of OH- overshadowing the flocculating effect of Ca2+ in soils predominantly positively charged. It should now be evident why liming to near neutrality is not generally necessary in tropical soil. In fact, there are good reasons why yields of crops may be depressed by so doing. In such cases, only the amount required to inactivate toxic substances and supply essential Ca appear to be needed, but plants differ in their tolerances to the toxic substance and soils differ as to the element most toxic and as to the level at which toxicity is eliminated. Hence for the time being, testing the soils by a lime requirement test and then adjusting the pH to the above-suggested range may be more satisfactory over all than identifying the toxic substance, determining the level to which it must be inactivated for optimum plant growth, and then liming accordingly.

In contrast to tropical conditions, much research has shown that yields of most temperate region crops, though responding markedly to elimination of toxic levels of Al or Mn at lower pH, continue to increase with lime increments much above that required to inactivate toxic substances. Others have cautioned against efforts to apply to tropical soils information gained from lime studies in the temperate regions. It seems equally important not to make the same mistake of misapplying to temperate soils the knowledge gained on tropical soils.

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