PLSOIL 120
ORGANIC FARMING AND GARDENING

GUIDE FOR FERTILIZATION OF HORTICULTURAL CROPS

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CHAPTER 8

LIMING

Development of Soil Acidity

Except in regions of relatively low precipitation, where leaching does not occur and sometimes where a net movement of bases is upward rather than downward, acidification of soils is a natural process or is a process associated with crop production. In humid regions where precipitation is high enough, soils become acid because of the leaching of calcium, magnesium, potassium, and other exchangeable bases from the surface layers. This leaching leaves behind exchangeable hydrogen and aluminum, which are acids in mineral soils. In cultivated land, acidity develops from the oxidation of complex organic matter into organic acids. Some fertilizers, particularly ammoniacal chemical fertilizers and carbon-based organic fertilizers lead to acidification of soils. The absorption of cations (Ca++, Mg++, K+, NH4+) in excess of absorption of anions (NO3-, SO4--, H2PO4-) by plants also leads to a net production of acidity by the roots.

Crop Responses to Acid Soils

Cultivated plants differ widely in growth responses to acid soils. Crops are classified roughly into categories of acid-tolerant and acid-sensitive species. Generally, crops that have origins in the tropical or humid temperate regions of the Western Hemisphere are acid-tolerant because they have become adapted to the acid soils of this region. Corn, tomatoes, potatoes, garden beans, and pumpkin are acid-tolerant crops with origins in the Western Hemisphere. Conversely, crops that originate from the neutral or alkaline soils of the Mediterranean Region, western and southern Europe, Asia Minor, and northern Africa are likely to be acid-sensitive. Many of our commonly grown vegetable crops, herbs, and flowering bulbs have been domesticated from neutral or alkaline soils. Even the acid-tolerant plants do better in soils that are in the range of pH 6 to 7 than in more acid soils, because of the favorable effects that this range of acidity has on the availability of plant nutrients.

In the United States, soils that are east of a line that extends southward from the eastern boundary of North Dakota to east central Texas will require liming, because precipitation in this region is sufficient to leach bases downward. Soils that are west of this line, with exceptions of some soils of the west coast, usually do not require liming.

Acid soil infertility is a syndrome of problems that affect plant growth in soils with low pH. This complex of problems arises from toxicities and deficiencies that occur in acid soil. Hydrogen ion concentration or acidity as such is not a major factor in poor plant growth in acid soils. A soil must be about pH 4 or lower for hydrogen ions to be toxic to plants. Among the toxic factors in acid soils are aluminum and manganese ions that increase in concentrations in the soil solution as pH falls. Plant roots particularly are sensitive to injury from elevated concentrations of aluminum and manganese. Seedlings and young plants are much more sensitive to soil acidity than older plants. In acid mineral soils, the detrimental effects of soluble aluminum and manganese are greater on plant growth than the effects of hydrogen ions, even at pH 4.

In certain soils with very low cation exchange capacities, calcium and magnesium deficiencies may develop, but in general, management practices such as adding fertilizer salts, manures, or composts, adequately supply these nutrients to crops so that even in acid soils these elements are seldom deficient. On the other hand, acidification lowers the availability of many plant nutrients in soil. This effect is pronounced in coarse-textured soils, which are inherently lower in nutrients and which are weakly buffered. Soil acidification may lead to dissolution of nutrients and to their subsequent leaching from the root zone. This effect becomes pronounced with time. In strongly acid soils, microbiological activities related to nitrogen transformations (mainly nitrification) may be inhibited, but the agricultural importance of these effects are unclear. In acid soils, the availability of phosphorus is limited by the formation of insoluble iron and aluminum phosphates. This reaction is phosphorus fixation, which is of major concern in growth and fertilization of crops in acid soils. Because of its low solubility, rock phosphate provides little phosphorus to crops in soils above pH 5.5. Also, molybedenum becomes insoluble in acid soils to the point that some crops may become deficient in this minor element.

Correction of soil acidity by application of agricultural liming materials (referred to commonly as lime) usually alleviates the problems that result from acid soil infertility (Table 32). Agricultural liming materials contain compounds that are carbonates, hydroxides, or oxides of calcium and magnesium. The anions of these compounds neutralize hydrogen ions in soil solution and on the exchange sites of soil colloids and remove toxic materials from solution by precipitation.

Table 32. Liming materials for agricultural soils.

________________________________________________________________________

 

Materials and formula

Calcium carbonate equivalency

Agricultural limestones:

Calcite



CaCO3



100

Dolomite

CaCO3MgCO3

108

Quicklime

CaO

179

Hydrated lime

Ca(OH)2

133

Wood ashes

(Calcium, potassium, and magnesium oxides and carbonates)

50

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Agricultural limestones. Agricultural limestone is the principal material used to adjust soil pH. It is usually a variable mixture of calcium carbonate (calcite) and magnesium carbonate (dolomite), so the actual calcium carbonate equivalency of a given limestone lies between that of calcite and dolomite. The various kinds of mixtures of limestones between calcite and dolomite are called intergrades. Specific intergrades are identified by their magnesium contents. Calcium carbonate equivalency is an expression of the total acid-neutralizing capacity of limes in units of a weight percent of calcium carbonate. The calcium carbonate equivalency of dolomite is slightly higher than that of calcite because of the lower molecular weight of magnesium carbonate relative to calcium carbonate. Reactivity, the rate at which limestone reacts in the soil, is a more important factor in assessing the value of limestones than calcium carbonate equivalency. Reactivity is governed by composition and by particle size. Since increases in crop yields have been correlated with quick neutralization of soil acitity, it is desirable to use limestone that reacts rapidly with the soil. The reaction of limestone with acid in the soil, using calcite as a model, is summarized by the following reaction.

 

CaCO3

+

2H+

--------->

Ca++

+

H2O

+

CO2

Calcite

Hydrogen
ions

Calcium ion

Water

Carbon dioxide

  

Limestone dissolves in water as expressed by the following reaction.

 

CaCO3

+

H2O

----------->

Ca++

+

HCO3-

+

OH-

Calcite

Water

Calcium ion

Bicarbonate

Hydroxide

Neutralization is a function of the reaction of acid with the carbonate of the limestone or with the bicarbonate or hydroxide formed by dissolution of limestone. The calcium (or magnesium) of limestone does not bring about neutralization, but these ions will replace hydrogen ions from soil colloids forcing these hydrogen ions into the soil solution where they will react with the limestone, bicarbonate, or hydroxide and be neutralized.

Dolomite is a harder substance and is slightly less reactive than calcite. In the short-run, equal sized particles of dolomite would not be as effective in neutralizing acidity as those of calcite. After 3 years, differences between limestones of different composition would be hardly noticable. Generally the reaction of limestone with the soil is complete in 2 or 3 years, but much of the reaction occurs within the first few weeks or months. Finely ground limestones neutralize soil acidity more rapidly and enhance crop yields more than coarse limestones. Pulverization adds to the cost of the product, and benefits of fineness and cost of pulverization must be balanced. Generally, limestone ground to pass a 60-mesh screen provides the benefits of quick neutralization of soil acidity and cost-effectiveness. Limestones ground more finely may be too expensive, and losses may occur through blowing of the limestone during spreading. Particles larger than 10-mesh have virtually no effect on soil pH at normal amounts of application.

The difference in neutralizing capacities between calcite and dolomite increases as the particles become coarser, but with particles of 60-mesh or finer, the differences are very small. In practice, most agricultural limestones are mixtures of calcite and dolomite so that if the particles are finer than 60-mesh, differences among types (intergrades) of limestones may be indetectable. Dolomitic limestones have the advantage of providing magnesium as well as calcium and would be the preferred choices. Analyses of calcium, magnesium, and calcium carbonate equivalency are reported on the bags of limestone and should be available from the vendor of bulk-spread limestones.

Quicklime and hydrated lime. Quicklime is sometimes called burnt lime or garden lime. The heat from burning materials decomposes the carbonates of limestone to oxides with loss of carbon dioxide to the air. Calcium and magnesium oxides will be formed in proportion to the amount of calcium and magnesium carbonate present.

The following equation demonstrates the reaction that occurs with calcite.

CaCO3

+

Heat

--------------->

CaO

+

CO2

Calcite

Calcium oxide
(Quicklime)

Carbon
dioxide

For rapid neutralization of acidity or to raise pH higher than 6.5, growers should consider using quicklime or hydrated lime. Hydrated kime is formed by reacting quicklime with water.

CaO

+

H2O

------------------>

Ca(OH)2

This process is called slaking; hence, the name slaked lime is used frequently for hydrated lime. Hydrated lime mixed with water is whitewash.

Basic reactions for the neutralization of acidity by these compounds are as follows.

Quicklime:

CaO + 2H+ --------------> Ca++ + H2O

Hydrated lime:

Ca(OH)2 + 2H+ --------------> Ca++ + H2O

The neutralization occurs by the reaction of the hydrogen ions (acid) with the oxide or hydroxide and the resulting formation of water. The calcium ions that are formed will replace adsorbed hydrogen ions on the colloids, forcing them into solution where they will be neutralized by the limes.

Before 1900, much of the lime used was quicklime. Burning served at least two purposes other than providing quick reactivity of the lime with soil. It reduced the weight for hauling (loss of carbon dioxide gave a 44% loss of weight), and since rock crushers were not conveniently located, burning reduced the product to an amorphous material with the proper state of subdivision. Quicklimes are not used often today, except when the grower has a need to accelerate the raising of soil pH.

Wood ashes. Burning of wood, bark, or paper products leaves ash, the composition of which depends on the amounts of alkaline metals (calcium, magnesium, potassium) present in the original materials. The first products are oxides. With aging, these oxides will be slaked and ultimately will react with carbon dioxide in the air to form carbonates. The effectiveness of wood ashes as lime depends on their composition, which varies with the product burned and with handling (aging, leaching). In general, wood ashes have about half the calcium carbonate equivalency of agricultural limestone.

Management related to application of limes

Amount of application. Response to limes varies for different crops and even for varieties of crops on the same soils, and this response can be determined best through experimentation. However, most growers are not interested in laying out a liming experiment to determine the lime requirements of their soils. Soil testing is a common means for estimating lime requirements Soil pH is the principal criterion used in determining the need for lime. Soil pH is an expression of active acidity, which is the hydrogen ions in the soil solution (Figure 16). However, most of the acidity of soils resides on the soil colloids, which are the clays and organic matter. This fraction, referred to as reserve acidity, constitutes the major portion of soil acidity. Only a small amount of limestone, possibly only a few humdred pounds, are needed to neutralize the active acidity, whereas thousands of pounds of limestone may be required to neutralize the reserve fraction. Because of this fact, lime requirement varies with soil texture. A fine-textured soil will require a larger amount of liming materials to reach a desired pH than a coarse-textured soil. Some guidelines for determining the lime requirement of soils of various textures are given below (Table 33).

 

Figure 16. Reserve acidity held to soil colloids (clay and organic matter) in equilibrium with active acidity in soil solution.

 

Table 33. Agricultural limestone needed to raise pH of soil one unit (recommendations are based on 60-mesh calcite).
___________________________________________________

Soil Textural

Class

Lime Requirement,

tons/acre

Sand

1.00

Sandy loam

1.33

Loam

2.00

Silt and silt loam

2.33

Clay and clay loam

2.50

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Examples of application of the above guidelines are as follows: A sandy loam of pH 5.0 would require 2.0 tons of agricultural limestone to raise it to pH 6.5 (1.5 units of pH times 1.33 tons per unit), and a clay loam for the same adjustment in pH would require 3.75 tons of limestone (1.5 units times 2.50 tons per unit). The more potent materials, quicklime and hydrated lime, would be used in lesser amounts than agricultural limestone. To lime the sandy loam in the previous example to pH 6.5 with quicklime, 1.1 tons would be used, for quicklime has 1.79 times the calcium carbonate equivalency of calcite (Table 32). The amount of hydrated lime needed would be 1.5 tons per acre, for hydrated lime has an equivalency 1.33 times that of calcite. Some other conversions are presented in the following table (Table 34).

 

Table 34. Examples of amounts of various limes needed to raise a soil from pH 5.0 to pH 6.5.

________________________________________________________________________

Texture of soil

-----------------------------------Amount of lime needed, tons per acre----------------------------------

Agricultural limestone|-------Quicklime---------|------Hydrated lime-----|-------Woodash-------

Sand

1.50

0.83

1.12

3.00

Sandy loam

2.00

1.12

1.50

4.00

Loam

3.00

1.67

2.25

6.00

Silty soils

3.50

1.96

2.63

7.00

Clayey soils

3.75

2.10

2.82

7.50

 Since the guidelines are only rules-of-thumb, more accurate methods of estimating lime requirements have been developed by soil-testing laboratories. The results of these tests are reported as buffer pH or the lime requirement (or as lime index) the amount of lime needed to raise the soil to pH 6.5 or so. The buffer pH is an expression of the sum of active and reserve acidities, or what is referred to as titratable acidity.

All methods to estimate lime requirements consider that only half to two-thirds of agricultural limestone reacts in the short term to neutralize acidity. Limestone is even less effective if it is coarse or mixed poorly with the soil, or if the pH is to be raised above 6.5. Also, recommendations are based on application of lime only in the top 3 inches of soil. If lime is mixed in the top 6 inches of soil, the lime requirement will be twice that of lime mixed in the top 3 inches of soil.

Time of Application

Lime can be applied to land at any time of the year that weather permits. Normally, lime should be applied as far ahead of seeding or planting of a new crop as practicable. Generally this amount of time is 3 months to a year. The fall before a spring crop is planted is a convenient time for liming. In the fall, fields are likely to be dry, making it easier to get equipment across the fields than in the spring. Commonly, growers fail to make the application until after the seedbed is prepared in the current year. In this case, quicklime or slaked lime is used. Generally, not more than 0.5 to 0.75 ton of quicklime or hydrated lime should be applied per acre per year to avoid overliming. Additionally, these materials work very quickly in the soil, but their effects also diminish faster than those of agricultural limestone. Ordinary agricultural limestone applied shortly before or at planting will have beneficial effects, and growers should not hesitate to use agricultural limestone in this fashion.

Methods of Application

Most commonly, liming materials are broadcasted to soil surfaces in amounts of 1 to 6 tons per acre and are mixed into the soil by tillage--disking or harrowing. Plowing under of limes should be avoided to prevent placement of lime at a depth at which it will have little benefit to shallowly rooted young seedlings. Often a lack of uniform mixing in the plow layer occurs with heavy applications. Poor mixing of lime in the soil may lead to poor neutralization of the soil in the short run. Over a period of time the neutralization becomes uniform as the lime is mixed by further tillage of the soil and by reaction of the limestone in the soil.

Uniform lateral application across the soil surface is important. This step is more important than uniform mixing vertically into the soil. Lime applied at recommended rates does not need to be mixed into the soil more than about 3 inches. It is important that the pH be adjusted correctly in this surface zone to protect germinating seeds from acid soil and for seedlings to become established. If the lime is mixed into the soil at greater depths, the amount of application should be increased to keep the weight of lime per unit weight of soil constant.

The principal problem of poor mixing of lime with soils is the incomplete neutralization of acidity and resultant slow change in pH. Because of poor mixing of lime or uneven spreading across the surface, pockets of high lime concentrations may occur. This condition occurs frequently when lime is mixed with moist soils, which hinder uniform dispersal of lime in the medium. These pockets may lead to a condition known as the overliming effect. Because of the possibility of the overliming effect, agronomists recommend that no more than 4 tons of limestone be applied per acre per year, even if the lime requirement exceeds 4 tons per acre. Lime requirements in excess of this amount should be met with applications split between two years. In many cases the effects of overliming go unnoted because these effects are less than the benefits of correcting soil acidity. The causes of the detrimental effects of overliming are not known but they are short-lived and usually are not apparent after 2 years, at which time the maximum benefits of liming will be expressed in crop yields.

Once a plot has been raised to about pH 6.5, it will remain at a constant level with moderate applications (2 tons/acre) of lime at intervals of up to 5 years. These applications will keep the pH more constant from year to year. Needs for reliming are governed by soil texture, leaching, cropping, cultivation practices, fertilization, and nature and fineness of the original liming materials, as well as by the crop that is to be grown. The pH should be checked every 2 to 3 years to ensure that the pH has not dropped into a range that would restrict crop yields. It is recommended commonly that soil be limed if pH falls below 6.0.

 

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Maroon Divider

Produced and maintained by Allen V. Barker
University of Massachusetts, Amherst.
last updated - March 23, 1999