# Chapter 19 pH, liming, and lime requirements.

OBJECTIVE

There are few measurements more important or useful in soil science than pH, and there are few practices more likely to increase plant yield than liming or (rarely) acidifying soil to an appropriate pH. In this chapter you will

* review the meaning of pH.

* practice calculations to determine the liming potential of various compounds.

* calculate lime requirements for soil at various pH.

Overview

Soils are unique physical, chemical, and biological environments. But it is important to recognize that soils are also managed environments--managed by crop selection, fertilization practices, and tillage operations. In terms of soil amendments then, it is critical to be able to calculate how much of an amendment to add to bring about the desired result. This is particularly important in the case of managing pH by liming, which is the topic of this chapter.

pH

It bears repeating that pH is the negative logarithm (base 10) of the hydronium ion ([H.sub.3][O.sup.+]) activity, which is very nearly the same as the [H.sub.3][O.sup.+] concentration in water.

pH = -[log.sub.10] [H.sub.3][O.sup.+] [19-1]

You are more likely see the hydronium ion represented as [H.sup.+], which is what we will do for the rest of this chapter.

A soil with a pH of 7 is considered neutral. A soil in which pH <7 is acidic. A soil in which pH >7 is basic or alkaline. The pH declines when the [[H.sup.+]] increases, and the pH rises when the [[H.sup.+] decreases (Table 19-1).

Example 19-1

What is the [[H.sup.+]] when the pH is 4?

Solution

[[H.sup.+]] = [10.sup.-4] M

Example 19-2

If the [[H.sup.+]] is [10.sup.-7], what is the pH?

Solution

pH = 7

Liming

Liming is synonymous with raising soil pH, although adding limestone (CaC[O.sub.3]) or lime (CaO) is not the only way to accomplish this. Limestone works on the basis of the following reaction:

CaC[O.sub.3] + 2[H.sup.+] [right arrow] [Ca.sup.2+] + [H.sub.2]O + C[O.sub.2] (g) [19-2]

Various materials have differing capacities to neutralize soil acidity (Table 19-2). For example, 1 ton of hydrated lime has the equivalent neutralizing capacity of approximately 1.4 ton of calcitic limestone. The effectiveness of liming materials is determined by its effective calcium carbonate equivalence (ECCE). You may also see this referred to as the relative neutralizing value (RNV) or effective neutralizing value (ENV). This is really a combination of two factors--the effectiveness of a material (how fine it is) x calcium carbonate equivalence (CCE) (how pure it is).

The smaller the size of the liming material, the more reactive it is. Fineness is measured by the ability to pass through meshes of different sizes--the larger the mesh size, the finer the particle (Table 19-3). Anything unable to pass through a 10 mesh sieve is chemically inert for the purpose of liming.

Liming materials, particularly limestone, are usually not completely pure but contain materials that do not contribute to neutralizing capacity. The CCE for a given liming material is adjusted to reflect its purity.

Example 19-3

What is the CCE of dolomitic limestone that is only 85 percent pure?

Solution

0.85 X 110% = 93.5%

It would require 1.07 ton of dolomitic limestone with this purity to have the same neutralizing capacity of 1 ton of calcitic limestone.

To calculate the ECCE, use the following equation:

ECCE = (%CCE/100) x 0.5 (%< 10 mesh + % < 50 mesh) [19-3]

Example 19-4

A supply of calcitic limestone is purchased that is 75 percent pure. Ninety five percent of the material passes through 10 mesh, while only 75 percent passes through 50 mesh. What is the ECCE?

Solution

ECCE = (75%/100) x 0.5 (95% + 75%) = 63.8%

How do you use the ECCE to calculate lime application rates?

Example 19-5

If the liming recommendation called for 3 ton of calcitic limestone, based on the ECCE from Example 19-4, how much should be added?

Solution

Actual lime desired rate = desired rate x CCE of liming material/ECCE of material applied

Actual lime rate = 3 ton [acre.sup.-1] x (100%/63.8%) = 4.7 ton [acre.sup.-1]

Lime Requirements

There are three kinds of acidity that contribute to the total acidity of soil--active acidity, exchangeable acidity, and residual or reserve acidity. Active acidity is what you measure when you measure the water pH of a soil. Exchangeable acidity reflects the exchangeable [Al.sup.3+] and [H.sup.+] on clay and organic matter surfaces. However, residual acidity represents the greatest contributor of acidity to soil (1000 to 10,000 times as much as the other two), so liming recommendations are based on neutralizing residual soil acidity. There is an equilibrium between [H.sup.+] and [Al.sup.3+] in soil solutions, which contributes to active acidity, and that remaining on the soil exchange surfaces as part of residual acidity, so lime requirements can be based on water pH alone. However, most soil testing services measure the pH in equilibrium with buffer (the buffer pH) and derive liming recommendations accordingly.

In most cases, if the water pH is greater than 6.4, liming is not required because there will often not be any demonstrable increase or decrease in yield. When lime is required, the relationship between water and buffer pH can be used to determine the lime needed, which typically assumes a CCE of 67 to 80 percent (based on the quality and size of most agricultural limestones). An example of a table that can be used to calculate lime requirements is shown in Table 19-4.

Example 19-6

How much lime is required to raise the soil pH to 6.4 if the water pH is 5.1 and the buffer pH is 6.3?

Solution

Read across Table 19-4 on the row containing a water pH of 5.1. Read down the table underneath the buffer pH of 6.3. Where row and column intersect, the value is 4.0. Therefore, the recommendation is for 4.0 ton [acre.sup.-1] of agricultural limestone (assuming a CCE of 67 percent) to raise the pH to 6.4.

Some tables do not require you to use water pH to calculate the lime requirement. They will be based on the buffer pH alone.

In addition to assuming a CCE of 67 percent, or some equivalent value, most lime recommendations assume that the material will be uniformly applied and incorporated to a depth of approximately 6 in (15 cm). The recommendation also assumes that approximately 4 years will be required for the limestone to completely react, although materials such as slaked or burnt lime react much more quickly. If liming a greater soil depth is required, then the recommended values must be increased accordingly.

Sample Questions

pH

1. What is the pH if the [[H.sup.+]] is 0.0005 M?

2. Which solution has more [H.sup.+], one at pH 7 or one at pH 9?

3. If the [[H.sup.+]] concentration is 0.01 M, what is the pH?

4. Calculate the [[H.sup.+]] at pH 9.

5. Calculate the [[H.sup.+]] at pH 6.5.

Liming

1. What is the CCE of hydrated lime that is only 95 percent pure?

2. What is the ECCE of pure calcitic limestone for which 75 percent passes a 10 mesh and 65 percent passes a 50 mesh?

3. What is the ECCE of dolomitic limestone that is 95 percent pure and is composed of particles that all pass through a 50 mesh screen?

4. If the liming recommendations of a state are based on an ECCE of 80 percent and the soil test suggests that 4 ton [acre.sup.-1] of lime is needed, how much material should you add if you have a source of dolomitic limestone that is 95 percent pure and for which 95 percent of the particles pass a 10 mesh, but only 50 percent pass a 50 mesh sieve?

5. If the ECCE of a liming material is 70 and the lime rate for the state is based on material with an ECCE of 83 percent how should the lime rate be adjusted if you determine that the ECCE of your liming source is 90 percent?

Lime Requirements

1. If the water pH is measured as 6.5, how much lime is required to raise the soil pH to 6.4?

2. If the measured water pH is 5.5 and the buffer pH is also 5.5, how much lime is required to raise the pH to 6.4?

3. If the water pH is 4.9 and the buffer pH is unknown, how much lime is required to raise the soil pH to approximately 6.4?

4. If the final soil pH is desired to be 6.4 and the buffer pH is 6.5, how much lime must be added to accomplish this goal if the water pH is 5.7?
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TABLE 19-1 Relationship of pH to H+ Concentration

3   0.001 M ([10.sup.-3] M)           Strongly acidic
4   0.0001 M ([10.sup.-4] M)
5   0.00001 M ([10.sup.-5] M)
6   0.000001 M ([10.sup.-6] M)        Slightly acidic
7   0.0000001 M ([10.sup.-7] M)       Neutral
8   0.00000001 M ([10.sup.-8] M)      Slightly alkaline
9   0.000000001 M ([10.sup.-9] M)
10   0.0000000001 M ([10.sup.-10] M)   Strongly alkaline

TABLE 19-2 Calcium Carbonate Equivalence (CCE) of Various
Liming Materials

Formula Weight
Type of Lime          Formula         (g [mol.sup.-1])   CCE Range (%)

Calcitic        CaC[O.sub.3]               100.1             80-100
limestone

Marl            Impure CaC[O.sub.3]                          70-90
(Mostly shells)

Dolomitic       CaMg [(C[O.sub.3])         184.4            110
limestone       .sub.2]

Quick lime      CaO                         56.1            150-180
(Burnt lime)

Hydrated lime   Ca[(OH).sub.2]              74.1            120-140
(Slaked lime)

Wood ash        Various oxides                               30-70
(e.g., MgO)

TABLE 19-3 Mesh Size and Relative Liming Effectiveness

Relative Liming
Mesh Size   Exclusion Range           Effectiveness (%)

>50         Passes through 50 mesh          100

10-50       Passes through 10 mesh,          50
but retained on 50 mesh

<10         Retained on 10 mesh               0

TABLE 19-4 Rate of Agricultural Limestone (tons
per acre) Needed to Raise Soil pH to 6.4

Buffer pH of Sample
Water pH                                           If Buffer pH
of Sample  5.5  5.7  5.9  6.1  6.3  6.5  6.7  6.9   is Unknown

Ag. Limestone Rate (tons per acre)

4.5        7.0  6.0  6.0  5.0  4.0  4.0  3.0  3.0      4.0
4.7        7.0  6.0  6.0  5.0  4.0  4.0  3.0  3.0      4.0
4.9        7.0  6.0  6.0  5.0  4.0  4.0  3.0  3.0      4.0
5.1        7.0  6.0  5.0  5.0  4.0  3.0  3.0  2.0      4.0
5.3        7.0  6.0  5.0  4.0  4.0  3.0  3.0  2.0      3.5
5.5        6.0  5.0  5.0  4.0  4.0  3.0  2.0  2.0      3.0
5.7        6.0  5.0  4.0  4.0  3.0  3.0  2.0  2.0      2.5
5.9         --  5.0  4.0  3.0  3.0  2.0  2.0  1.0      2.0
6.1         --   --  3.0  3.0  2.0  2.0  1.0  1.0      1.5
6.3         --   --   --  2.0  1.0  1.0  1.0  1.0      1.0

Note. 2001-2002 Lime and Fertilizer Recommendations, AGR-1.
Lexington, KY: University of Kentucky, Cooperative Extension
Service.
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