lab 8 determination of ph and lime … 8 determination of soil ph...sst3005 – fundamentals of soil...

SST3005 – Fundamentals of Soil Science PJJ, PPL / UPM

LAB 8

DETERMINATION OF pH AND LIME REQUIREMENT OF SOIL

Learning outcomes

The student is able to:

1. determine soil pH and its nature

2. measure lime requirement of a soil

Introduction

We are interested in soil pH because it plays an important role in plant growth.

Soil pH influences many facets of crop production and soil chemistry, including

availabilities of nutrients and toxic substances, activities and nature of microbial

populations, solubility of heavy metals, and activities of certain pesticides. The soil

pH is easily determined and, like taking your temperature when you are sick, it gives

us some quick, valuable information that will enable the "Plant Doctor" to prescribe

corrective procedures.

pH is defined as the negative logarithm of the hydrogen ion (H+)

concentration. When water ionizes to H+ and OH- (a neutral solution), both H+ and

OH- ions are in equal concentrations of 0.0000001 moles per liter. That is a very

small concentration.

HOH <—> H+ + OH-

[H+] = [OH-] = 1 x 10-7 moles/liter. The H+ ion and OH- concentrations in water are

very small.

The pH scale has been devised for conveniently expressing these small

concentrations by expressing

pH = -Log [H+]


When the hydrogen concentration is greater, such as 0.0001 moles per liter,

the pH is 4; when it is smaller, such as 0.00000001, the pH is 8. One thing to

remember is that when the pH changes from one unit to another, the change in the

hydrogen ion concentration is a ten-fold change, not just one. So a pH of 5 is ten

times more acid than a pH of 6 and 100 times more acid than a pH of 7.

Causes for Acid Soils

The pH of a soil is dependent on the parent material, the climate, the native

vegetation, the cropping history (for agricultural soils), and the fertilizer or liming

practices.

The pH range for most mineral soils would be from 5.5 to 7.5. This is also the

range for most soils found in Malaysia.

Exchangeable hydrogen is the principal source of H+ until the pH of the soil

goes below 6. Below 6, exchangeable aluminum becomes the source of hydrogen

ions, due to the dissociation of Al from clay minerals. For simplicity, we will use the

term "exchangeable H" for the cause of acid soils.

Soils tend to become acidic as a result of: (1) rainwater leaching away basic

ions (calcium, magnesium, potassium and sodium); (2) carbon dioxide from

decomposing organic matter and root respiration dissolving in soil water to form a

weak organic acid; (3) formation of strong organic and inorganic acids, such as nitric

and sulfuric acid, from decaying organic matter and oxidation of ammonium and

sulfur fertilizers. Strongly acid soils are usually the result of the action of these strong

organic and inorganic acids.

Sources of H+ ions in the soil:

1) dissociation of carbonic acid, which forms readily in soils when CO2 is

present;

2) organic acids formed during the decomposition of organic matter;


3) the burning of coal in electrical power plants releases sulfur to the

atmosphere which is added to soils during precipitation as sulfuric acid, and

fertilizers containing sulfur, which adds H+ ;

4) the conversion of NH4+ to NO3

- releases H+ during the nitrogen cycle or

when nitrogen fertilizers are added to soils.

pH is < 4.0 =indicates that the soil contains free acids probably as a result of

sulfide oxidation.

pH is < 5.5 =indicates that the soil's exchange complex is dominated by Al

pH is < 7.8 =soil pH is controlled by a range of factors

pH is > 7.8 =indicates that the soil contains CaCO3

Where leaching is minimal, the concentration of basic cations (Ca++, Mg++, K+,

and Na+) on the exchange complex will be large. These basic cations will come from

the weathering of rocks and minerals, from dust blown on soils, from irrigation water

or runoff water. When basic cations dissociate in the soil solution, they will produce

hydroxyl ions (OH-). This will raise the pH of the soil.

The "pH of the soil" refers to the concentration of hydrogen ions in the soil

solution--not on the exchange complex.

Descriptive terms commonly associated with certain ranges in soil pH are:

Table 8.1 : The descriptive terms associated with certain pH Ranges.

Acidity pH Examples

Extremely acid <4.5 Lemon(pH=2.5); vinegar (pH=3.0); stomach acid(pH =2.0); soda

(pH = 2-4)

Very strongly acid 4.5 – 5.0 Tomatoes (pH=4.5)

Strongly acid 5.1 – 5,5 Carrots (pH = 5.0); asparagus (pH = 5.5), boric acid )pH = 5.3);

cabbage (pH=5.3)

Moderately acid 5.6 – 6.0 Potatoes (pH = 5.6

Slightly acid 6.1 6.5 Salmon (pH = 6.2); cow milk (pH = 6.5)

Neutral 6.6 – 7.3 Saliva (pH = 6.6 -7.3); blood (pH=7.3); shrimp (pH=7.0)

Slightly alkaline 7.4 – 7.8 Eggs (pH= 7.6 – 7.8)

Moderately alkaline 7.9 – 8.4 Sea water (pH = 8.2); sodium bicarbonate (pH = 8.4)

Strongly alkaline 8.5 – 9.0 Borax (pH = 9.0)

Very strongly

alkaline >9.1 Milk of magnesia (pH=10.5); ammonia (pH=11); lime (pH=12)


Significant of Soil pH

The effect of soil pH is great on the solubility of minerals or nutrients.

Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before

a nutrient can be used by plants it must be dissolved in the soil solution.

Most minerals and nutrients are more soluble or available in acid soils than in neutral

or slightly alkaline soils. Phosphorus is never readily soluble in the soil but is most

available in soil with a pH range centered around 6.5. Extremely and strongly acid

soils (pH 4.0 - 5.0) can have high concentrations of soluble aluminum, iron and

manganese which may be toxic to the growth of some plants. A pH range of

approximately 6 to 7 promotes the most ready availability of plant nutrients.

There are of course exceptions, such as azaleas, rhododendrons, blueberries,

white potatoes and conifer trees, which tolerate strong acid soils and grow well. Also,

some plants do well only in slightly acid to moderately alkaline soils. However, a

slightly alkaline (pH 7.4 - 7.8) or higher pH soil can cause a problem with the

availability of iron to pine oak and a few other trees causing chlorosis of the leaves

which will put the tree under stress leading to tree decline and eventual mortality.

This is also a problem on beans and strawberries.

The soil pH can also influence plant growth by its effect on activity of

beneficial microorganisms. Bacteria that decompose soil organic matter are hindered

in strong acid soils. This prevents organic matter from breaking down, resulting in an

accumulation of organic matter and the tie up of nutrients, particularly nitrogen, that

are held in the organic matter.

Changing Soil pH

The pH of the soil is dependent on the quantity of hydrogen ions in the soil

solution. If we want to raise soil pH, we need to increase the quantity of OH- ions in

solution. However, when more H+ ions are removed from the solution, they are

replaced by hydrogen ions that were held on the cation exchange sites.

This ability of the soil to withstand rapid changes in pH is important for plant

growth. However, it means that the total amount of bases needed to raise the pH is

dependent on the total amount of hydrogen ions held on the reserve. This is referred

to as buffering capacity.


This diagram shows a convenient way to think of the capacity of the soil to

resist changes in pH (the soil's buffering capacity). The active acidity (soil solution

hydrogen ions) is in the small outside spout. The reserve acidity is in the big tank.

When we remove some active acidity by adding a base such as CaCO3, we remove

some hydrogen ions from the active acidity. However, the original soil acidity of the

soil solution is very rapidly restored. So, in order to change soil pH, we need to

change the percent base saturation.

It is common practice in soil testing labs to make direct measurements of lime

requirements by using a buffer solution. A buffer with a pH of 7.5 is mixed with a

known quantity of soil. The exchange acidity is replaced from the exchange sites,

and the depression in pH from 7.5 is a measure of the total acidity

Liming Soils

Raising soil pH requires a quantity of agricultural liming material that is

determined by the amount of acidity in the soil and the quality of the liming material.

Soil acidity is measured by soil testing; the quality of agricultural liming material is

determined by its purity and particle size distribution.

An agricultural liming material is defined as a material containing calcium (Ca)

and/or magnesium (Mg) compounds capable of neutralizing soil acidity. These

materials include: limestone (both calcitic and dolomitic), burned lime, slaked lime,

marl, shells, and by-products such

as sugar beet sludge, and sludge

from water treatment plants.

Fluid lime is a term that is generally

used to describe the concept of

suspending liming materials of

various types in either water or

fertilizer solutions. Frequently, the

liming material in fluid lime is finely

ground agricultural limestone with a high neutralizing value. Advantages include

rapid availability and application with existing fluid fertilizer equipment. Drawbacks

are low rates of application and relatively high cost for the lime applied.


Lime Neutralizes Soil Acidity

When added to the soil, calcium and/or magnesium dissolved from the liming

materials displaces hydrogen (H+) from the clay particles. Remember it is the

hydrogen ion (H+) that makes soils acid. The displaced hydrogen then reacts with

carbonate.

Carbonate dissolved from the limestone materials forms carbonic acid.

Carbonic acid is not stable in soils and quickly forms carbon dioxide and water. With

this chemical process, the hydrogen (H+) has been converted from an ion on a clay

particle to a neutral molecule of water, thereby reducing soil acidity.

The chemical reaction for this process is

H+Clay + CaCO3 (Limestone) ——> Ca++Clay + H2CO3 (Carbonic Acid)

Then: H2CO3 ——> H2O (Water) + CO2 (Carbon Dioxide)

Everything that contains calcium or magnesium is not necessarily a liming

material. Gypsum, for example, is calcium sulfate (CaSO4 • H2O). When added to the

soil, the calcium in the gypsum can displace the hydrogen on a clay particle. The

hydrogen, however, would remain in the soil solution and the pH would not change

because of the absence of carbonate

Materials

Plastic vials

Distilled water

1 M KCl solution

0.01 M CaCl2 solution

pH meter

0.04N (0.02M) Ca(OH)2 solution


Activities

Soil pH Determination

One of the methods to determine soil pH is by using pH color indicator. It is

simple but not so accurate. The most accurate determination can be made using a

pH meter and glass electrode. The electrical conductance of the solution is

measured using the meter. The conductance is correlated in the machine to pH

values which are read directly.

There are three main internationally accepted methods available for measuring

soil pH. All of them rely on shaking (or stirring) soil with a solution for 1-2 hours and

then determining the pH of the resultant soil slurry.

1. Weigh out 10 g of soil into labeled 50 ml plastic (polypropylene) vials

2. Add one of the following 3 solutions

a) 25 mL of distilled water. (This is the simplest method and normally OK for

most soils. It doesn't remove H+ from the exchange sites and is not very good

for soils with high salt content)

or

b) 25 mL of 1 M KCl (used to mask differences in soil's salt content). Useful if

determining exchangable cations as both cations and pH can be done on the

same sample. It does displace H+ from the soil's cation exchange sites, so the

results are usually slightly lower than obtained with methods (a) and (c).

or

c) 25 mL of 0.01 M CaCl2. This is an intermediate between methods (a) and (c)

and masks small differences in the soil's salt content.

3. Shake for 1 hr at room temperature (25°C)

4. Let the soil settle for a few minutes (e.g. 3 min) and measure the pH after a two

point (pH 4 and pH 7) calibration of the pH meter.

5. Normally 2 replicates are performed for each soil sample

6. Field moist soil (store at 5°C) should preferably be used .

.


Lime Requirement Determination

1. Weigh 10 g of soil in each 6 plastic vials.

2. Add in distilled water and 0.04 N (0.02M) Ca(OH)2 solution according to

the volume ratio in Table 8.1.

Table 8.2 : Ratio of distilled water and 0.04 N Ca(OH)2 solution in each vial.

Vial No. Distilled water (mL) 0.04N Ca(OH)2 (mL) pH

1 0 25

2 5 20

3 10 15

4 15 10

5 20 5

6 25 0

3. Shake the vials for 30 minutes and let it settle down for another 1/2 hour

before pH is measured.

4. Draw a graph of pH vs. mL 0.04N (0.02M) Ca(OH)2 on a graph paper and

determine lime requirement for the soil if we want to raise its pH to a

certain level.

Worked Example

Suppose that our soil has a pH of 5.5 in water and we want to raise its pH to 6

according to crop or plant requirement. From the graph that we have sketched, it

was found that our soil needs 10 mL of 0.04N (0.02M) Ca(OH)2 to raise it. Calculate

the lime requirement of this soil if:

1. the bulk density of the top 15 cm of the soil is 1.5 g cm-3, and

2. and use calcite (CaCO3) with 90% purity is used.

For every hectare of the land.

1. Calculating the amount of Ca(OH)2 that are needed

Milliequivalent (me) of Ca(OH)2 needed = Normality x volume (mL)

= 0.04 x 10

= 0.40 me Ca(OH)2/10 g soil


or = 0.40 me CaCO3 / 10 g soil

2. Calculating the weight of Ca(OH)2 needed

Weight of Ca(OH)2 needed = 0.40 me /10 g soil

1 me Ca(OH)2 = 40 + 2(16 + 1) mg

2

= 37 mg

Thus, 0.40 me Ca(OH)2 = 37 x 0.4 mg

= 14.8 mg/10 g soil

3. Changing to unit of part per million (ppm)

10 g (104 mg) soil needs 14.8 mg of Ca(OH)2

Thus, 106 mg (a million mg) soil needs = 106 x 14.8 mg Ca(OH)2

104

= 1480 mg Ca(OH)2

106 mg soil

= 1480 ppm

4. Calculating the amount of Ca(OH)2 in the top 15 cm soil in a hectare

of land.

Soil bulk density is 1.5 g cm-3 (1.5 Mg m-3 or 1500 kg m-3)

Weight of 1 hectare soil is = 1500 x 100 x 100 x 0.15 kg

= 2.25 x 106 kg

But 106 kg of soil needs 1480 kg of Ca(OH)2 (see 3 above)

Therefore, 2.25x106 kg soil needs = 2.25 x 106 x 1480 kg of

106

Ca(OH)2

= 3330 kg Ca(OH)2/ha

= 3.33 metric ton Ca(OH)2/ha

5. Calculating the lime (CaCO3) used in the field

1 me CaCO3 = 50mg of CaCO3

Thus, 0.4 me CaCO3 = 0.4 x 50 mg CaCO3

= 20 mg CaCO3/10 g soil

That is, 104 mg soil needs 20 mg CaCO3

Thus, 106 mg soil needs 2000 mg CaCO3


Thus, 1 hectare soil needs = 2.25 x 106 x 2000 kg CaCO3

106

= 4500 kg CaCO3

But the purity of CaCO3 is only 90%, and thus the actual quantity of

CaCO3 needed is = 4500 x 100 kg CaCO3

90

= 5000 kg CaCO3/ha soil

= 5 metric ton CaCO3 /ha.

lab 8 determination of ph and lime … 8 determination of soil ph...sst3005 – fundamentals of soil...

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