TESTING COMMON SUBSTANCESFOR pH
Student Learning Objectives. Instruction in this lesson should result in studentsachieving the following objectives:
1 Define pH and discuss its role in plant nutrition.
2 Explain how soils become acidic.
3 Explain how soil pH is measured.
4 Explain why lime is applied to acid soils.
5 Discuss the effectiveness of lime on acidic soils.
Anticipated Problem: What is pH and how does it affect plant nutrition?
I. Soil pH is the measure of acidity or alkalinity of the soil.
A. The pH scale is a fourteen point scale used to measure pH.
1. A neutral pH is 7.0.
2. A solution with a pH between zero and 6.9 is considered acid.
3. A solution with a pH between 7.1 and 14.0 is considered alkaline or base.
4. The scale is expressed in logarithmic terms.
a. Each unit change in pHcorresponds to a tenfold change in acidity or alkalinity.
b. A pH of 6.0 is 10 times more acidic than a pH or 7.0.
B. The pH valve of soil is important to agriculturalists because certain nutrients become
unavailable to plants if the pH value is too high or too low.
1. The amount of nutrients that are available is dependent upon soil pH.
Anticipated Problem: How do soils become acidic?
II. pH is determined by the concentration of hydrogen (H+) ions and hydroxyl ions (OH-) inthe soil solution.
A. A sample of pure water has an equal number of H+ and OH- and is neutral.
1. An acid is a substance that releases hydrogen ions. When saturated with H+, a soil
behaves as a weak acid. The moreH+held on the exchange complex, the greater the
soil’s acidity.
B. Several factors influence soil pH.
1. Soil organic matter is continuously being decomposed by micro organisms into
organic acids, carbon dioxide, and water, forming carbonic acid. Carbonic acidreacts with Ca and Mg carbonates in the soil to form more soluble bicarbonates,
which are leached away, leaving the soil more acid.
2. As water from rainfall passes through the soil, basic nutrients such as calcium and
magnesium are leached. They are replaced by acidic element including aluminum,
hydrogen, and manganese.
3. Soils formed under forest vegetation tend to be more acidic than those developed
under grasslands.
4. Soils often become more acidic when crops are harvested because bases are
removed. Legumes generally contain higher levels of bases than grasses. Legumes
also release H+ ions into their rhizosphere when actively fixing atmospheric N.
5. Nitrogen from fertilizer, organic matter, manure, and legume N fixation produces
acidity. Nitrogen fertilization speeds up the rate at which acidity develops. At lower
N rates, acidification rate is slow, but is accelerated as N fertilizer rates increase.
Anticipated Problem: How is soil pH measured?
III. The two most commonly accepted methods of measuring soil pHare indicator dyes and thepH meter.
A. Indicators are frequently used in the field to make a rapid pH determination and must
be used by a trained hand to avoid major error.
B. The more accurate and widely used method is the pH meter used in soil testing laboratories.
Anticipated Problem: Why is lime applied to acidic soils?
IV. Since various plants require different pH levels for optimum growth, growers must attemptto adjust soil pH to suit the crop or plant being grown. This involves the use of limestone toraise pH or the use of alum to lower pH.
A. Lime requirement is the amount of agricultural limestone needed to establish the
desired pH range.
1. Soil pH is an excellent single indicator of soil acidity, it does not determine lime
requirement.
2. Lime requirement of a soil is not only related to the pH but also to its buffer capacity
or cation exchange capacity.
a. Cation exchange capacity is the total number of exchangeable cations, an ion with
a positive charge, a soil can adsorb.
b. The relative amount of the cation exchange capacity filled with basic cations is
called percent base saturation. SoilpHis a measure of the percent base saturation.
3. Lime replaces hydrogen and aluminum on the cation exchange sites with calcium
and changes hydro gem ions to water.
4. Four factors tell growers how much lime is required: the present pH, the desired
pH, the cation exchange capacity of the soil, and the liming material to be used.
Anticipated Problem: What determines the effectiveness of lime on acid soils?
V. The effectiveness of lime depends upon its purity, fineness, and the rate it dissolves.
A. The neutralizing power of lime depends upon its purity, measured as the calcium carbonateequivalent.
1. Neutralizing values of all liming materials are determined by comparing them to the
neutralizing value of pure claim carbonate.
2. Setting the neutralizing value of calcium carbonate at 100, a value for other materials
can be assigned.
B. When a given quantity of lime is mixed with the soil, its reaction rate and degree of reactivityare affected by particle size.
1. Coarse live particles react more slowly and less fully.
2. Fine lime particles react more rapidly and much more completely.
3. Cost of lime increases with the fineness of grind. The goal is a material that requires
a minimum of grinding, yet contains enough fine material to cause a rapid pH
change.
C. There are other important factors determining the effectiveness of lime.
1. Placement for maximum contact with the soil in the tilled layer is essential.
2. Even when properly mixed with the soil, lime will have little effect on pH if the soil
is dry. Moisture is essential for lime-soil reaction to occur.
3. The full benefit of lime is not seen until the second or third year after application.
Lime does not react with acidic soil very far from the lime particle.
Illinois Physical Science Applications in Agriculture Lesson B2–2 • Page 5
TESTING COMMON SUBSTANCES FOR pH
Part One: Matching
Instructions: Match the word with the correct definition.
a. Acid e. Lime requirement
b. Calcium carbonate equivalent f. Percent base saturation
c. Cation g. pH scale
d. Cation exchange capacity h. Soil pH
______1. The measure of acidity or alkalinity of the soil.
______2. A fourteen point scale used to measure pH.
______3. A substance that releases hydrogen ions.
______4. The amount of agricultural limestone needed to establish the desired pH range.
______5. The total number of exchangeable cations.
______6. An ion with a positive charge.
______7. The relative amount of the cation exchange capacity filled with basic cations.
______8. The measuring of the neutralizing power of lime depending upon its purity.
Part Two: Fill-in-the-Blank
Instructions: Complete the following statements.
1. A solution with a pH between zero and 6.9 is considered ______.
2. A ______pH is 7.0.
3. A solution with a pH between 7.1 and 14.0 is considered _____.
4. A pH of 6.0 is ___ times more ____ than a pH of 7.0.
5. The use of ______is used to raise the pH level of the soil.
6. The use of ______is used to lower the pH level of the soil.
Illinois Physical Science Applications in Agriculture Lesson B2–2 • Page 8
Part Three: Multiple Choice
Instructions: Write the letter of the correct answer.
______1. Which of the following pH readings would indicate that a soil is acidic?
a. 6.0
b. 7.0
c. 8.0
d. 9.0
______2. Plants grow best in what pH range?
a. 7.0–8.0
b. 5.0–7.0
c. 6.0–7.0
d. 5.0–6.0
______3. Which of the following is used to adjust soil pH?
a. lime
b. sulfur
c. organic matter
d. both a and b
______4. Which of the following plants grow well in strongly acidic soils?
a. corn
b. spinach
c. cabbage
d. watermelon
______5. Which of the following is a neutral pH reading?
a. 5.0
b. 6.0
c. 7.0
d. 8.0
______6. Information commonly found on a soil test report includes P1 test results, pH test results and ______.
a. potassium results
b. county soil test was collected
c. soil textural class
d. soil texture type
______7. How is lime usually applied?
a. as a liquid
b. in the row
c. broadcast in a granular form
d. after a crop has emerged
______8. Which of the following is not a reason why soils become acidic?
a. acid rainfall
b. harvested crops deplete the soil of calcium and magnesium
c. the most popular fertilizers are the most acidic
d. adding too much lime
Illinois Physical Science Applications in Agriculture Lesson B2–2 • Page 9
______9. The downward movement of water through soil is known as ______.
a. mass action
b. percolation
c. mass flow
d. hydrolysis
______10. The exchange between a cation in solution and one adsorbed on a soil colloid is called ______.
a. percolation
b. cation exchange
c. mass flow
d. residual acidity
______11. Which of the following is not a factor in how much lime growers decide to put on their fields?
a. the present pH
b. the pH it was last year
c. the desired pH
d. the cation exchange capacity of the soil
______12. Which of the following forms of lime would act the fastest on acidic soil?
a. finely ground
b. moderately finely ground
c. medium ground
d. coarsely ground
______13. Which of the following statements is not true?
a. Different lime products have different capacities to neutralize acidity.
b. The best results in using lime occur when there is not close contact between the grains of lime andthe soil.
c. Lime should be spread evenly over the soil for best results.
d. As hydrated lime dissolves it replaces calcium and hydroxyl ions.
______14. Lime converts hydrogen to ______.
a. oxygen
b. water
c. iron
d. calcium
______15. If lime is present placing small drops of a strong ______will cause bubbles to form and the substancewill fizz if calcium is presence.
a. base
b. alkaline
c. acid
d. none of the above
______16. Which of the following statements is not true?
a. Liming acid soils adds aluminum toxicity.
b. Liming soil promotes important activities of desired organisms such as those that fix nitrogen forlegumes.
c. Liming soils improves crop responses to fertilizers by improving the uptake of primary nutrients.
d. Agricultural lime is a soil conditioner.
Illinois Physical Science Applications in Agriculture Lesson B2–2 • Page 10
Part Four: Short Answer
Instructions: Answer the following questions.
1. What are the four factors that tell growers how much lime is required to apply to their fields?
2. The neutralizing power of limestone depends upon what?
Illinois Physical Science Applications in Agriculture Lesson B2–2 • Page 11
Assessment
TS–A
Technical Supplement
TESTING COMMONSUBSTANCESFOR pH
1. What is the definition of pH?
pH is a measure of the amount of hydrogen ion activity in a solution. It is defined as
the negative log of hydrogen ion activity. For our purposes hydrogen ion activity and hydrogen ion concentration can be considered to be equal. Let us consider abeaker of water with a pHof 6.0. This means the hydrogen ion concentration in thatwater is .000001 moles/liter (the negative logarithm of .00001 equals 6), or this watercontains .000001 moles/liter of hydrogen ions. Since pH is a logarithmic termanother beaker of water at pH 3 would contain one thousand times more hydrogenions or a concentration equal to .001 moles/liter. Hydrogen ions in solution may bethe result of dissolved salts of strong acids in solution.
2. What is meant by strong acids and strong bases?
Strong acids and bases are those acids and bases that are completely dissociatedinto their respective cations and anions in solution. Weak acids and bases on theother hand are those acids and bases that are not completely dissociated into cationsand anions in solutions. For example H2SO4 is a strong acid. In a water solutionthis acid is almost completely ionized to H+ ions and SO42- ions. BecauseH2SO4 is almost completely dissociated, all of the hydrogen ions contribute to themeasured pH. On the other hand a weak acid like H2CO3 in water will only partiallydissociate toH+and CO32- and may also exist as undissociated H2CO3- orpartially dissociated HCO3-. Undissociated species will not contribute to hydrogenions in solution or to a lower solution pH.
Illinois Physical Science Applications in Agriculture Lesson B2–2 • Page 28
3. What happens to pH when you mix two solutions of differing acidity?
Because pH is logarithmic you cannot use a mathematical average of pH to determine
the final pH of a mixture of two solutions. Consider a mixture of one liter ofwater at pH 6 and one liter of water at pH 3. The first solution by definition contains.000001 moles of hydrogen ions (H+) and the second contains .001 moles of hydrogenions. When we mix the two we get two liters of water containing .001001 molesof hydrogen ions (.001 + .000001). The concentration of hydrogen ions in the finalmixture then is .001001/2 (two liters) or .0005005 moles/liter. A calculator will tellyou that the negative log of .005005 is a pHof 3.3. That is much closer to pH 3 thanto pH 6 even though we mixed equal amounts of water to start with. This little exercisedemonstrates the logarithmic function of pH. It also explains why average pHvalues should be used with caution.
4. What is soil pH?
We have defined pH as a measurement of hydrogen ions in solution. This means
when we measure soil pH we are actually measuring the hydrogen ion concentration
in soil solution. You cannot measure the pH of a dry soil but must first mix itwith water. If the soil solution has a low pH it reflects the dominance of acidic cations(H+ and Al3+) on the soil exchange sites also because they will be in equilibriumwith soil solution. Soils with a solution pH of 4.0 will have a majority of theirexchange sites occupied by acidic cations (hydrogen and aluminum) while soils withsolution pH 7 or higher will have a dominance of basic cations (Ca2+, Mg2+, K+,and Na+) rather than hydrogen or aluminum.
5. Why is soil pH important?
Soil pH values reflect the mineral content of the material the soil formed from, the
length of time and amount of weathering in soil formation, the amount of basic cations
such as Ca+ +, and Mg + + that have been leached from the soil. Soil pH isimportant because plant availability of elements such as iron, copper, phosphorus,and zinc as well as that of toxic substances are controlled in a large part by soil pH.Some potentially toxic substances in soils, aluminum and lead have little effect onplant growth in alkaline soils (high pH) but can be serious concerns in acid soils.Many nutrients, such as phosphorus are most available to plants in soils with slightlyacid to neutral pH values. Soil pH may also serve as an indicator of serious soil problems.For example, soil pH values above 8.5 are indicative of excess sodium in soilsand pH values below 4 suggest the presence of oxidized sulfur compounds.
6. What is acid rain and how does it affect soils?
Acid rain is rainfall that has been acidified by natural or manmade emissions. During
formation rainfall equilibrates with the earths’ atmosphere which contains about
350 ppm carbon dioxide. This carbon dioxide dissolves to a limited extent in rainfall
and produces carbonic acid. Natural CO2 levels theoretically result in an “unpolluted”
rainfall pH of approximately 5.6. Measured pH levels are often lower thanthis especially in the northeastern U.S. in part because of sulfur and nitrogen emissions.Sulfur, largely from coal burning power plants, is emitted from smokestacksas SO2. This combines with water in the atmosphere forming sulfuric acid. Nitrogenemissions from automobiles and industrial sources also combine with water inthe air to form metric acid. This causes rainfall in Illinois, for example, to have amean annual pH of about 4.3. While there is no question about the presence of acidrain the larger question is what the effects of this increased acidity on soils are. Rainfallis only slightly buffered which means it is relatively easy to change its pH. Soilson the other hand are highly buffered, meaning they resist changes in pH. Bufferingis discussed in more detail in the unit entitled “Observing Limestone Reaction”, butin short soils rapidly change the pH of rainfall to approximately the native soil pH sothe effects of acid rain on soils are very minimal. Rainfall moving through a soil willcause the soil to become more acid over very long time periods, because it leachesout basic cations like Ca2+ and Mg2+, which may be replaced by acidic cations likeH+, and Al3+. Acid rain will tend to accelerate this process slightly, but the differencebetween “normal rain” and acid rain in terms of soil acidification is quite small.Because acid rain is very slightly buffered it is also readily neutralized by crop plants.Research on the effects of acid rain on crop production has also shown very little evidenceof detrimental effects of acid rain on crop production.
Illinois Physical Science Applications in Agriculture Lesson B2–2 • Page 30