Deck 9: Soil Acidity, Alkalinity, Aridity and Salinity

Question 1

If you want to quickly (in just a few days) increase the pH of a soil from 5.5 to 7.0, which of the following would you use?

A) sawdust
B) calcitic limestone
C) dolomitic limestone
D) burned lime
E) ammonium nitrate

Answer 1

D

Question 2

Which of the following would occur when lime is added to the surface of a soil at pH 6.0?

A) Earthworms would help move the limestone down into the soil profile.
B) Conservation tillage would increase the pH of the subsoil.
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C) CO₂ from root and microbial respiration would quickly solubilize the CaCO₃ and increase its rate of downward movement.
D) Aluminum toxicity would increase in the vicinity of the limestone.
E) The release of H⁺ ions from the exchange complex would help reduce the soil pH.

Answer 2

A

Question 3

You are using pure limestone (CaCO₃) to provide the calcium needed to exchange with acid cations in a soil with a maximum potential cation exchange capacity of 20 cmol_c/kg. How many kg of this limestone would you need to add to a hectare furrow slice (2.0 x 10⁶ kg) to reduce the percentage acid saturation from 40 to 10%, assuming that all the calcium added exchanged with aluminum or hydrogen ions?

A) 20,000
B) 50,000
C) 12,000
D) 6,000
E) 8,000

Answer 3

D

Question 4

Poor plant growth in a well-drained irrigated soil in an arid region (pH=8.0) is most likely due to ________.

A) manganese toxicity
B) iron deficiency
C) phosphate toxicity
D) calcium toxicity
E) molybdenum deficiency

Question 5

The exchange complexes of strongly acid (pH 4.5) mineral soils are saturated mainly with _____.

A) exchangeable H⁺ ions
B) exchangeable Al³⁺ ions
C) exchangeable Al(OH₂)⁺ ions
D) tightly bound H⁺ ions
E) tightly bound Al³⁺ ions

Question 6

How does the amount of lime needed to neutralize the residual acidity in a clay loam soil compare to that needed to neutralize the active acidity in that soil?

A) residual requires considerably less
B) residual requires about the same
C) residual requires about double
D) residual requires 10 times greater
E) residual requires 5,000 times greater

Question 7

In humid regions repeat applications of limestone are needed after a few years primarily because of ____.

A) the release of aluminum ions from the structure of silicate clays
B) the annual loss of calcium and magnesium in drainage waters
C) increases in soil acidity from acid rain
D) increased rates of formation of carbonic acid (H₂CO₃)
E) increases in phosphate fertilization

Question 8

The drainage of certain wetlands has resulted in extreme soil acidity. This is due primarily to which of the following processes that take place when the soil is drained?

A) oxidation of organic materials to produce organic acids
B) reduction of iron to low valent states that stimulate acidity
C) release of nitric acid due to oxidation of nitrogen compounds
D) high redox potential characteristic of aerated soils
E) oxidation of sulfur-bearing minerals that produces sulfuric acid

Question 9

Under which of the following conditions would you favor a finely ground dolomitic limestone as your choice of a liming material?

A) need for a rapid reaction with the soil
B) need to provide magnesium in addition to calcium
C) need for a material with a low shipping cost
D) need to obtain a high soil pH
E) need to significantly reduce the exchangeable Al³⁺ level in the soil

Question 10

The ill effects of acidity in subsoils can be ameliorated by adding gypsum (CaSO_4-2H₂O) to the soil surface. Which of the following most likely helps account for this effect?

A) The gypsum replaces hydrogen from colloids which moves down the profile and decreases the pH of the subsoil layers.
B) Calcium from the gypsum replaces hydrogen from colloids in the subsoil thereby increasing the soil pH.
C) Gypsum dissolves and leaches downward, increasing the Ca/Al ratio in the soil solution in subsoil layers.
D) Sulfate ions from the gypsum react with H ions in the subsoil to form H₂SO₄.
E) Gypsum stimulates root growth in the surface soil horizons.

Question 11

Among the soils of which order would you most likely find the highest buffering capacities?

A) Alfisols
B) Vertisols
C) Oxisols
D) Spodosols
E) Ultisols

Question 12

Assume you want to grow azaleas in a soil with a pH of 6.0. The application of which of the following would be most appropriate to make this soil suitable for this ornamental?

A) chicken manure
B) limestone
C) elemental sulfur
D) iron oxide
E) hydrated lime

Question 13

An acid soil is known to contain toxic quantities of a certain plant nutrient. Which of these essential elements is it most likely to be?

A) nitrogen
B) manganese
C) phosphorus
D) molybdenum
E) boron

Question 14

Which of the following trees would you expect to fare most poorly in a soil at pH 5.0?

A) loblolly pine
B) aspen
C) sumac
D) walnut
E) white spruce

Question 15

A soil has a maximum potential cation exchange capacity of 30 cmol_c/kg and has a 50% acid saturation. If you want to decrease the percentage acid saturation to 10%, how many cmol_c of calcium would it take to replace the exchangeable acid cations per kg of soil? Assume all the calcium exchanged with aluminum.

A) 30
B) 40
C) 6
D) 12
E) 5

Question 16

Phosphorus availability in acid soils is constrained by the presence of significant quantities of reactive ______.

A) iron
B) calcium
C) magnesium
D) sulfur
E) boron.

Question 17

Which of the following would likely grow best on a soil with a pH of 6.0?

A) alfalfa
B) sweet clover
C) tomatoes
D) blueberries
E) cranberries

Question 18

The very high pH levels found in some arid region soils are most likely due to high levels of exchangeable _____.

A) Al³⁺ B. Al(OH)₂₊
C) Ca²⁺ D. Mg²⁺ E. Na⁺

Question 19

Which of the following human actions is most apt to result in a long term increase in soil pH?

A) application of NH_4- containing fertilizers
B) application of farm manure
C) irrigation with high sodium salt-containing waters
D) emission of gases from automobiles
E) application of unlimed sewage sludge

Question 20

Which of the following does not describe aluminum hydroxy ions (such as Al(OH)₂₊) in the soil?

A) They are more prominent in acid than in neutral soils.
B) They affect the pH of the soil solution through hydrolysis.
C) They block negative sites on some clays thereby reducing the cation exchange capacity.
D) They influence the swelling of some clays by entering into the interlayer spaces.
E) They are commonly applied to help reduce the soil pH for acid-loving plants.

Question 21

The cation exchange capacities of alkaline soils are generally higher than those of acid soils (Vertisols excepted). Which of the following characteristics of alkaline soils most likely accounts for this high CEC?

A) high humus content
B) high content of hydrous oxides of iron and aluminum
C) high exchangeable calcium
D) high exchangeable sodium content
E) high content of 2:1 type clays

Question 22

Saline soils are characterized by _______.

A) high CaCO₃ levels near the soil surface
B) sodium adsorption ratios of 13 or higher
C) electrical conductivity of standard saturated paste of less than 4 dS/m
D) pH values of less than 8.5
E) low chloride and sulfate contents

Question 23

If you wanted to reduce the percentage sodium saturation from 30% to 5% in a sodic soil with a CEC of 20 cmol_c/kg, how many cmol_c's of CaSO₄ would need to react with 1 kg of this soil?

A) 30
B) 20
C) 10
D) 5
E) 2

Question 24

A farmer started irrigating a soil some 10 years ago but in recent years yields have declined and the soil physical condition has deteriorated. Which of the following most likely accounts for this situation?

A) high content of Ca and Mg in the irrigation water
B) failure to use conservation tillage to reduce salt accumulation at the soil surface
C) high SAR of the irrigation water
D) excessive drainage of the soil
E) high gypsum content in the soil

Question 25

Which of the following plants would be most apt to tolerate high soil salinity?

A) cotton
B) alfalfa
C) apples
D) corn
E) tomato

Question 26

A homeowner in New Mexico attempted to grow azaleas in a well drained soil (pH 7.5) but the plants were stunted and the leaves were yellow. What is the most likely cause of this constraint?

A) P deficiency
B) Mn toxicity
C) Fe deficiency
D) Ca toxicity
E) Na

Question 27

The growth of plants on sodic soils is constrained by all but one of the following:

A) caustic influence of high pH caused by sodium carbonate and bicarbonate
B) toxicity of bicarbonate and other anions
C) low micronutrient availability due to high pH
D) oxygen deficiency due to breakdown of soil structure or
E) toxicity of very high levels of calcium carbonate

Question 28

The degradation of irrigated soils across the once very productive "Fertile Crescent" of the Middle East was due primarily to _____.

A) high calcium and magnesium levels in the irrigation waters
B) high levels of calcium-containing salt in the irrigation water
C) inadequate equipment and power to properly cultivate the soil
D) poor internal drainage of the soils
E) destructive wars in the region

Question 29

Which of the following comparisons of the three different classes of salt-affected soils is correct?

A) Sodic soils generally have the lowest pH.
B) Saline soils are generally higher in soluble salts than saline-sodic soils.
C) Sodic soils generally have the highest EC levels.
D) The pH of saline soils is generally above 8.5.
E) Sodic soils are commonly formed by leaching saline-sodic soils.

Question 30

Which of the following characteristics of irrigation water is most apt to stimulate the formation of a sodic soil?

A) high selenium content
B) high Ca²⁺ and Mg²⁺ content
C) high content of chlorides
D) low pH
E) high SAR values

Question 31

Saline-sodic soils are characterized by all but which one of the following?

A) soil pH less than 8.5
B) exchangeable sodium percentage greater than 15
C) EC 4 dS/m or higher
D) sodium adsorption ratio greater than 13
E) CaCO₃ layer near the soil surface.

Question 32

Available phosphorus levels are constrained in alkaline and salt-affected soils because of _____.

A) high iron and aluminum levels
B) high exchangeable sodium levels
C) high calcium and magnesium levels
D) high manganese levels
E) low hydrogen ion levels

Question 33

Normal alkaline soils are characterized by all but which one of the following?

A) low levels of available iron
B) high levels of molybdenum
C) high levels of exchangeable sodium
D) low levels of available manganese
E) low levels of available boron

Question 34

Sodic soils generally have a poor physical condition. To what is this most likely due?

A) low organic matter content
B) impact of raindrops on the soil surface
C) dispersal of the Na saturated soil colloids
D) precipitation of the Ca²⁺ and Mg²⁺ ions as carbonates
E) high content of smectite in the soils

Question 35

Attempts to leach the excess salts from a saline-sodic soil resulted in a marked reduction of crop yields. To what is this most likely due?

A) deficiency of iron and manganese
B) increase in percentage sodium saturation
C) removal of Ca²⁺ and Mg²⁺ ions from the exchange complex
D) increase in the soil pH
E) increase in the Al³⁺ concentration

Question 36

Which of the following correctly describes the relationship among the several methods available to measure electrical conductivity (EC) of soils?

A) The saturated paste extract method is the best field method.
B) The four electrode field sensor gives EC values about 2x those of the saturated paste method.
C) The saturation extract procedure is more accurate than the saturated paste method.
D) Electromagnetic induction method requires prongs to penetrate the soil.
E) The saturated paste method takes more time than the four electrode field sensor.

Question 37

Gypsum is the most widely used chemical for the reclamation of sodic soils. What characteristic likely accounts for this popularity?

A) Gypsum is abundant and low priced.
B) Gypsum is quite insoluble so it remains in the soil for long periods of time.
C) Gypsum provides sulfates that attract Na⁺ from the exchange complex.
D) Gypsum forms sulfuric acid that helps reduce the soil pH.
E) Gypsum reacts with Na+ ions to form insoluble Na₂SO₄.

Question 38

Some irrigation systems have stimulated the formation of saline and saline-sodic soils. Which of the following contributes to this process?

A) the low sodium adsorption ratio in the irrigation water
B) high Ca and Mg contents in the irrigation water
C) inadequate drainage systems to remove soluble salts
D) high contents of sulfates and chlorides in the irrigation water
E) low pH of the irrigation water

Question 39

You were advised to use elemental sulfur in the reclamation of a saline-sodic soil. Indicate how the sulfur helps bring about this reclamation.

A) Sulfur leaches down into the soil and increases the soil pH.
B) Sulfur is oxidized to sulfates that will precipitate much of the exchangeable Na⁺.
C) Sulfur helps maintain high salt content while reducing exchangeable Na+ levels.
D) Sulfur is oxidized and forms sulfuric acid which, in turn, removes the exchangeable Na^+.
E) Sulfur stimulates the formation of gypsum in the soil.

Question 40

If you want to reclaim a saline soil, which of the following practices would you most likely use?

A) Add CaCO₃ to reduce the exchangeable Na⁺ level.
B) Leach the soil with low salt containing water.
C) Leach the soil with water having a high SAR ratio.
D) Leach the soil with water high in Ca²⁺ and Mg^2+.
E) Leach the soil with water high in bicarbonates.

Question 41

The acidity of "acid rain" can be traced to the reaction of CO₂ in the atmosphere forming carbonic acid (H₂CO₃).

Question 42

Loblolly pine tolerates higher levels of acidity than beech or maple trees.

Question 43

Residual soil acidity is that acidity remaining in the soil after limestone has been applied.

Question 44

In extremely acid soils (pH 3.0 to 4.0) most of the Al³⁺ and H⁺ ions are bound tightly by organic matter and silicate clays and are not readily exchangeable.

Question 45

Hydrogen ions dominate the exchange complex of very acid soils (pH<5).

Question 46

The drainage of some coastal wetland soils results in high pH due to underlying lime deposits.

Question 47

If you want to grow azaleas in a soil that initially has a pH of 6.5 and have been told that you should apply sulfur to reduce the soil pH, you would likely need more sulfur if the soil is a clay Vertisol than if it is an clay Ultisol.

Question 48

Fungi are much more sensitive to strong acid conditions than are bacteria or actinomycetes.

Question 49

Active soil acidity is a measure of the H⁺ ion activity in the soil solution.

Question 50

The cation exchange capacity of humus increases if pH increases.

Question 51

Soil reaction indicates the degree of acidity or alkalinity in a soil.

Question 52

If you wanted to quickly increase the pH of a soil to 7.5 to combat a disease in cabbages you are about to plant you should use hydrated lime rather than ground limestone.

Question 53

The amount of exchangeable acidity is much smaller than the residual acidity.

Question 54

Sulfur can be used to increase soil acidity and to reduce alkalinity.

Question 55

The presence of neutral salts in an alkaline soil has a tendency to increase the pH of the soil.

Question 56

The buildup of excess salts in irrigated soils can result in increased pH if the salts are high in sodium bicarbonate.

Question 57

The amount of limestone required to increase the pH from 5 to 7 would likely be higher for an Oxisol than for an Alfisol.

Question 58

The higher the cation exchange capacity of a soil, the greater is its buffer capacity.

Question 59

Nitrogen fertilization is a significant cause of acidity in many cultivated soils.

Question 60

Iron and manganese are commonly deficient in very acid soils.

Question 61

Normal alkaline soils often exhibit deficiencies of available iron and manganese.

Question 62

The presence of neutral salts in salt-affected soils helps keep the pH from rising above 8.5.

Question 63

The high pH of sodic soils is due to the high saturation of the colloidal complex with calcium.

Question 64

The source of salts for some naturally occurring saline soils are ancient fossil deposits of salts in old lake beds.

Question 65

Irrigation of a saline-sodic soil with water very low in salts could lead to the formation of a sodic soil.

Question 66

Sodic soils are sometimes referred to as white alkalis because of the white salt crusts that appear on the surface.

Question 67

Saline-sodic soils have electrical conductivities greater than 4 dS/m and a sodium adsorption ratio of less than 13.

Question 68

The cation exchange capacities of alkaline soils are commonly lower than those of comparable soils in humid regions.

Question 69

The electrical conductivity (EC) of a paste of a saline soil and water is generally less than 4 dS/m.

Question 70

Lack of adequate drainage in irrigation projects leads to the accumulation of CaCO₃ that is responsible for the high pH values above 8.5.

Question 71

Gypsum plus irrigation reduces the pH of sodic soils by replacing sodium from the exchange complex and removing it from the soil.

Question 72

Saline soils are characteristic of arid and semiarid areas with poor internal drainage.

Question 73

When the soil pH = 5.3, manganese is more likely than aluminum to be toxic to plants.

Question 74

Removal of excess salts from the soil profile is a requisite for the reclamation of a saline soil.

Question 75

Irrigation waters with high sodium adsorption (SAR) ratios are preferred over those with lower ratios.

Question 76

Sodic soils generally have better physical conditions than either saline or saline-sodic soils.

Question 77

Conservation tillage can help reduce salt build up by reducing surface evaporation.

Question 78

Bright brownish or reddish colored soil or saprolite layers encountered during excavation are likely to be result in acid sulfate soil problems if exposed to the air.

Question 79

Elemental sulfur is the most widely used chemical for the reclamation of sodic soils.

Question 80

Phosphate availability is constrained in alkaline soils by the presence of iron and aluminum oxides.