Deck 9: Soil Acidity

Question 1

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

Answer 1

B

Question 2

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

Answer 2

C

Question 3

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

Answer 3

E

Question 4

The ill effects of acidity in subsoils can be ameliorated by adding gypsum (CaSO₄^-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 5

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

A)application of NH₄^- 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 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

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 8

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 9

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 10

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 11

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 12

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 13

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

Question 14

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 15

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

Question 16

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 17

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 18

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.
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.

Question 19

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 20

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 21

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

Question 22

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

Question 23

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 24

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

Question 25

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 26

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

Question 27

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

Question 28

Iron and manganese are commonly deficient in very acid soils.

Question 29

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

Question 30

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

Question 31

The cation exchange capacity of humus increases if pH increases.

Question 32

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

Question 33

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

Question 34

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

Question 35

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

Question 36

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

Question 37

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

Question 38

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

Question 39

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 40

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 41

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 42

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