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

Accepted Answer

A pH of 8.0 is indicative of alkaline soil conditions, which can lead to iron deficiency in plants. Iron becomes less available to plants as the pH of the soil increases, and symptoms of iron deficiency include stunted growth and yellowing of leaves. Additionally, the fact that the soil is well-drained and irrigated suggests that water may be evaporating or leaching out nutrients before the plants can access them, exacerbating any nutrient deficiencies.

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

Accepted Answer

Tomatoes prefer a slightly acidic soil with a pH range of 6.0-7.0. Alfalfa, sweet clover, blueberries, and cranberries all prefer a soil with a pH lower than 6.0.

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

Accepted Answer

The oxidation of sulfur-bearing minerals, such as pyrite, in soil produces sulfuric acid, which can lead to extreme soil acidity.

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.

Accepted Answer

The answer of The ill effects of acidity in subsoils...

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

Accepted Answer

The answer of Which of the following human actions is...

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

Accepted Answer

The answer of How does the amount of lime needed...

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.

Accepted Answer

Reactive iron in acid soils can bind to phosphorus and make it unavailable for plant uptake.

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

Accepted Answer

Manganese toxicity is more common in acid soils. In acidic conditions, manganese becomes more soluble and can reach levels that are toxic to plants, leading to symptoms such as brown spots on leaves and stunted growth.

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

Accepted Answer

The soil's maximum potential cation exchange capacity is 30 cmol_c/kg, with a 50% acid saturation, meaning 15 cmol_c/kg are acid cations. To reduce acid saturation to 10%, only 3 cmol_c/kg (10% of 30 cmol_c/kg) should be acid cations. Therefore, to replace the acid cations from 15 cmol_c/kg to 3 cmol_c/kg, 12 cmol_c/kg of acid cations need to be replaced. However, since calcium has a valency of 2, it takes half the amount of calcium to replace the same number of monovalent acid cations. Thus, 6 cmol_c/kg of calcium is needed.

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

Accepted Answer

Azaleas prefer acidic soil, and elemental sulfur is used to lower the pH of soil, making it more acidic. This would be appropriate if the existing soil pH is above 6.0 and needs to be adjusted to suit azaleas.

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

Accepted Answer

Walnut trees prefer a soil pH between 6.0 and 7.5, so a soil pH of 5.0 would not be conducive to their growth. The other four trees listed can tolerate a wider range of soil pH and may fare better in acidic soil.

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⁺

Accepted Answer

High pH levels in soils are indicative of alkalinity, which is often caused by high levels of exchangeable sodium (Na). Sodium ions can displace other cations, leading to a higher soil pH.

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

Accepted Answer

1 hectare furrow slice = 2.0 x 10^16 kg
Maximum potential CEC = 20 cmol/kg

To determine the amount of limestone needed, we need to use the following formula:

Limestone needed (kg) = ((initial % acid saturation - final % acid saturation) / 100) x CEC x 10,000

Initial % acid saturation = 40%
Final % acid saturation = 10%
CEC = 20 cmol/kg = 200 mol/ha

Limestone needed (kg) = ((40 - 10) / 100) x 200 x 10,000
Limestone needed (kg) = 6,000

Therefore, we would need 6,000 kg of limestone to reduce the percentage acid saturation from 40 to 10%.

The correct answer is D.

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

Accepted Answer

Dolomitic limestone contains both calcium and magnesium, which are both essential nutrients for plant growth. If the soil is deficient in magnesium, then using a finely ground dolomitic limestone would be a good choice.

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

Accepted Answer

Burned lime (also known as quicklime) is the most effective at quickly increasing soil pH. It has a very high pH and reacts quickly with soil to neutralize acidity. However, it is important to use with caution as it can be potentially harmful to plants and should be applied sparingly. The other options listed (sawdust, calcitic limestone, dolomitic limestone, and ammonium nitrate) can also raise soil pH, but not as rapidly or effectively as burned lime.

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

Accepted Answer

In humid regions, frequent rainfall can cause the leaching of calcium and magnesium from the soil, leading to a decrease in soil pH and a need for repeat applications of limestone to maintain soil fertility. Option A is not a factor in the need for limestone applications, and option C primarily affects pH changes in areas with higher levels of acid rain. Option D refers to carbonic acid formation, which is not directly related to the need for limestone applications. Option E, while potentially affecting soil fertility, is not specific to humid regions and is not the primary cause of the need for repeat limestone applications.

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.

Accepted Answer

Aluminum hydroxy ions are not commonly used to reduce soil pH for acid-loving plants. Instead, elemental sulfur, ammonium sulfate, or acidifying fertilizers are commonly used for this purpose.

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.

Accepted Answer

The answer of Which of the following would occur when...

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

Accepted Answer

The answer of Among the soils of which order would...

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

Accepted Answer

Strongly acid soils have a high concentration of hydrogen ions (low pH), which promotes the formation of aluminum (Al) ions. These aluminum ions can replace other ions on the exchange complex, leading to the saturation of the exchange complex with exchangeable Al ions. This is particularly true for strongly acid mineral soils, where the dominant forms of exchangeable cations are Al and hydrogen (H) ions. Therefore, the best choice is B, which represents exchangeable Al ions combined with other ions such as sulfate (S), phosphate (P), and monovalent or univalent cations (U or B). Choice A (exchangeable HS, which likely represents hydrogen sulfide) is not typically found in mineral soils, while choice C (exchangeable Al hydroxide) is more common in soils with higher pH. Choice D and E (tightly bound HS and Al ions) do not represent forms that are typically found on the exchange complex.

Quiz 9: Soil Acidity

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

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

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?

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?

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

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?

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

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?

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?

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

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

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

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?

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

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

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

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

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