Chapter 9 Homework

In the following octahedral sheet, what is the net charge?

-1

In the following octahedral sheet, what is the charge in one of the oxygen planes?

-3

Arrange the following cations starting from the one most likely to be bound the strongest to a soil colloid to the one most likely to be bound the weakest.

1. Al 2. Ca 3. Mg 4. K 5. N

Arrange the following exchangers in order of smallest CEC to largest CEC.

1. Loamy sand soils 2. Silt loam soils 3. Clay loam soils 3. Clay loam soils 4. Vertisols 5. Histosols 6. Soil humus

Consider the figure below and arrange the soils in order of deceasing ability to bind Cd.

1. Inceptisol 2. Alfisol 3. Ultisol 4. Oxisol

An SAR over what value indicates that a soil is sodic?

12

An alkaline soil contains 20% clay and 3% humus. The pure clay has a CEC of 40 cmolc kg-1 and the humus 200 cmolc kg-1. What is the CEC of the soil?

14

At what EC would you expect to find halophytes?

14

An ESP over what value indicates that a soil is sodic?

15

An alkaline soil contains 30% clay and 4% humus. The pure clay has a CEC of 60 cmolc kg-1 and the humus 250 cmolc kg-1 . What is the CEC of the soil?

28

At what pH would you expect the highest amount of dissolved aluminum to be present in soil porewater?

4.2

In the following octahedral sheet, what is the charge in the Mg/Fe plane?

5

What is the ESP of a soil with the following composition:

5%

At what soil pH would you expect root length to be the shortest for plants that are sensitive to aluminum?

5.75

A set of soil samples has about 55 g kg-1 of organic carbon. However, all the soils have different pH values. The soil with what pH value will likely have the largest CEC?

8.2

Select the non-silicate colloids from the list below.

A. Gibbsite D. Goethite E. Hematite F. Ferrihydrite

Based on the structure of organic matter (humic substances), select all the chemical functional groups that undergo protonation and deprotonation.

A. Phenolic groups F. Hydroxyl groups G. Carboxylic groups

From the discussion in class, select the principles of cation exchange from the list below:

A. Reversibility C.Charge equivalence G. Cation Selectivity

Select the compounds that we discussed which will help lower soil pH.

A.Elemental sulfur B.Iron sulfate C.Ammonium sulfate E.Aluminum sulfate F.Organic matter from acidic sources

From the following list of soil organisms, select those which are classified as microorganisms.

A.fungi F.bacteria H.algae

Which of the following soil additives will likely have the greatest effect at decreasing soil pH?

Ammonium sulfate

Select three ways in which a cation exchange reaction can be pushed to one direction

B. Strong association of cations with anions E. Formation of precipitates F. Volatilization of products

Select the crops/plants that favor strongly acidic soils.

B.azaleas D.blueberries H.coffee

Select the base cations from the following list

C. Na D.K F.Ca H.Mg

Which compounds are often responsible for the formation of acid rain?

C.N2O H.SO2

These microorganisms obtain their energy from the biochemical oxidation of organic carbon

Chemoheterotrophs and Earthworms

Select all appropriate acidic cations commonly typically found in soils.

D.Aluminum H.Hydrogen

Select the cations that do not hydrolyze easily in soils, and thus do not produce protons or cause soil acidity. These are the "nonacid" or "base" cations.

D.Mg2+ E.K+ F.Ca2+ G.Na+

What is the principal value used to determine soil salinity?

EC

What does the term ESP stand for?

Exchangeable Sodium Percentage

Calcium and sodium are the principle causes of acidity in soils.

False

During a cation exchange reaction of calcium for hydrogen on a soil colloid surface, because of charge equivalence, it will take two calcium atoms replace one hydrogen atom.

False

For soil colloids with pH dependent charge, an increase in soil pH will cause the surface to be more positively charged.

False

In the laboratory method to determine CEC, the volume of the saturation solution and the mass of the soil sample are not used in the calculation.

False

Silicate clay minerals with a 2:1 structure have two octahedral sheets and one tetrahedral sheet.

False

The difference between an outer sphere surface complex and an inner sphere surface complex is that the inner sphere surface complex has the presence of water between the cation and the surface.

False

Under acidic conditions, variable charge minerals will exhibit a negative charge.

False

When a soil has high ESP, the hydraulic conductivity will also increase.

False

When considering a material to use as a layer to a landfill, sand displays a much lower hydraulic conductivity than clay; thus, sand would be preferred.

False

Select the type of soil colloid whose negative charge is entirely pH dependent.

Goethite

CaSO4

Gypsum

What are the ions that plants typically exude to balance charge when obtaining cations and anions from the soil solution?

H+ Bicarbonate

Select the colloid that can possess the most net negative charge.

Humus

In non-expanding 2:1 silicate clays, which atom or molecule is fixed between the layers (this can prevent expansion between the layers):

K

Which of the following cations is most likely to cause dispersal of soil colloids?

Na+

What does the term SAR stand for?

Sodium Adsorption Ratio

Aluminum is abundant in soils, and it is highly toxic to plants.

True

At a high pH, Histosols have higher CEC than Oxisols.

True

Chlorosis and dead leaves are some forms of plant stress and damage that can result from excessive salt in the soil.

True

During cation exchange, in addition to exchange of cations on soil colloid surfaces, the dissolved cations and anions can form complexes in the soil pore water which enhances their leaching from the soil profile.

True

If a soil had ammonium fertilizer applied to as a source of nitrogen (versus nitrate fertilizer), the soil pH is likely going to decrease.

True

Iron and aluminum oxides can exhibit positive charge in soils.

True

Not all cation exchange reaction are reversible.

True

One of the principles of cation exchange is the ratio law, which states that for cations of similar selectivity, the ratio of cations on the soil colloid surface will be similar to the ratio of cations in the soil solution.

True

Plant roots can exude protons in an effort to liberate plant nutrients.

True

Plants need to maintain charge equivalency with respect to the ions they take in and exude from their roots.

True

The point of zero charge of a soil colloid is the pH at which there is no net surface charge.

True

What is the principle method used to identify clay minerals in soils?

X-ray diffraction

NH4+

ammonium ion

From the following list of soil organisms, select those which are classified as macroorganisms.

ants earthworms mites

One main reason that salt can accumulate on the surface of soils is because ____________.

capillary action draws water with salt to the surface and only the water evaporates, leaving the salt in the soil

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

dispersal of the Na saturated soil colloids

A non-sodic soil has a higher ESP than a sodic soil.

false

As we decrease in the size of microorganisms, their relative abundance in soil also decreases.

false

If, for example, a humid region arable soil received more rainfall than an arable soil in a semiarid region, the soil in the semiarid region will likely have a lower pH.

false

Over the past several decades, there as been a decrease in the amount of land area worldwide that is irrigated for agricultural purposes.

false

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

false

The methods for determining soil salinity are primarily lab based - meaning, soil salinity cannot be determined in the field.

false

The pH of saline and sodic soils are both generally above 8.5.

false

If liming materials are not available, what other kinds of soil additives have we discussed to help alleviate aluminum toxicity?

gypsum soil organic matter

The principal reason that aluminum causes acidity in soils is because it _______.

hydrolyzes

When we measure pH in soil we are measuring _____________.

only active acidity

Soil organisms that utilize carbon dioxide or carbonate minerals as their primary source of carbon AND sunlight as their energy source are called __________.

photoautotrophs

Why would certain soil colloids exhibit positive charge at low pH and not high pH?

protonation of some hydroxy groups by excess H+ ions

Sodic soils differ from saline soils in that:

sodic soils have lots of sodium while saline soils have lots of other salts

A soil solution with pH 6 is more acidic than a soil solution with pH 8.

true

Adding calcium carbonate is an effective way to raise soil pH.

true

Adding sulfuric acid to saline-sodic soils can be one method to help leach excess sodium out of the soil profile.

true

As the hydroxyl concentration in soil pore water increases, so does the bicarbonate concentration.

true

Atmospheric carbon dioxide decreases the pH of rain. So, the pH of rain is typically between 5 and 6.

true

Because the reaction to raise soil pH is not immediate after applying lime, you should apply lime well before planting (e.g., 6-12 months before).

true

Chlorosis of leaves, which is the yellowing of leaves under plant stress conditions, can be due to nutrient deficiencies caused by high soil pH.

true

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

true

Irrigation water that contains high levels of dissolve salts can damage the soil over time.

true

Many soil microorganisms are microscopic, and viruses are about the same particle size as clay particles.

true

Nematodes are very common in soils and can be plant pathogens.

true

One risk with applying too much lime is the resulting decrease in nutrient bioavailability.

true

Oxidation reactions generally produce H+ while reduction reactions tend to consume H+.

true

Providing calcium in the form of gypsum is an effective way to remove sodium from the soil.

true

Saline soils are more likely to occur in arid and semiarid environments than in humid environments.

true

Salinization of a soil indicates that the soil is accumulating soluble salts.

true

Salts can accumulate on the soil surface when water evaporates.

true

Soils that receive lots of rainfall tend to leach more cations such as Ca and Mg. This will effectively decrease soil pH over time.

true

The buffering capacity of soils is an important property to maintain stable pH conditions.

true

The carbonic acid (H2CO3) formed as a result of liming a soil can breakdown to form H2O and CO2, and CO2 can be lost from the soil as a gas.

true

The majority of soil organisms are heterotrophs.

true

The most common bases applied to soils are called agricultural limes.

true

The salinity of water that leaches from irrigated field generally has a higher salt content than the original irrigation water because the leached water carries dissolved cations from the soil with it.

true

What color does calcium carbonate have in a soil profile?

white