Deck 10: Soils of Dry Regions: Alkalinity, Salinity, and Sodicity

Question 1

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

Answer 1

C

Question 2

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

Answer 2

D

Question 3

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

Answer 3

E

Question 4

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 5

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 6

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 7

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 8

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 9

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 10

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 toxicity

Question 11

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 12

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 13

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 14

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 15

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 16

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 17

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 18

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 19

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 20

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 21

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 22

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

Question 23

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

Question 24

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

Question 25

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

Question 26

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

Question 27

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

Question 28

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

Question 29

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

Question 30

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

Question 31

Sulfuric acid is sometimes added to irrigation water to help in the reclamation of sodic soils.

Question 32

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

Question 33

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

Question 34

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

Question 35

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

Question 36

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

Question 37

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

Question 38

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

Question 39

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

Question 40

Soils with high sodium adsorption ratios commonly have very stable soil structures.

Question 41

In desert areas, the soil under scattered clumps of natural vegetation is usually depleted of nutrients compared to the unvegetated soil between clumps.