Deck 11: Soil Organic Matter Overview

Question 1

Which of the following is a greenhouse gas contributed to the atmosphere by soils?

A) nitrous oxides
B) carbon dioxide
C) methane
D) all of the above
E) none of the above

Answer 1

D

Question 2

When fresh plant residues are added to soils, the "priming effect" may result in the decomposition of _______.

A) stable humus
B) sugars and starch
C) waxes and fats
D) lignin and cellulose
E) proteins and other nitrogen-rich compounds

Answer 2

A

Question 3

Green plant residues such as grass clippings or a green manure crop generally have a water content of about ____% by weight.

A) 10
B) 25
C) 45
D) 75
E) 95

Answer 3

D

Question 4

Which of the following is (are) major sources of the carbon dioxide now being added to the atmosphere in excess of the amount taken out by plants?

A) loss of soil organic matter
B) burning of fossil fuels
C) destruction of natural forests
D) all of the above
E) none of the above

Question 5

After drying to eliminate the water in plant litter, which two elements account for about 80% of the dry weight?

A) N and C
B) N and S
C) C and H
D) C and O
E) Ca and C

Question 6

Nearly all organic compounds from which living beings are made have as their backbone, chains or rings of _____ atoms.

A) aluminum
B) oxygen
C) carbon
D) nitrogen
E) none of the above

Question 7

Soil organic matter consists of ______.

A) living organisms and cells
B) dead tissues and wastes
C) identifiable biomolecules
D) polymerized, non-identifiable molecules
E) all of the above

Question 8

Which of the following pools contain the largest amount of the world's carbon?

A) soil
B) vegetation
C) atmosphere
D) animals

Question 9

Under natural vegetation, about 60 to 90% of soil organic matter is in the _____fraction.

A) active
B) slow
C) passive

Question 10

Given this Table of plant residue analyses, choose the "highest quality" residue in terms of suitability to support an active community of soil organisms and rapid nutrient cycling.

Question 11

Soils with thick horizons consisting mainly (> 30% by weight) of organic matter and that form in topographic depressions or low-lying places usually belong to the order ______.

A) Alfisols
B) Vertisols
C) Histosols
D) Umbrasols
E) Spodosols

Question 12

Alcohols, methane and organic acids of various kinds often accumulate as a result of ______ decomposition of residues in soil.

A) nitrogen-limited
B) acidic
C) fungal
D) anaerobic
E) abiotic

Question 13

Humic substances consist of _____.

A) living organisms and cells
B) dead plant & animal tissues
C) identifiable biomolecules
D) polymerized, non-identifiable molecules
E) all of the above

Question 14

Which of the following added to a soil would be likely to result in the greatest level of soil organic matter after several years of decomposition?

A) 5 Mg of carbon in plant roots
B) 5 Mg of carbon in plant tops
C) the manure produced by feeding cattle 5 Mg carbon in plant residues
D) the compost produced by composting 5 Mg of plant leaf material.

Question 15

To avoid a nitrate depression period when it is added to a garden soil, about how many kg of nitrogen must be added to a load of grass clippings that contains 1000 kg of C and 25 kg of N?

A) 15
B) 35
C) 50
D) 75
E) 100

Question 16

Biomass consists of ______.

A) living organisms and cells
C) identifiable biomolecules
E) all of the above

Question 17

One would expect well drained soils to have the highest organic matter contents where the climate is _______.

A) warm and dry
B) warm and humid
C) cool and dry
D) cool and humid

Question 18

Suppose leaves containing 1000 kg of carbon fell to the forest floor in a humid temperate environment. After a year or so, how many kg of that carbon would you expect to remain in the soil as microbial biomass and humus?

A) 10
B) 20
C) 200
D) 400
E) 800

Question 19

The active fraction of soil organic matter largely accounts for which of the benefits of adding organic matter to soils?

A) increased CEC
B) increased water holding capacity
C) increased microbial activity
D) increased aggregate stability
E) both c and d

Question 20

Detritus consists of ______.

A) living organisms and cells
C) identifiable biomolecules
E) all of the above

Question 21

Fulvic acids are a major type of non-humic substances.

Question 22

Removing all the straw with the harvest of paddy rice would increase the amount of methane produced by the paddy soil microorganisms.

Question 23

Non-humic substances formed during organic matter decomposition probably play a more important role than do humic compounds in the mineralization of N and S as well as in the solubilization of iron and other micronutrients.

Question 24

Mineralization of soil organic matter results in the addition of inorganic nutrient ions to the soil solution.

Question 25

As decomposition of added organic material proceeds, the C/N ratio in the remaining undecomposed material steadily increases.

Question 26

When native prairie soils are brought under cultivation, the fraction of soil organic matter which disappears most quickly is the passive fraction.

Question 27

Allelopathic chemicals, once added to soil, usually make the affected soil unsuitable for any plant life for periods of years.

Question 28

Humus indirectly affects plant growth through its effects on soil physical and chemical properties, but humic substances have not been shown to affect plant growth processes directly.

Question 29

The cation capacity of soil humus is two to three times greater than that of an equal volume of high CEC silicate clays.

Question 30

Practices that increase the soil organic matter level generally have favorable effects on the chemical and biological properties of soils, but have little or no effect on soil physical properties.

Question 31

Aridisols generally contain more organic matter than Mollisols.

Question 32

The species of microorganisms that are able to sustain themselves by slowly attacking stable soil organic matter are the most active decomposers of freshly added organic materials because of the head start they enjoy over other organisms which were present only in inactive forms prior to the addition.

Question 33

The soils of the world contain more carbon than all the world's vegetation and the entire atmosphere, combined.

Question 34

40 In the United States, a national goal of maintaining 5% organic matter in all agricultural soils would be a practical and beneficial policy in the long term.

Question 35

Poorly drained soils generally contain more organic matter than well drained soils.

Question 36

The main contributions of the active fraction of soil organic matter are increased CEC and water holding capacity of the soil.

Question 37

The priming effect refers to the excretion of growth promoting substance by certain bacteria during decomposition of fresh organic residues.

Question 38

Sandy soils generally are higher in organic matter than fine textured soils.

Question 39

Microbial metabolism in the guts of termites and in waterlogged soils are two major sources of excess methane that is contributing to the global greenhouse effect.

Question 40

Humus is an important nutrient, essential for healthy plants.

Question 41

Well aerated soils enriched with organic amendments can help reduce the methane load in the atmosphere.