Exam 1 Soil Chemical Properties: Soil Organic Matter

Question 1

What is the difference between organic and inorganic molecules?

Answer 1

Organic: contain C-C or C-H bonds
Inorganic: do not contain either bond

Question 2

Soil biomass

Answer 2

Organic Compounds present in live microbial tissues

Question 3

Litter/Light Fraction

Answer 3

Undecayed plant/animal tissues and their partial decomposition products.
Litter: on surface
Light fraction: in soil

Question 4

Soil organic matter or “humus”

Answer 4

Total of the non-living organic compounds except for undecayed organic matter or its recognizable partial decay products

Question 5

Non-humic Substances

Answer 5

Chemically recognizable compounds dervied from plant or microbial tissue (category of SOM)

Question 6

Humic Substances

Answer 6

High molecular weight (complex) brown to black colored substances formed in the soil and distinctive to the soil environment. Dissimilar to the biomolecules of microorganisms and higher plants.

Question 7

Carbohydrates

Answer 7

Energy storage; structural molecules

Question 8

Cellulose

Answer 8

Carbohydrate: polysaccharides; most abundant bipolymer on earth

Question 9

Proteins

Answer 9

Linked chains of amino acids; major source of organic N in soils; perform critical functions

Question 10

Lipids

Answer 10

Energy storage; structural components of cell membrane; fat and water soluble components

Question 11

Nucleic Acids

Answer 11

Cellular information storage and energy currency; DNA, RNA, ADP ATP; contains N and P

Question 12

Ligin

Answer 12

Structural component of woody plant tissues; 2nd most abundant bipolymer

Question 13

Tissue Death

Answer 13

Entry of organic compounds into soils as non-living organic matter

Question 14

Determining Organic Matter (%) by Loss On Ignition (LOI)

Answer 14

1. Weigh oven dried soil
2. Burn in muffle furnace at 375-400 C
3. Reweigh sample
4. Difference in weight (mass) in the OM

Question 15

C:N:P:S ratio

Answer 15

100:10:1:1 (140:10:1.3:1.3)

Question 16

Percent C in soil organic matter

Answer 16

50%

Question 17

Other 50% of soil organic matter

Answer 17

O and H (but N,P,S are important as well)

Question 18

Percent microbial Biomass of total soil organic matter

Answer 18

1-12% (avg 5%)

Question 19

Percent Humus and non-humic compounds

Answer 19

Humus: 40-80% (avg 50%)

Non Humic: 50%; mostly light fraction and microbial biomass

Question 20

Organic matter tends to accumulate and “stick around” longer in soils when:

Answer 20

1. Microbial activity is low (cold/wet)
2. OM is closely associated with mineral surfaces of fine particles
3. OM exists as complex, difficult to degrade molecules

Question 21

Source of reds, browns, yellows, and oranges in soils

Answer 21

Iron Oxides

Question 22

Source of dark browns to black in soils

Answer 22

Organic matter

Question 23

Source of gray to colorless soils

Answer 23

Mineral Particles

Question 24

Properties and Effects of OM in Soils

Answer 24

• Water retention (can hold 20x its weight in h20)
• Structure/Aggregation (promotes both granular and macropores)
• Negative charge (>4 pH, weakly acidic)
• Fertility (P, N, S availability)

Question 25

Soils with 1-10% OM

Answer 25

Mineral Soils

Question 26

Soils with >30% OM

Answer 26

Organic Soils