11. Organic Matter

Question 1

soils are a (3)… of carbon. globally, oceans buffer CO2 levels but fossil fuel combustion has accelerated increases in the…

Answer 1

store
source
sink

atmosphere

Question 2

bulk of global carbon is in….They contain… the carbon of vegetation and atmosphere combined

Answer 2

soils

twice

Question 3

Soil organic matter values may be estimated as 2.0 times soil organic C, although the multiplier traditionally used is …

Organic N may also be estimated from organic C values by dividing by ….for most soils,

Answer 3

1.72.

12

Question 4

Organic compounds may be listed in terms of ease of decomposition as follows:

Answer 4

1. Sugars, starches, and simple proteins
2. Crude proteins
3. Hemicellulose
4. Cellulose
5. Fats and waxes
6. Lignins and phenolic compounds

Question 5

Decomposition involves the …. When organic tissue is added to an aerobic soil, four general processes take place:

Answer 5

breakdown of large organic molecules into smaller, simpler components.

oxidation, release, synthesis, and protection.

Question 6

It may take from days to years for the just described processes of decomposition and mineralization to run their course, depending mainly on two broad factors:

Answer 6

(1) the environmental conditions in the soil

(2) the quality of the added residues as a food source for soil organisms.

Question 7

The environmental conditions conducive to rapid decomposition and mineralization

Answer 7

1. sufficient soil moisture
2. good aeration

–> 60% pores = water

3. warm temperature
4. near neutral pH

*periodic stresses such as episodes of severe drying actually accelerate overall mineralization due to the dramatic burst of microbial activity that occurs each time the soil re-wets

Question 8

green manures—

Answer 8

plant residues used to enrich agricultural soils

Question 9

The ratio of carbon to nitrogen (C/N) in organic residues applied to soils is important for two reasons:

Answer 9

(1) intense competition among microorganisms for available soil N occurs when residues having a high C/N ratio are added to soils,

(2) the residue C/N ratio helps determine the rate of decay and the rate at which N is made available to plants.

Question 10

green plants are … water
dry plant matter is 90% of… (3)

Answer 10

60-90%

C, H, O

Question 11

…(3) quick to decompose
…(4) slow to decompose

Answer 11

sugars, starch, protein

cellulose, lignin, hemicellulose, fats

Question 12

anaerobic conditions = incomplete decomposition … (3) remains

Answer 12

alcohols
CH4
organics

Question 13

C:N ratio in organics has a major influence on decomposition rates:

Answer 13

> 25 = N depletion and slow break down = poor quality SOM
–>wood, sawdusts, bark mulch, straw

<20 = rapid decomposition and N release = good quality SOM
–> higher N boosts primary production, C builds-up and stabilizes
–> hay

Question 14

litter quality

Answer 14

Lignin ratio = good guide to litter quality
–> more lignin = more breakdown needed = slower decomposition

High quality litter: low lignin/polyphenol content, narrow C:N ratio
–> quick o.m. increase, quick decomposition
–> low C:N = always getting N
–> low lignin

Poor litter = slow decomposition, C build-up
–>always short of N until o.m. decomposes

Question 15

Polymerization:

Answer 15

complex, resistant to further break down = humic substances

Question 16

Humus

Answer 16

amorphous, colloidal from decomposition and synthesis by microorganisms

–> adds CEC
–> adds water-holding capacity
–> not a nutrient, but it enhances fertility

Question 17

Humics vs non-humics

Answer 17

60-80% of SOM
complexing with clays (joining with others colloidal)

Non-humics: less resistance, identifiable

Question 18

composting

Answer 18

creates humus-like material from raw residues

requires N, water, air, high surface area for more rapid decomposition

heats up to 70C when active –> sterilizes compost

= excellent mulch and potting medium

Question 19

commercial example of composting:

Answer 19

composting straw for mushroom farming

–> add water, nutrients then aerate (mixing-windrow)

Question 20

amount of SOM in soils varies. SOM depends on balance: …vs….

Answer 20

humification vs oxidation

Deserts - 20-30% by mass

Typical arable soil -1-4%

Mollisols and Andisols - high om in A horizon (deeper horizons = lower om)

Forests and grasslands = large stores because of higher productivity = greater SOM build up

Question 21

SOM build up determined by environment:

Answer 21

-temperature and rainfall
(low temperatures and high rainfall = more vegetation growth = more om)

-lush growth (grasslands within fixed environment)

-fine textured soils (higher surface area, complexiation with om, clay particles (and silts) = more likely that om retained in soil instead of being oxidized)

-restricted drainage (water saturation = slower decomp.)
-little to no tillage
-sod crop rotations
-high soil fertility (especially N)

Question 22

aim of mgmt of SOM = improve both…. of SOM

Answer 22

quality and quantity of SOM

increase active SOC by regular additions (green manures, cover crops, manures, mulch)

adequate N to boost productivity

return roots and shoots to soil

minimum tillage

perennial vegetation&raquo_space; annuals

Question 23

soils and greenhouse effect

Answer 23

soil = important sink for carbon
CO2 produced by all soils
HNO2 + CH4 produced by anaerobic soils

–> increase SOM to store more CO2

–> too much inorganic N-fertilizer stimulates NH4 oxidizing bacteria and suppress bacteria oxidizing CH4 so even more methane production (rice paddies, marshes)

Question 24

organic soils contain more than….om depending on clay

Answer 24

20-30%
Histosols (US)
Organics (CAD)

-marshy depressions, anaerobic conditions
-low bulk density
-high C:N ratios
-high water and nutrient holding capacity

-oxidation by drainage = rapid break down
–> peat mined for fuel and horticulture

Question 25

SOM is a complex and dynamic (why dynamic?) soil component that influences soil …. (3). It has 3 pools:…..

Answer 25

overtime changes with decay vs. buildup
–> decay and mineralization governs N + S cycling
–> build up and decay controlled by enviro. and om quality (C:N ratio + composition)

behaviour, properties and functions

active, slow and passive (stable)

Question 26

C:N ratio stabilizes at ….so add N to ….to increase productivity

Answer 26

12:1

increase C