Biological

Question 1

List Sources of Organic Matter (6)

Answer 1

• plant / crop residues
• green manures / cover crops
• animal manures
• Composts from ag, industry, municipalities
• Biosolids
• Soil animals (earthworms)

Question 2

Pools of organic matter in soils

Answer 2

• organic materials decomposing
  
  • roots, residues, manures
• Biomass
  
  • microorganisms
• Decomposed Organic materials

Humus is the most stable form of these pools

• accounts for most of the organic matter in soil

Question 3

When Organic Materials undergo decomp they also ________

Answer 3

• contribute to biomass and humus pools
  
  • 80% of added materials converted to CO₂ and lost from soil
  • 15% becomes humus
  • 5% is biomass
• difficult to build up humus

Question 4

Soil Organic Matter can retain Nutrients

Answer 4

• retain nutrients by adsorption
  
  • Cations held by CEC
• ALL negative charges on soil organic matter are pH dependent
  
  • as pH increases, the negative charge on organic matter increases

Question 5

Soil Organic Matter and Buffering

Answer 5

• soil organic matter contributes to buffering against changes in pH

Question 6

Beneficial Effects of Soil Organic Matter

Answer 6

• as SOM increases, soil aggregates become more stable
• SOM source of nutrients (primarily N, also P, S) when decomposes
  
  • humus contributes to CEC (retains cations)
• as SOM increases, water holding of soil increases
  
  • SOM can hold large amounts of water
  • avail to plants so plant avail water increased
    
    • surface crusting reduced , infiltration increased
• stable agregates also help with drainage
  
  • soil aeration improves

Question 7

Crop Rotation, Cover Crops, and tillage / residue

management affect Carbon in soil

Answer 7

• maintaining / increasing organic matter content of soil requires additions to offset losses through decomposition
  
  • Grow cover crops
    
    • increasing cover crop complexity can increase carbon
  • manures
  • returning crop residues
  • recycling biosolids and other organic wastes
• Most effective way to sequester Carbon is to:
  
  • reduce or eliminate tillage

Question 8

Tillage Affect on Soil Organic matter

Answer 8

• Takes only 5 years or less to reduce organic soil matter by tilling
  
  • tilling speeds up decomposition
• takes decades to build Organic Matter back up
• Only a fraction of organic matter additions are converted to humus
  
  • Takes 10 tons / acre of humus to increase soil organic matter by 1%

Question 9

How following Factor Impacts Soil Microbial Activity

Temp

Answer 9

• as temp increases above freezing, increase in microbial activity up to optimum temp, then decrease
  
  • around two fold increase in activity for every 10 EC increase in temp from 15 C to 35C
    
    • above 35, decline usually occurs

Question 10

How following Factor Impacts Soil Microbial Activity

Moisture

Answer 10

• aerobic microbial activity usually optimum at 40 to 60 percent of a soils water holding capacity (drier than field capacity)
• as soil dries, small declines occur, followed by rapid declines at critical soil water content
  
  • critical level varies by microbial
• anerobic microbial acitivity occurs in saturated soil, ceases in presence of 0₂
  
  • facultative anaerobes can function in presence or absence of 0₂

Question 11

How following Factor Impacts Soil Microbial Activity

Soil pH

Answer 11

• Fungi are more active in acid soils
• bacteria more active in nuetral to basic soils
• Rates of reactions carried out by specific microorganisms change as pH changes
  
  • minimum, optimum, and maximum value

Question 12

How following Factor Impacts Soil Microbial Activity

Organic Matter

Answer 12

• additions of organic C to soil stimulate microbial activity
  
  • the more decomp the C is, the more rapid the increase in activity
    
    • green manure has high decomp C, Sawdust low

Question 13

How following Factor Impacts Soil Microbial Activity

Salinity

Answer 13

• as soil salinity increases, microbial activity declines
  
  • amount depends on microbes and salinity
  • microbes spend more energy to absorb water as salinity increases
    
    • enzymatic processes within microbial cell can be impaired by soluble salts

Question 14

How following Factor Impacts Soil Microbial Activity

Nitrogen Application

Answer 14

• Wide range of impacts
  
  • in general, better soil fertility increases microbial activity

Question 15

How following Factor Impacts Soil Microbial Activity

Tillage

Answer 15

• tillage increases aerobic soil microbial activity
  
  • lasts days, weeks
  • long term can decrease organic matter content in soil

Question 16

C:N Ratio affects Organic Matter Decomp

Answer 16

• N rich (low C/N ratio) organic materials release large amounts of inorganic N as NH₄⁺ to the soil when they are decomposed by microorganisms
  
  • organic materials such as green manures, animal manures, etc
  • microorganisms do not need the organic N to build new cells
    
    • the released NH₄⁺ is avail to plants (mineralization)
• N poor (high C:N ratio) cause microorganisms to remove large amounts of inorganic N from soil, decreasing N avail
  
  • Immobilization

Question 17

Soil Health

Answer 17

• holistic approach to evaluating how well soil performs at supporting various finctions such as
  
  • crop production,
  • animal production,
  • water quality.
  • air quality,
  • carbon sequestration

Question 18

Physical, Chemical, Bio properties of soil health

Answer 18

• major factor is organic matter content of a soil, and efforts to increase organic matter levels
• organic matter important component of
  
  • physical
    
    • structure, aeration, infiltration, drainage, avail water
  • chemical
    
    • CEC, retention of nutrients
  • biological
    
    • hebicide degradation, N fixation, N and P release during decomp