Soil Nutrient Management Exam #2

What enzyme is involved with BNF?

What are the two soil factors that affect BNF?

soil pH and soil available nitrogen content

What agricultural crop can generally fix the most nitrogen?

How is N distributed in the plant in legume and forage?

the nitrogen is distributed evenly throughout the whole plants (40-60% in the above ground parts and 40-60% in the below ground biomass)

What factors are accounted for when taking a legume credit in WI (A2809 table)?

1.) the quality of the stand

• good stand means more N

• poor stance means less N (but still substantial)

2.) Harvest Time

• summer cuts allow for substantial regrowth and N fixing (>8'“)

• late cut has less time for substantial regrowth (<8”)

3.) Variety

• some varieties of alfalfa have been bred to fix more N than others

4.) Soil Type

• sandy soils lower N credits

What factors are not accounted for but are likely a factor?

What is a green manure?

Crops grown, not harvested, and plowed under

How does planting of green manures affect credits?

The age of the stand will have a considerable effect on N credits from green manures - this takes into consideration spring vs. summer/fall seeding

How is N distributed in the plant in the field crop legumes (soybeans, peas, ect.)?

Describe the residue theory and its relationship to field crop nitrogen

• Soybeans harvest results in less residue left on the field

• this allows the soils to “wake up” sooner in the spring

• warmer temperatures, more microbial activity, more N mineralization from OM

• overall increase in N mineralization

What soil test can be used to confirm forage legume credits?

Pre-side dress nitrate tests (PSNT) is recommended because pre-plant tests may not be adequate enough for legume N credits, but they can be used following applications of soybeans

How were book values were developed and why are they generally not recommended over sampling?

• Book values were developed from research-based best guesses (from historical data) that combine the average total nutrient content with availability factors

  • these factors are influenced by the specific livestock type

  • this is the default value in snap plus

How often should manure samples be taken?

Samples should be taken annually for 3-4 years to develop a sample history - this will establish a baseline

• Every 3-5 years after baseline is established will be sufficient if the management is consistent

• after any significant management changes

• CAFOs - whatever permit says (typically annually)

How many samples should be taken when sampling solid storage for a representative sample?

• should take at least 10 samples

  • from several locations throughout pile or pack areas

Should samples be taken from the top of solid storage? Why or why not?

• samples should be collected from at least 16 inches below the surface and away from the outer edges

What is stratification and why is it an issue in liquid manures storages?

The settling of solids and nutrients

• nutrients can vary between the liquid manure and the solid manure that will “stratify” or settle out in the bottom of the storage container

• Phosphorus is the most susceptible nutrient to stratification

The enzyme required for biological nitrogen fixation to occur is called: 

Nitrogenases