Soil Biology
These are the most abundant microorganisms in most soils.
What are bacteria?
This term describes the zone of soil directly influenced by plant roots.
What is the rhizosphere?
This process converts organic nutrients into inorganic forms available for plant uptake.
What is mineralization?
Water held around soil particles that supplies most plant needs.
What is capillary water?
Large soil pores responsible for aeration and rapid drainage.
What are macropores?
These microorganisms help regulate bacterial populations by feeding on them.
What are protozoa?
These compounds released by roots provide food for microorganisms.
Answer: What are root exudates?
This process temporarily ties nutrients (like N) up in microbial biomass.
What is immobilization?
Water that drains downward through the soil due to gravity.
What is gravitational water?
Small soil pores responsible for water storage.
What are micropores?
These thread-like structures make up the body of a fungus.
What are hyphae?
The dark, stable, highly decomposed organic matter remaining after decomposition.
What is humus?
Microorganisms require this nutrient to build proteins, enzymes, and cells during decomposition.
What is nitrogen?
Water held so tightly to soil particles that plants cannot use it.
What is hygroscopic water?
Compared with clay soils, these soils have larger pores, drain faster, and require more frequent irrigation.
What are sandy soils?
This group of filamentous bacteria produces many antibiotics and gives soil its characteristic earthy smell.
What are Actinomycetes (Actinobacteria)?
Explain why microbial populations are often greater in the rhizosphere than in surrounding soil.
Roots release sugars, amino acids, and other compounds that serve as food sources for microorganisms.
What is the desirable carbon-to-nitrogen ratio range for organic materials?
What is 20:1 to 30:1?
Water moves from areas of _____ soil water potential to _____ soil water potential.
What are higher and lower soil water potential?
This management practice long-term can reduce microbial populations, damage fungal networks, and lower organic matter.
What is excessive tillage?
Name two reasons Actinomycetes are important in soil ecosystems.
They decompose resistant organic materials and contribute to nutrient cycling. (Also would accept production of antibiotics or earthy soil odor.)
List three benefits of soil organic matter.
Improved water retention
Increased nutrient-holding capacity
Improved aggregation and soil structure
Enhanced biological activity
When microorganisms decompose materials such as wood chips, straw, or dry leaves, they may temporarily remove available nitrogen from the soil and store it in their biomass. What is this process called, and how does it affect plants?
What is immobilization? During immobilization, microorganisms use available soil nitrogen to support decomposition, temporarily reducing the amount of nitrogen available for plant uptake.
Name and define the three major soil moisture states discussed in class.
Saturation: all pores filled with water
Field Capacity: water remaining after gravitational drainage
Permanent Wilting Point: moisture level at which plants cannot recover from wilting
Why do grassland soils in arid regions often contain surprisingly high levels of organic matter?
Extensive root systems continuously add organic matter belowground, and root turnover contributes large amounts of carbon to the soil.