Soil Biology & Nutrients

What are the three main categories of soil organisms?

Macrofauna (e.g., earthworms), mesofauna/microfauna (e.g., nematodes), and microorganisms (e.g., bacteria, fungi).

What is the rhizosphere?

The narrow region of soil directly influenced by root secretions and associated microbial activity.

What are mycorrhizae?

Symbiotic fungi that associate with plant roots, enhancing nutrient (especially P) and water uptake.

How do earthworms benefit soil?

They improve structure, aeration, and organic matter mixing, and enhance microbial activity through their casts.

What is biological nitrogen fixation?

The conversion of atmospheric N₂ into ammonium (NH₄⁺) by bacteria, making nitrogen available to plants.

Name a symbiotic nitrogen-fixing organism.

Rhizobium bacteria (forms nodules on legume roots).

What do soil fungi do?

They decompose complex organic materials, form mycorrhizal associations, and help aggregate soil particles.

What is mineralization?

The microbial conversion of organic nutrients into inorganic forms that plants can absorb.

What is immobilization?

The conversion of inorganic nutrients into microbial biomass, temporarily making them unavailable to plants.

When does immobilization dominate over mineralization?

When organic material has a high carbon-to-nitrogen (C/N) ratio (>30:1).

What are the three main nutrient uptake processes?

Mass flow, diffusion, and root interception.

Describe mass flow.

Nutrients dissolved in water move to roots as water is taken up by the plant.

Describe diffusion (in the context of soil).

Nutrients move from areas of high concentration to low concentration near the root surface.

What is root interception?

Roots grow into contact with nutrients present on or near soil particles.

What are the key forms of nitrogen plants absorb?

Ammonium (NH₄⁺) and nitrate (NO₃⁻).

What is nitrification?

The conversion of NH₄⁺ → NO₂⁻ → NO₃⁻ by nitrifying bacteria.

What is denitrification?

The reduction of NO₃⁻ to N₂ gas by bacteria under anaerobic conditions, resulting in nitrogen loss to the atmosphere.

Why is nitrate (NO₃⁻) easily lost from soil?

Because it is negatively charged and not held by soil particles; it leaches easily with water.

In what forms is phosphorus taken up by plants?

H₂PO₄⁻ and HPO₄²⁻.

Why is phosphorus often unavailable in soil?

It forms insoluble compounds with Ca at high pH and with Fe/Al at low pH.

In what form is potassium taken up by plants?

As K⁺ (potassium ion).

What are two cations important for plant structure and chlorophyll production?

Calcium (Ca²⁺) and Magnesium (Mg²⁺).

What nutrient is the central atom in the chlorophyll molecule?

Magnesium (Mg²⁺).

Name two essential macronutrients involved in protein synthesis and energy transfer.

Nitrogen (N) and Phosphorus (P).