Lecture 5 Soil Physical Properties Lecture Review

Flashcards covering key concepts on soil physical properties, including soil texture, structure, bulk density, porosity, soil-water relationships, and the impact of human activities like tillage and compaction.

What are the three core properties of soil?

Soil texture, mineralogy, and soil organic matter.

What are the important soil physical characteristics that affect water movement and availability to plants?

Soil structure, bulk density, and porosity.

Define soil structure.

The spatial arrangement of primary soil particles into groups called aggregates or peds that are separated from adjoining aggregates by surfaces of weakness.

What are the main classes of soil structure?

Granular, Blocky (angular and subangular), Prismatic (prismatic and columnar), Platy, and Massive.

How do aggregates form in soil?

Through flocculation of clay colloids, clay-organic matter complexes, fungal hyphae and fine roots enmeshing mineral particles and organic debris, and many microaggregates binding together.

What type of soil structure is characteristic of surface (A) horizons?

Spheroidal (Granular or Crumb).

What type of soil structure is common in B-horizons, especially in humid regions?

Block-like (Angular blocky or Subangular blocky).

What type of soil structure is usually found in B-horizons in arid and semi-arid regions?

Prism-like (Columnar or Prismatic).

What are the two main types of processes that strongly influence soil structure?

Physical-chemical processes (e.g., clay attraction, shrinking/swelling) and biological processes (e.g., activity of living organisms, binding properties of soil organic matter).

What are the benefits of good soil structure?

Reduced risk of erosion, improved root penetration, improved emergence of seedlings, and greater water infiltration, retention, and availability.

What are some threats to good soil structure?

Disturbance that exposes soil to erosion, tillage (hastens organic matter oxidation, physically breaks aggregates), crop residue harvest/burning, grazing, vehicle traffic, timber harvesting, and drainage.

Define bulk density.

The mass of a unit volume of dry soil, including both solids and pores, serving as an indicator of soil porosity.

What is the relationship between soil texture and bulk density?

Fine textured soils generally have lower bulk density than sandy soils because sandy soils tend to have less organic matter and are less aggregated, while fine textured soils organize into a porous granular structure.

How does depth in the soil profile affect bulk density?

Bulk densities are generally greater deeper in the soil profile due to compaction, lower soil organic matter, less aggregation, and fewer biopores.

How do human activities like tillage and vehicle traffic affect bulk density and soil porosity?

They cause compaction, which condenses solid soil particles into pore spaces, reducing porosity and increasing bulk density. Compacted soil has high bulk density and lower porosity.

Define soil porosity and its relationship to bulk density.

The volume percentage of pores in a soil. It is inversely related to bulk density; a compacted soil has lower porosity and greater bulk density, while a looser soil has greater porosity and lower bulk density.

What are macropores and micropores?

Macropores are large pores that allow ready movement of air and drainage of water, accommodating plant roots. Micropores are small pores usually filled with water, where water movement is very slow and air movement is minimal.

How do sandy soils and clay soils generally differ in pore characteristics?

Sandy soils typically have larger individual pores but less total pore space compared to clay soils.

What is considered an ideal combination of pores in soil?

An ideal soil, regardless of texture, will have a combination of macro- and micropores.

Describe the different types of tillage based on crop residue retention.

Conservation tillage leaves >30% undisturbed residue; Minimum tillage leaves >70% undisturbed; Zero-till (No-till) involves direct seeding with minimal soil disturbance.

What is Control Traffic Farming?

A farming practice that uses permanent tramlines, requiring compatible equipment, and leads to better soil structure, infiltration, and water-holding capacity, particularly advantageous in extreme years.