Lecture 6 Soil Physical Properties (Part 2) & Soil Water Storage and Movement

This set of flashcards covers key concepts related to soil physical properties such as structure, bulk density, porosity, compaction, and their impact on water movement and storage, as well as definitions for saturation, field capacity, and permanent wilting point.

What is 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.

List common types of soil structure classes.

Granular, Blocky, Prismatic, Platy, Massive.

How is bulk density defined?

The mass of dry soil per unit volume, usually expressed in g per cm³, which includes both the solid phase and the pore space.

Which soil type generally has lower bulk density?

Clay soils tend to have lower bulk density than sandy soils.

What is soil compaction?

Compaction is the increase in bulk density and concurrent decrease in porosity by the application of mechanical or other forces to the soil.

How is soil porosity defined?

The proportion of the volume of pore space in a volume of soil.

How does soil compaction affect bulk density and soil porosity?

Soil compaction increases bulk density and decreases soil porosity.

Define conservation tillage.

A practice where more than 30% of the soil surface remains undisturbed.

Define minimum tillage.

More than 70% of the soil surface remains undisturbed.

What is zero-till or no-till farming?

A direct seeding practice where the soil is left undisturbed.

What are the benefits of Control Traffic Farming (CTF)?

Leads to better soil structure, better infiltration, and better water holding capacity, particularly advantageous in extreme years.

How does soil texture impact soil structure?

Microaggregates first form from electrostatic charges between clay particles and ions in solution and with organic matter, making aggregation more difficult in sandy soils due to less clay particles.

What is the relationship between bulk density and porosity for the same soil?

For the same soil, bulk density and porosity are inversely related; a compacted soil will have higher bulk density and lower porosity.

What three factors primarily affect soil's ability to store water?

The unique chemical structure of water, the negative charge characteristic of clay and organic matter, and the combined influence of adhesion and cohesion.

Describe the partial charges of a water molecule.

The unique V-shaped configuration causes the water molecule to possess a partial positive charge on its hydrogen side and a partial negative charge on the oxygen side.

What is adhesion in the context of soil water?

The attraction of water molecules to the surface of soil particles, where the positively charged side of a water molecule is attracted to the negatively charged surface of soil particles.

What is cohesion in the context of soil water?

The force of attraction of like water molecules to each other, resulting from its dipole character (the negative side of one water molecule is attracted to the positive side of another).

How are saturation, field capacity, and permanent wilting point commonly expressed?

Volumetric water content (volume of water per volume of soil).

What does it mean when a soil reaches saturation?

When additional water fills all pore spaces, leaving no air in the pore system, making the soil anaerobic. The volumetric water content equals porosity.

What is field capacity?

The moisture content where the forces attracting the outer layers of water to soil particles are just balanced by the force of gravity after excess water has drained. It is the maximum amount of water a soil can hold against gravity and is an aerobic state.

What is the permanent wilting point?

The soil moisture content at which plants can no longer make use of the remaining water because it is held with such strong force, leading plants to wilt and die if moisture is not added.

How is the water holding capacity of a soil (plant available water) calculated?

It is the difference between the water present at field capacity and the water remaining when the crop is unable to extract more (Field capacity – Permanent Wilting Point).

List other factors that influence soil water holding capacity besides texture.

% Soil organic matter, compaction, osmotic potential (i.e., presence of salts), and soil depth.