MR 1 Study Guide - Soil Physical Properties

Question 1

What are the main soil particle sizes and their size ranges?

Answer 1

Sand, silt, and clay

Sand is 0.05-2 mm
Silt is 0.002-0.05 mm
Clay is <0.002 mm

Question 2

What is soil texture?

Answer 2

The relative proportion of particle sizes in a soil

Question 3

What is soil structure?

Answer 3

Defines how soil mineral particles (sand, silt and clay) are bound together and arranged in the three dimensional space

Question 4

What is the textural class of a soil with 20% clay, 40% silt, 40% sand?

Answer 4

Loam

Question 5

Which soil texture has larger total soil pore space (or pore volume), a sandy or a clayey soil? Why?

Answer 5

A clayey soil because it has many more pores despite them being small

Question 6

Which soil texture has larger surface area, a sandy or a clayey soil? Why?

Answer 6

Clayey soil because it has more particles, resulting in more surface area

Question 7

What soil texture results in a larger amount of plant available water? Sand, sandy loam, loam, clay loam or clay?

Answer 7

Loam, but clay and sandy loam are second best

Question 8

What is granular soil structure and where it is most typically found within the soil profile?

Answer 8

A healthy soil texture found at the top of the soil profile

Aggregates less than 0.5 cm diameter

Particles held together by organic substances

Question 9

Describe platy soil structure

Answer 9

Thin, flat horizontal plates formed by compaction

Question 10

Describe wedge soil structure

Answer 10

Elongated shape; polished surface (slickensides)
Caused by the shrink-swell process

Question 11

Describe blocky soil structure

What horizons is it commonly found in?

Answer 11

Irregular blocks
1.5-5.0 cm in diameter
Particles held together by clay
Commonly found in B horizons (also A and C)

Question 12

Describe prismatic soil structure

Answer 12

Vertical columns
Particles cemented by shrink/swell (smectitic) clay

Question 13

Describe columnar soil structure

Answer 13

Vertical columns with rounded tops
Found in soils of arid climates with high content of salts

Question 14

Describe structureless soil structure

Answer 14

Have no observable aggregation or structural units

Question 15

Soil bulk density

Answer 15

The mass per unit volume of dry soil, including pore space

(dry meaning no water)

Soil bulk density = weight oven dry soil (g)/total volume soil (cm3)

Question 16

Typical soil bulk density

Answer 16

Mineral soils: 1-1.8 g/cm^3 or g cm^-3
Organic soils: 0.5 g /cm^3 or g cm^-3

Question 17

Soil porosity

Answer 17

The percentage of the soil volume that is pore space

Soil porosity (%) = [volume of soil pores (cm^3)/soil volume (cm^3)] x 100

or

[1-(Bulk density/particle density)] x 100

Question 18

How does soil structure drive infiltration and permeability?

Answer 18

Depending on structure, soil can have good or poor infiltration/permeability

Question 19

Which soil structure will have higher infiltration rates: Granular or platy? Why?

Answer 19

Granular because there’s a lot more pore space and it hasn’t been compacted

Question 20

What causes soil compaction? How can you determine an issue of soil compaction?

Answer 20

Mostly related to ‘traffic’, which can be many things

Question 21

How does compaction affect soil bulk density and porosity?

Answer 21

Increases soil bulk density but decreases porosity