What is soil

Question 1

Soil definition for microbio

Answer 1

the unconsolidated, thin, variable layer of mineral and organic material that covers most of earth’s
land surface. It is usually biologically active. Particle size is smaller than 2mm

Question 2

components of soil

Answer 2

50% soil solids- 45% mineral, 5% organic

● 50% pore space- 20-30% air, 20-30% water

Question 3

soil as a 3 phase system

Answer 3

Solid phase- minerals, OM
● Liquid phase- mostly water in pore space
● Gases- oxygen, nitrogen, other atmospheric

Question 4

Organic matter to organic carbon conversion

Answer 4

OM content = 1.724 x organic C content

Question 5

Organic carbon to organic matter conversion

Answer 5

Organic C content = 58% x OM

Question 6

Sources of negative charge in soil

Answer 6

Isomorphous substitution (substitution of one element for another in ionic crystals without
change of the structure)
○ Broken edge of clay minerals
○ Organic matter
■ Typical formula (R-COOH) → R-COO + H
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Question 7

Cation Exchange Capacity (CEC)

Answer 7

total capacity of a soil to hold exchangeable cations

Question 8

Anion Exchange Capacity (AEC)

Answer 8

the degree to which a soil can adsorb and exchange anions

○ Increases as soil pH decreases

Question 9

A molecule like Al-OH2 may have different charges at different pH

Answer 9

Positive at lower pH: Al-OH2
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○ Neutral at a higher pH: Al-OH
○ Negative at a high pH: AL-O
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Question 10

Particle density

Answer 10

dry mass of soil per unit volume of soil solids

○ Ranges 2.6-2.75 g/cm3

Question 11

Bulk density

Answer 11

dry mass of soil per unit bulk volume of a soil

○ Ranges 0.9-1.8 g/cm3

Question 12

MASS WATER CONTENT (θm

)

Answer 12

θm = mass of water / dry soil mass

Question 13

VOLUMETRIC WATER CONTENT (θ or θV

)

Answer 13

● θ = water volume / dry soil volume
● θ = θm
* bulk density

Question 14

SOIL WATER POTENTIAL (𝜓)

Answer 14

● Relative energy level of water in a given amount of soil
● Moves from high potential to low potential
● In unsaturated soil, 𝜓 would be negative
● In saturated soil, 𝜓 would be 0
● If there is standing water on top of the soil, 𝜓 would be positive
● 𝜓 is positively related to θ

Question 15

Hygroscopic water

Answer 15

𝜓 < -31 bar
○ Held by adhesion
○ Can’t just air dry to remove. Must heat at 105C

Question 16

Capillary water

Answer 16

-31 bar < 𝜓 < -0.33 bar
○ A result of cohesion, held together by H bonds
○ Available for plant uptake

Question 17

Gravitational water

Answer 17

○ Doesn’t stay in soil long

○ Not much gets used by plants or microbes

Question 18

Field capacity

Answer 18

soil moisture or water content held in the soil after excess water has drained away and
the rate of downward movement has decreased ○ -0.1 ~ -0.3 bar (-10 ~ -30 kPa)

Question 19

Permanent wilting point

Answer 19

the minimal point of soil moisture the plant requires not to wilt. If moisture
decreases to this or any lower point a plant wilts and can no longer recover its turgidity
-15 bar (-1500 kPa)

Question 20

Optimal pressure for microbial activity

Answer 20

-0.1 bar and -10kPa

Microbial life can be present at -140 bar (-14 MPa)

Question 21

% N2, O2, and CO2 in atomosphere

Answer 21

78% N2, 21% O2, 0.04% CO2

Question 22

% N2, O2, and CO2 in soil air

Answer 22

78% N2, 18-20% O2, 0.07 - 1% CO2

Question 23

Why does soil air have less O2 than the atmosphere

Answer 23

MICROBES! use O2 and emit CO2 in respiration

Question 24

Why don’t soil microbes move?

Answer 24

physical heterogeneity of soil, different size particles and aggregates, nutrients on surface of particles, not energy efficient