Module 1

Question 1

Discuss the features of clay soil that make it so reactive

Answer 1

Clay particles are shaped like thin and flat platelets, they are also very small which means they have a very large surface area. Clay also has a charge which is mostly negative which can attract positively charged ions

Question 2

What is Vt?

Answer 2

The total volume of wet soil

Question 3

What is Pb?

Answer 3

Bulk density. Large bulk density maybe because it is a sandy soil or it is very compact

Question 4

What is Pw?

Answer 4

Density of water

Question 5

What is Ms?

Answer 5

The mass of the oven-dry soil

Question 6

What is Mw?

Answer 6

The mass of water. Can calculate by subtracting Ms from the mass of the wet soil

Question 7

What is w?

Answer 7

The gravimetric water content

Question 8

What is θsat?

Answer 8

The volumetric water content of soil when it is saturated. To calculate θsat = f

Question 9

What is f?

Answer 9

Porosity

Question 10

What is θfc?

Answer 10

The volumetric water content of soil when it is at field capacity. To calculate θfc = θsat - θmp

Question 11

What is θmp?

Answer 11

The macroporosity. The volume of all the macropores

Question 12

What is θ?

Answer 12

The volumetric water content

Question 13

What is the minimum value of macroporosity for good aeration?

Answer 13

10%

Question 14

What is the field capacity?

Answer 14

The micropores are full of water but the macropores have successfully drained

Question 15

What is saturation?

Answer 15

All the pores are full of water

Question 16

Which two soil constituents have the largest surface area?

Answer 16

Clay and organic matter (hummus)

Question 17

What is z?

Answer 17

The effective root depth and should always be converted to mm

Question 18

How do you calculate the readily available water holding capacity of soil?

Answer 18

(θfc - θsp) x z

Question 19

How do you calculate the total available water holding capacity of soil?

Answer 19

(θfc - θpwp) x z

Question 20

What is θsp?

Answer 20

The volumetric water content at the stress point

Question 21

What is θpwp

Answer 21

The volumetric water content at the permanent wilting point

Question 22

How is soil structure assessed in the field?

Answer 22

The extent to which aggregates have formed and stability of these aggregates to a force applied. The shape of the aggregates. The size of peds.

Question 23

What is the permanent wilting point?

Answer 23

When the plants have extracted all the water they are capable of taking from the soil

Question 24

Features of good soil structure are…

Answer 24

Small aggregates that are stable

Question 25

What is the stress point?

Answer 25

When the soil dries to a point where the roots cannot extract water at the rate required by the plant

Question 26

What is mottling?

Answer 26

The switching between saturated and aerated conditions gives rise to mottling, a speckled pattern of orange or rusty colored flecks

Question 27

What is porosity?

Answer 27

The fraction of the soil volume that consists of holes

Question 28

Why do coarse-textured soils have smaller total pore volumes but good drainage?

Answer 28

Coarse-textured have more macropores than fine-textured soils, however, fine-textured soils have many micropores which means they have a larger surface area of pores and therefore a larger total pore volume. Coarse-textured soils have more macropores meaning they have better drainage

Question 29

Explain why it is harder to work a clay soil than sandy soil?

Answer 29

Because clay soils are more likely to become waterlogged which makes them very hard to cultivate

Question 30

What is the phase lag of soil temperatures?

Answer 30

Energy is absorbed by the soil more while moving down the profile and the difference between the max and min temperatures will be smaller at depth. This delay in
reaching max and mins is the phase lag

Question 31

Difference between soil texture and soil structure?

Answer 31

Soil texture is the proportion of sand, silt, and clay in a soil whereas soil structure is the arrangement of soil particles into aggregates and the stability of these aggregates to stress

Question 32

The soil water balance equation is…

Answer 32

Wn(water stored at end of day) = Wn-1(water stored at end of previous day) + P(rainfall) + I(irrigation) - R(drainage) - D(discharge from drains) - S(surface runoff) - E(evaporation)

Question 33

Simplified soil water balance equation is…

Answer 33

Wn = Wn-1 + P + I - E

Question 34

Water required for irrigation can be calculated by…

Answer 34

I = E - P

Question 35

How does soil structure develop?

Answer 35

Bigger aggregates may be broken up and stabilized
Or substituents (sand,silt,clay) are brought into close proximity to form new aggregates

Question 36

How do wetting and drying cycles create good soil structure?

Answer 36

Wetting breaks up as outside expands from absrobing water, creating a pressure difference that causes it to explode
Drying brings soil particles together in greater alignment, bonding and stablising the peds

Question 37

Why should you not apply force to a soil when it is wetter than the plastic limit?

Answer 37

It will compact, forming pans, ruining aggregates impeding drainage and aeration

Question 38

Why is particle density very constant across most soils

Answer 38

.

Question 39

Why is water at the permanent wilting point unavailable?

Answer 39

It is bonded too tightly inside molecules or in micro-pores whose capillary action is too strong for plants to overcome

Question 40

What is soil consistence

Answer 40

The soils resistance to deformation or rupture, and is dependent on the water content of the soil