Soils –205 General Soils

Study questions- Quiz 2

 

 

1)       How does the structure of a water molecule influence its properties and behavior in soils?

Water is a polar molecule.  Polarity is the uneven distribution of charge within the water molecule while H-bonding is the attraction of the H atom of one water molecule to the O atom of another.  These properties result in bonding of water molecules to charged sites on soil surfaces, and to charged materials such as cations and anions (adhesion) as well as to each other (cohesion).  This bonding results in greatly lowered energy of water molecules.   

 

2)      A moist soil sample is collected in the field- the weight of the sample is 910 g and it has a volume of 500 cm3.  After oven drying, the sample weighs 675 g.  What is the gravimetric water content?  The volumetric water content?  What would the volumetric water content of this soil be at saturation?

qg = mass of water/mass of oven dry soil = (910g – 675g)/675g = 0.35 or 35%

qv = vol. of water/total vol. of soil = (910 cm3 – 675 cm3)/500 cm3 = 0.47 or 47%

 

alternatively

qg x Db = qv

Db = 675g/500cm3 = 1.35g/cm3

qg = 0.35

qv = 0.35 x 1.35 = 0.47 or 47%

 

At saturation qv = % pore space

%PS = 100 – (Db/Dp x 100%) = 100 – (1.35/2.65) x 100 = 49%

so at saturation (all pores are filled with water) qv = 49%

 

3)      What is meant by “soil water potential”?  What are the three components that determine the total soil water potential (yt)?

Soil water potential refers to the energy status of soil water relative to free, pure water at some reference elevation.  The three components are:

Ym = matric potential.  Matric potential arises from the association of soil water with solid surfaces, it is important under all moisture conditions up to saturation. 

Yo = osmotic potential.  Osmotic potential arises due to the attraction of water molecules to solutes.  The association lowers the water’s free energy.  Osmotic potential is important in soils with high concentrations of salts.  It is also important when considering plant uptake of water at the root (a semi-permeable membrane).

Yg = The gravitational potential is important in soils at or very near saturation.  It arises from the higher potential energy of water above some arbitrary reference point.

 

4)      What is the importance of using soil water energy status to describe soil water?

Differences in potential create gradients.  Water flow is in response to gradients, where water will flow from a higher to lower energy level.

 

 

 

 

5)     

qv
 
Draw a graph showing the relationship between soil water potential and soil water content for a sand, loam, and clay.

Soil water potential Y (kPa)
 

 

 

 


Note:  at a potential of 0 (saturation) the pore space is equal to qv.  Clay has a higher amount of pore space (0.56 or 56%) than does the sand (43%) or loam (50%).  The curve for sand decreases more rapidly than do the other curves due to the loss of water from the large macropores.  Be sure to be able to find the water content at saturation, field capacity, and permanent wilting point given a graph like this one.  For example:  the loam has about 32% water at field capacity and about 21% at permanent wilting point.

 

6)      Consider the following statement- water always moves downward in a soil.  Do you agree or disagree with this statement?  Why or why not?  If water can flow upwards, what forces are involved?

Water can flow upward or horizontally in soil.  Capillarity, which is a result of both adhesion and cohesion, causes water to move into very small pores or capillaries even against the force of gravity.  It is the principle way in which water moves under non-saturated conditions.

 

7)      Consider a soil profile in which a loamy sand E horizon overlies a sandy clay Bt horizon.  If the soil is allowed to wet up as irrigation water is continually added at the surface, what will happen when the wetting front reaches the contact between these 2 horizons?  Why?

The water will move quickly downward away from the saturated zone.  Due to the larger pores in the E horizon there will not be significant horizontal flow away from the saturated zone.  When water reaches the clay horizon, it will slow and start to pool due to the low hydraulic conductivity of the material.

 

 

8)      Consider a soil profile in which a sandy clay Bt horizon overlies a loamy sand C horizon.  If the soil is allowed to wet up as irrigation water is continually added at the surface, what will happen when the wetting front reaches the contact between these 2 horizons?  Why?

Water will move both horizontally and downward in the clay horizon do to capillary flow (small pores).  As the water reaches the sand, it will start to collect under the clay layer is completely saturated.  Once it is saturated, gravity will be able to pull the water downward into the larger pores found in the sand.

 

9)     

qv
 
Define the following terms:  saturation, field capacity, permanent wilting point, and hydroscopic coefficient.  What is the approximate soil water potential at each states defined above?

Saturation- all pores are filled with water; Y = 0

Field capacity- drainage due to gravity has ceased, water is retained due to matric forces; this is one boundary of what is considered plant available water. Y = -1/10 to –1/3 bar or -10 to –3 kPa

Permanent wilting point- point at which soil water is held so tightly (thin water films) that most plants can no longer reach low enough potentials needed for water to flow upwards into the plant.  Y < -1500 kPa or –15 bars

 

10)  What is plant available water?  How does it change with soil texture?

Soil water potential Y (kPa)
 
 


Plant available water is defined as the difference in water content between field capacity and permanent wilting point.  It is highest in silt loam type textures.  This is because clay soils hold a significant amount of water at potentials more negative than –1500kPa.

 

11)  Compare and contrast saturated and unsaturated flow in soil.  Be sure to address the driving forces and approximate water potentials over which each occur.

In saturated flow, the driving force is gravity.  Potentials are > -33 kPa.  Saturated flow is more rapid in coarse textured soils.

In unsaturated flow, pores are filled with both water and air.  The main driving force is the matric potential.  Potentials are < -33 kPa.

12)    In the following diagram, will water flow from soil A to B, or soil B to A?

 

 

 

 


Soil A                                       Soil B

qv = 14%                                 qv = 20%

 

 

Although you can guess that water will flow from soil B to soil A (the drier soil), it is the potential that drives water movement so you can not say for sure.  If soil A had a total potential of –12 bars and soil B had a potential of –8 bars, water would flow from B to A, or from higher to lower energy.

 

 

13)  How does hydraulic conductivity (K) change with water content in a sand?  In a clay?

Hydraulic conductivity is highest under saturated conditions, in the sand.  As the large pores dry out in the sand, K will become higher in the clay.  Micropores are more important to water movement under unsaturated conditions.

K(q)
 

Wet                                               Dry
 

q
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


14)  What are the possible fates of water that has infiltrated the soil?

 

     Infiltrated water can be:

      1)  lost through evaporation

      2)  transpired by plants

      3)  lost from the root zone (leaching)

 

 

15)  Approximately what fraction of the earth’s water is actively cycling?  What implications does this have on importance of good soil management?

 

About 0.05% of the earth’s water is actively cycling.  A large percentage of this water is associated with soil.  Through better management we can improve water use efficiency.  Examples include adopting practices that increase organic matter, aggregation, and infiltration.  By increasing infiltration, we will decrease runoff and protect surface water quality.

 

16)  What does the term evapotranspiration mean.  What is PET?

Since the loss of water vapor from evaporation and transpiration is difficult to separate, we often combine the terms and measure evapotranspiration.  Evapotranspiration therefore, is the combined loss of water from a given area, by evaporation from the soil surface and by transpiration from plants. 

PET stands for potential evaporatranspiration.  It is calculated by applying a correction factor to the amount of water evaporated from an open pan of water of standard design.  If actual ET is lower than PET, it indicates that water is limited.

 

17)  What factors influence evaporation?  When the surface of the soil is wet, the evaporation rate is determined mostly by climatic variables.  When the soil surface dries, the evaporation rate is controlled by the rate at which soil can transmit water upwards through capillary flow.  This depends on texture.  Plant characteristics influence evaporation from the soil surface by limiting the amount of solar radiation that reaches the soil.

 

18)  How are soil aeration, water content and microbial activity related? 

Microbial respiration uses oxygen and produces carbon dioxide.  As the water content increases, oxygen has a harder time moving into the soil and may become depleted.  Carbon dioxide will also build up.  This will decrease microbial respiration and organic matter decomposition.

 

19)  What are three ways of characterizing soil aeration?

            1) oxygen concentration in soil air

            2)  the air filled porosity (air filled pores = total porosity – water filled pores)

 

20)  What is the oxidation-reduction or redox potential?

The redox potential (Eh) is a measure of a substance’s tendency to either gain or lose electrons.  High Eh indicates oxidizing conditions, low Eh values indicate reducing conditions

 

21)  What are the functions of wetlands?  Wetlands provide species habitat, water filtration, flooding reduction, shoreline protection, commercial and recreational activities, natural products.

 

22)  What are three features that scientists look for when defining a wetland?

1)  wetland hydrology

2) hydrophytic plants

3) hydric soils

 

23)  Is every seasonally saturated soil a Hydric soil?  Why are why not?

No, to be a hydric soil, reducing conditions must occur at times when temperatures are high enough to have sufficient biological activity to deplete oxygen.

 

24)  What are three features of hydric soils?

1)  accumulation of organic matter due to reduced microbial respiration and decomposition rates

2)  redoximorphic features (black nodules= reduced Mn and redox depletions or mottles)

3)  gleyed colors throughout the profile occurs in extreme cases

 

25)  What three factors (in terms of energy balance) that influence changes in soil temperature?

1) the total amount of energy reaching the soil- depends on soil cover and albedo

2)  the amount of energy required to change the soil temperature- depends on water content and compaction.

3)  The energy lost due to evaporation.

 

26)  If all other factors are equal, do wet soils or dry soils warm up first in the spring?  Why?

Dry soils will warm up faster due to the high specific heat of water.

 

 


27)  How do mulches influence soil temperature?  A mulch will keep the soil cooler in the summer and warmer in the winter.  Due to the air pockets in the mulch layer the thermal conductivity is greatly reduced compared to soil.

 

29)  Describe how fire affects soil:

a.  chemical properties:  decreases organic matter

b.  physical properties:  can change soil texture and alter water repellency

c.  biological properties: can kill organisms and decreases organic matter which serves as a food for microorganisms.

 

 

30)  What is sheet, rill, and gully erosion?  How are they related?

Sheet erosion:  The removal of a fairly uniform layer of soil from the land surface by runoff water.  This is the form of erosion that we generally concentrate on preventing.

Rill erosion: Erosion process where numerous small channels of only several centimeters in depth are formed.  It occurs when water moving uniformly across a soil starts to concentrate and downcut into the soil.

Gully erosion: process where water accumulates in narrow channels and removes the soil from the to considerable depths.  Gullies are larger than rills and can be up to 100 ft.

They are all three types of water erosion and differ in their severity.  If it is not prevented, sheet erosion can lead to rill erosion, which can lead to gully erosion.

 

Note:  Remember that erosion is a natural process that involves detachment and transport.  The problem is accelerated erosion.

 

31)  What are the components of the Universal Soil Loss Equation and how do they affect water erosion?

A = RKLSCP

 

R = rainfall erosivity- related to amount and intensity

K = soil erodibility- related to the structural stability and infiltration capacity

L = slope length

S = slope gradient or steepness – L and S are considered together       

C = cover and management- ratio of soil lost from a given area under a certain vegetation relative to the soil lost from the same area of bare soil.  Vegetation can reduce erosion by protecting the soil from the detachment caused by raindrop impaction and also by enhancing macropores and infiltration.  Remember that manipulation of vegetative cover is perhaps the easiest way to control erosion. The erosion rate from undisturbed forest and grassland soils, for example, is much lower than in agricultural lands.

P = erosion-control practices- conservation practices that decrease the erosion rates relative to land where no erosion control practices are being used.  Examples include

Terraces (decreases the slope length and gradient)

Grassed waterways (carry water off the land without the formation of gullies)

Contour farming (layout of crops along the hillslope contours instead of up and down the slope)