1. Indicate, with a + or - ,  the potential energy relationships of water falling as rain on the soil, water held in soil pores, and flood water moving in a stream channel.

                Rainfall               Water held in soil pores                           Flood waters

                    (    )                                (    )                                                (    )
 
 

2. Define each of the following terms:
 

 a. matric potential
 

 b. osmotic potential
 

 c. gravitational potential
 

 d. pressure potential
 

 e. total water potential

           f.  Write an equation that shows the relationship among all of the above energy potentials
                    on water in soil.
 

          g.  Which of the above potentials has the greatest significance for plants obtaining water from
                     soil?
 
 
 
 
 
 
 
 
 

 Draw two soil particles "X" and "Z" and surround each by water films of different thickness.

3. a. The water potential is largest for particle _____ and lowest  for particle _____. Explain.
 
 
 

 b. Why is the water potential for films of water around soil particles  assigned a negative water potential value ( - kPa)?
 
 

 c. What three mechanisms do plants use to obtain water from soil?

 Table 2.1  The Influence of Texture on the Percent Moisture Held in the Top 60-cm of Four Soils
at Selected Water Potentials.

  Texture*               Saturation         Field Capacity         Wilting Point             Air Dry
                                   0.0 kPa          -33 kPa                   -1500 kPa              -3100 kPa
                                       __________________ % moisture_______________
Sandy Loam                 16                   11                               5                          2
Loam                            28                   22                               8                          4
Clay Loam                    32                   27                             11                          7
Clay                              38                   33                             14                          8
                                                                                                                                         
 *Modal clay content:  sandy loam p= 10%, loam = 20%, clay loam = 35%, clay = 60%
 

 The depth of water in a give depth of soil can be found using the following  relationship.
  Dw  =  Pm/100  X  Ds/Dw  X  ds
 
 Where:   Dw  is the depth water
               Pm  is percent moisture by mass
               Ds  is soil bulk density
               Dw  is the density of water
               ds  is depth of soil

 Find the depth of water in 30 cm of soil with a bulk density of 1.56 g/cm3 that  contains 21% moisture
at saturation.

  9.8 cm  =  21%/100  x  1.56 g/ cm3 /1.0 g/cm3 x 30 cm
 

 4.  Find the cm of available water in 60 cm of each of the above soils.
See Section 5.6-5.8 in the text and the above example.  The Bulk Density values for the four texture
conditions are 1.53, 1.34, 1.27, and 1.12 g/cm3 respectively.
 
   Texture  cm
   Sandy loam  ______
   Loam           ______
   Clay loam      12.2
   Clay             ______
 

5. When allowable depletion is 45%, find the days between irrigations for each soil texture if the rate of
water use is 0.8 cm/day.  (Show all math)

   Texture  Days
   Sandy loam  ______
   Loam           ______
   Clay loam     6.8  
   Clay             ______
 

6. Go to the following URL:
 http://www.cimis.water.ca.gov/

 Find the Eto and precipitation data for San Luis Obispo during the months of April, May, June, and July.  If
sweet corn was grown during that period, find the cost of the water the grower needed to apply to the crop.
Remember rainfall is free and corn requires 1.4 times more water than Eto.  Irrigation water costs $32 per acre-foot.
 

7. What are five methods commonly used to determine soil moisture content? (See section 4.6 in the text).
 

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