Soil Science 100 ID______________________

Fall 2002 Lab. Section____________________

Homework 3

Due Monday October 14

Value 10 points



1. Add the master horizon and subordinate horizon symbols in the blank space to the left of the horizon depths. (1 point each)

O--1/4 to 0; light grayish brown, annual grass and forb litter; loose when dry, mats weakly when moist.


A--0 to 7 inches; grayish brown (10YR 5/2) coarse sandy loam, dark brown (10YR 3/3) moist, moderate fine and medium granular structure; slightly hard, friable; nonsticky and nonplastic; many fine roots; many fine tubular pores; slightly acid (pH 6.2); clear smooth boundary. (5 to 12 inches thick)


Bt--16 to 35 inches; brown (10YR 5/3) light sandy clay loam, dark yellowish brown (10YR 4/4) moist; moderate coarse and very coarse angular blocky structure; very hard, firm, slightly sticky and plastic; few fine and medium roots; few fine tubular pores; common moderately thick clay films; strongly acid (pH 5.3); abrupt smooth boundary.


BtC--35 to 42 inches; pale brown (10YR 6/3) coarse sandy loam, yellowish brown (10YR 5/4) moist; weak medium and fine subangular blocky structure; slightly hard, friable, slightly sticky and plastic; highly micaceous; few thin clay films; medium acid (pH 5.6); abrupt wavy boundary. (0 to 13 inches thick)


Cr--42 to 60 inches; very pale brown peppered with dark gray, strongly weathered parent rock, varied yellowish brown and dark gray moist; granitic rock fabric clearly visible; easily excavated material crushes to loamy coarse sand; neutral (pH 6.6).


TYPE LOCATION: Fresno County, California; approximately 1/2 mile west of New Auberry on the Auberry-North Fork Road in the NW1/4 NE1/4 section 6, T.10S., R.23E.


2. What weight of soil do you need to fill a square pot that is 10 cm on a side to a depth of 8 cm, in order to have a soil bulk density of 1.27 g/cm3? (1 point)


Volume of the pot to be filled is 10 x 10 x 8 cm =



Bulk density = Mass/Volume = 1.27 g/cm3 = mass/800 cm3


Mass = 1.27 g/cm3 * 800 cm3 = 1,016 g



Homework 3 Page 2



3. What is the porosity of the soil sample in the pot from question 2 if the average particle density is 2.65 g/cm3? (1 point)



Porisity = 1 bulk density/particle density = 1 1.27/2.65 = 1-0.479 = 0.521


Note that porosity has no units because it is the ratio of two densities.





4. What volume of water will a 225 cm3 soil sample hold when it is saturated if the sample has a porosity of 0.427? (1 point)


Volume of pores = 225 cm3 * 0.427 = 96.1 cm3


Assume that a saturated sample will have all of the pores full of water. The volume of water that this soil sample will hold when saturated is 96.1 cm3.




5. What is the bulk density of a soil sample that has a percent pore space of 41%? Assume that the average mineral density is 2.65 g/cm3. (1 point)


porosity = 1-bulk density/particle density


porosity = 100 * percent pore space = 100*41% = 0.41


0.41 = 1-bulk density/2.65 g/cm3


bulk density = (1-0.41) *2.65 = 1.56 g/cm3


6.                  What is the weight of 1 hectare of soil that is 20 cm thick with a density of 1.35 Mg/m3? (1 point)


1 hectare is 100 meters on a side or 1 x 104 m2


Note that a Mg (mega gram) is 1*106 grams


The volume of a 20 cm thick slice of that hectare is 0.2 m * 1x104 m2 = 0.2x104 m3


Bulk density = mass/volume


1.35 Mg/m3 = mass/0.2x104 m3 = 2,700 Mg