LABORATORY EXERCISES 103 



burning with burning wood. What kind of soil contained more water? 

 More organic matter? Do the surface or the subsoils contain more 

 organic matter? 



35. Per Cent of Air in Soils. 



Materials. Beakers or jars, graduate, soil samples collected in 

 No. 33. 



Put a measured amount of soil into each beaker. Pour in water 

 from a graduate containing a measured amount of water until it just 

 rises to the surface of the soil. How much water does it take in each 

 case? Repeat this for a very dry soil. The amount of water required 

 is an approximate measure of the air-space. We shall then have the 

 air space in a dry soil and in those collected under field conditions. 

 The dryer the soil the more air-space it contains. The total space in 

 a dry soil is the pore-space. Record results for each soil as follows: 



Volume of soil 



Volume of water added 



Per cent of air-space 



36. Soil Particles and Their Separation. 



Materials. Two beakers, three fruit -jars, pan, samples of soil. 



(a) Put about a tablespoonful of sand in one beaker and clay in 

 another. Shake each one and allow to settle. Which settles more 

 rapidly? Why? Which would be deposited first when a swift stream 

 is checked? Where in your neighborhood is there evidence of this 

 sorting power of water? Did the glaciers thus sort soil? How could 

 we separate the different-sized particles in a soil? 



(6) The day before this lesson is given, put about four tablespoon- 

 fuls of a loam soil in a fruit-jar and nearly fill with water. Co_yer. Shake 

 occasionally. After standing a day, shake thoroughly. Allow to settle 

 one minute. Pour the rily water into another jar; allow this to stand 

 one hour. Pour off the rily water and evaporate by setting on a stove 

 or over a flame. When dry (which will probably be for the next lesson), 

 examine the dry separates. The part that settled out first is sand, the 

 second is silt, and the finest material is clay. Examine them to find 

 differences in texture. Which ones stick together? Would pure clay 

 or pure sand make a good soil? Why? Save the materials for number 37. 



37. Microscopical Examination of Soil Particles. 



Materials. Compound microscope. Sand, silt and clay from No. 35. 

 Examine the sand particles (X50), i. e., use the combination of eye- 

 pieces and objectives that magnify fifty diameters. Mix the silt with 



