Plants in Relation to Man 455 



A study may be carried on in the following manner and should 

 include individual reports to the class of the results of obser- 

 vation. 



Exercise. Select a small area of ground about two square feet, 

 uncultivated if possible. Dig a hole three to five feet deep. Cut one 

 side straight and sharp. Note and record the differences in the size 

 and character of the particles of the several layers from top to bottom. 

 Note the changes in color, in solidity, and in compactness. What ex- 

 planation can you give of the color, size, and character of the particles? 

 How far down does what is called the soil reach ? What is the general 

 appearance of the subsoil? How does it differ from the soil? If you 

 cannot answer this question now, keep it in mind for later considera- 

 tion. What reasons can you give for the difference in compactness 

 of the various layers? 



Note the moisture at different levels. Collect carefully samples of 

 each of the various layers of earth through which you dig, putting 

 each in a glass jar with closely fitting cover. If you have a thermom- 

 eter, you might measure the temperature of the surface soil and that 

 of the lowest layer. 



You may find some decaying and decayed organic matter, such as 

 roots, leaves, and stalks of plants, etc. What do you find as regards 

 the color, moisture, and compactness of this material? What is the 

 character of the soil surrounding it? Do you find decayed vegetable 

 matter at the lower levels of the hole? Why? Does this material 

 seem to hold more moisture than the other kinds of soil ? Such vege- 

 table matter when saturated with water, as that accumulated in a 

 marsh or a filled-in lake, is changed into what is commonly called 

 muck, and this when partly dried out is known as peat. In the prairies, 

 however, where roots of grasses have accumulated for centuries, or in 

 woodlands, one may find great quantities of vegetable matter which 

 decayed in the ground, became black like tar, but different from muck, 

 and is known as humus. 



The quantity of humus in the soil examined may be measured 

 roughly. Weigh out a small quantity of thoroughly dried soil, put 

 it into an iron pan that can be placed on the stove over a hot fire and 

 left there for about an hour. Do not let the fire reach the soil directly. 

 When thoroughly burned, remove and weigh it. What you have 

 left is mineral matter. The difference between the two weights indi- 

 cates the amount of humus contained in the soil. 



The depth of the soil or that part of the earth's surface containing 

 humus is not on the average more than fourteen inches. In cultivated 



