PREPARATION AND CULTIVATION OF THE SOIL 73 



is needed whenever a surface crust forms on the land, 

 as after a rain. 



By breaking this crust and the adjacent parts of the 

 soil with a cultivating implement, a layer of loose soil is 

 formed that contains many large air-spaces. Across tnese 

 air-spaces moisture cannot move, but must remain in the 

 lower layers near the root. A crust must not be allowed 

 to form ; cultivation will prevent it. 



EXERCISE. Take two pieces of chalk of the same length. Break 

 one in half. Pour a thin layer of ink into a shallow tin can or can top. 

 At the same moment stand upright in this ink on their flat ends the 

 unbroken and the broken piece of chalk. Carefully place the upper por- 

 tion of the broken piece on its lower part, in the position it occupied 

 before being broken. Watch the ink rise upward into both. Notice 

 that when the liquid reaches the crack, its rise is checked by the air- 

 space between the two broken pieces of chalk. This shows how air- 

 spaces in cultivated soil keep moisture from rising rapidly to the sur- 

 face, where it would be evaporated. 



A similar experiment can be made with entire and broken lumps of 

 sugar placed in a thin layer of coffee. 



NOTE TO THE TEACHER. Designate two or three pupils to make 

 the following experiment : 



Fill five similar open cans with the same amount of damp soil, 

 packed in equally. Leave one as it is, thoroughly cultivate one to a 

 depth of one inch, cover the others respectively with mulches (one inch 

 deep) of leaves, dry sand, and dust from under the house. As soon as 

 prepared, and again after a few days, weigh all cans and see hqw much 

 water each has lost, so as to learn which best retains the moisture in 

 the lower layer of soil. See also Crosby's Exercises 42 and 44. 



