THE TILLAGE OP SOILS 37 



There are two ways in which the particles of soil may 

 fit together, and hence two soil structures are commonly 

 recognized : 



First : that in which the particles are stuck together in 

 tiny masses, forming what is known as the crumb structure. 



Second : that in which the particles are run together 

 in large masses, forming lumps, or clods. This is known 

 as the jmddled structure. 



The formation of lumps and clods; i.e., the puddled 

 structure, may be brought about in four ways: (1) by 

 plowing the ground when it is too wet ; (2) by allowing 

 animals to trample the fields when the soil is full of mois- 

 ture ; (3) by leaving the fields too long in one crop ; and 

 (4) by neglecting to add organic matter. 



EXERCISE IS 



Object. — To show the effect of freezing upon the 

 structure of soils. 



Procedure. — Make four mud pies out of clay such as 

 was obtained in Exercise 1. These pies should be about 

 an inch and a half in diameter, one half inch thick, and 

 contain enough water to prevent cracking when molded. 

 Set one pie on a window sill or on a shelf to dry. It 

 should not be allowed to freeze. Place the other three 

 outside the schoolroom where they will freeze thoroughly. 

 Bring them into the schoolroom after they have been frozen 

 solid for a day or two, and thaw them out carefully. As 

 soon as the frost is out of them, but while they are still 

 wet, place one upon the window sill or shelf inside the 

 schoolroom, and the other two outside to freeze thoroughly 

 again. Bring the two frozen pies in again later; and 

 when they have thawed out a second time, set a third 

 one on the shelf, and put the last one out to freeze once 



