HOW THE SOIL WAS FORMED 59 



the soil is too wet, the plow breaks up these groups and 

 packs close together the tiny particles that before formed 

 the granule. Great clods are then formed, so that a single 

 plowing when the clay soil is too wet may injure the field 

 for many years. 



Coarse- and fine-grained soils. Soils may be arranged 

 in the following order, according to the coarseness of 

 the particles of which they consist, beginning with the 

 coarsest and ending with the finest : 



Gravel. Fine sand. . Silt loam. 



Gravelly loam. Sandy loam. Clay loam. 



Coarse sand. Fine sandy loam. Clay. 



Sand. Loam. 



The most satisfactory soils are those consisting of a mix- 

 ture of sand and clay. These are called loam soils. They 

 have enough sand to make them pulverize easily and drain 

 well, together with enough clay to hold sufficient moisture 

 for plants and furnish a gradual supply of certain kinds of 

 plant-food. The coarsest soils become "worn out" soonest. 

 Clay soils usually last longer because they contain the 

 largest amount of total plant-food. They require more 

 tillage, however, to make this plant-food available. 



Treatment of sandy and clay soils. You have just 

 learned that a clay soil must not be plowed when wet. 

 But if a soil consists almost wholly of sand, plowing it 

 when rather wet does little harm. After plowing a clay 

 soil the large lumps must be broken with a harrow 

 before they dry and become hard clods. Live-stock 

 should not be allowed to pass over clay soil while it 



