CONDITIONS AFFECTING SOIL FERTILITY 39 



broken down into their original particles, the soil is said to 

 be puddled. The roots of plants, and the water and air nec- 

 essary for their growth, have difficulty in penetrating pud- 

 dled soils, and the farmer is careful not to work his land 

 when doing so may induce this condition. Puddled clay is 

 so impervious to water that it is often placed in the bottom 

 of artificial ponds to prevent the water from leaking out. 





FIG. 18. A stony field in southern New York, showing glacial debris 

 At the time the photograph was taken this field was planted with corn 



The presence of lime and humus also has considerable 

 effect on the texture of the soil. Lime has the faculty of 

 flocculating clay soils, but in sandy soils small amounts 

 of lime serve to bind the particles together ; in addition, it 

 serves to correct acid soils and promotes the growth of the 

 soil bacteria. Humus makes heavy soils more open and pro- 

 motes the movement of air in them, thus making them 

 warmer. It also absorbs and holds for the plants nearly 



