Soils ' 37 



remains in hard clods that are difficult to reduce in size. The 

 tilth of a heavy soil is often spoiled for a whole season or even 

 longer by working it when it is too wet. 



Granular structure. In a soil having a granular structure, 

 the particles are cemented together in small grains. A fine- 

 grained soil with this kind of structure is easily brought into 

 good tilth. A loam soil is usually of a granular structure. 



Conditions affecting soil structure. The operations of till- 

 age have much to do with the structure of soil. The use of 

 tillage implements tends to pulverize soil and to reduce it to a 

 granular condition, provided the work is done when the soil 

 contains the proper proportion of moisture. Notice the pul- 

 verized condition of the soil shown in Fig. 15. As stated pre- 

 viously, if a soil of fine texture is tilled when it is too wet, a 

 puddled structure results. Also, a heavy soil, if plowed when 

 it is too dry, does not pulverize well, but is likely to remain 

 cloddy. 



Heavy soils, if subjected to alternate wetting and drying, 

 become granular. The alternate expanding and contracting 

 that occurs causes the soil to break into lines of fracture in all 

 directions and this aids in the formation of the much desired 

 granular structure. 



The proportion of humus in a soil has an influence on the 

 structure. Humus is somewhat plastic and tends to bind the 

 soil particles together ; in the case of a clay soil this helps to 

 form a granular structure. 



Lime benefits the structure of a clay soil. It has a floccu- 

 lating action, that is, the drawing together into granules of 

 the fine particles of a soil mass. If quicklime is placed in water 

 that contains soil particles in suspension, there is a change in 

 arrangement of the particles. At first they draw together in 

 groups that soon settle to the bottom of the vessel. This is 

 flocculation. It occurs when lime is placed on a field and is 

 one of the beneficial effects of such an application. 



Freezing and thawing of water in the soil influence soil 



