THE PHYSICAL CHANGES 99 



The surface of a plowed field is a few inches higher than before 

 it was plowed. This difference is due to the looseness of the parti- 

 cles and the added pore space among the grains. These pores are 

 first filled with air. As snows melt, or as rains fall upon such a 

 field, much of the pore space may be filled with water. The use 

 of the harrow (Fig. 74) and the roller will repack the surface to 

 some extent. 



Heavy soils are often plowed in late fall, particularly in the 

 middle and northern states. This has a marked physical effect 

 upon such soils. The weathering of under-soil thus exposed on the 

 surface is beneficial and the effect may be readily observed by 



FIG. 74. The common spike-tooth harrow is sometimes called a drag harrow. It 

 is one of the best implements for smoothing the surface of the soil and producing a dust 

 mulch to save the soil moisture. (New Jersey Station.) 



any one. Fall plowing may be practiced on any soils in any climate 

 for the purpose of destroying certain forms of injurious insect life, 

 by exposing the hibernating forms of the destructive insects to 

 winter weather and to attacks of winter birds. Fields which have 

 been plowed in the fall can be prepared for crops early in the 

 spring. On level fields, not subject to erosion, winter moisture 

 may be conserved best by fall plowing. The field may be harrowed 

 before spring winds cause the ground to become crusty. This 

 harrowing will allow the air to enter and the temperature is made 

 warmer. 



Late fall plowing cannot well be practiced where it is important 

 to have a winter cover- crop growing. The farm manager must 



