2Z8 Pliysics of the Soil 



interferes with quick germination. This danger is greatest 

 where grain has been sown with a drill. 



The use of the roller when the soil is a little too wet may 

 also interfere with the formation of nitric acid in ths soil 

 by making it too close and too wet. In such a case the im- 

 mediate use of a light harrow would only retain the moist- 

 ure and make the rate of nitrification slower. 



294. The Plow The plow as a tillage tool is used for 

 two distinct purposes, 1st, to alter the texture, forming 



FIG. TQ. Showing the principle of the pulverizing action of the plow. 



from a comparatively hard soil a deep and mellow layer of 

 earth; 2d, to bury beneath the surface weeds and other 

 vegetation or manure where it may decay rapidly and be 

 converted into available plant food. 



If you will open a book, placing the fingers upon the fly 

 leaf in front and the thumbs under the fly leaf in the back 

 and abruptly bend up the corner it will be seen that every 

 leaf is slipped over its neighbor. What takes place is rep- 

 resented in Fig. 76. Had pins been put through the book 

 before attempting to bend the leaves the bending would 



