234 Physics of the Soil 



broken down to fill the spaces between the larger simple 

 grains and finer granules not yet broken down to make a 

 close textured, impervious material in which no plant can 

 thrive, and through which neither water nor air can move. 



286. Early Spring. Tillage. The early stirring of the soil 

 in the spring preparatory to seeding has for its main object 

 the changing of the soil "texture so that it, will become 1st, 

 warmer, 2d, dryer, 3d, better aerated, 4th, better suited to 

 lessen the ra^e of evaporation of the deeper soil water, and 

 5th, to hasten the development of weed seeds so they may 

 be destroyed before the crop is in the way of killing them. 



FIG. 71. The disc harrow. 



287. The Disc Harrow. One of the best tillage tools yet 

 devised is the disc harrow represented in Fig. 71. There 

 is no harrow which so thoroughly pulverizes a soil in the 

 spring after fall plowing as this tool. When set to work 

 deep the draft is heavy but the amount of work it is doing 



