128 



EXPERIMENTS WITH PLANTS 



sents a single soil -crumb greatly magnified. The 

 particles are held together by the water (w), just as 

 are two plates of glass or the hairs of a brush when 



wetted. If the brush be dry or 

 submerged in water the hairs 

 tend to fall apart; the same is 

 true of the soil-particles. . 



At the same time that the 

 soil is broken up it is mixed with 

 air. It will be noticed that the 

 surface of the plowed land is 

 much higher than that of the 

 unplowed: there has been a 

 noticeable increase of bulk due 

 to the spaces between the soil- 

 crumbs formed by tillage. These 

 spaces are filled with air, so that the increase in bulk 

 represents the amount of air which has been mixed 

 into the soil in the plowing. 



Surface tillage is accomplished by the harrowing 

 which follows plowing, and also by placing the plants 

 in rows several inches apart and running a cultivator 

 between them. This serves the triple purpose of 

 destroying weeds, breaking up and aerating the soil, 

 and maintaining a surface mulch. 



It is important not to till the soil when it is too 

 wet, since in that case the tendency is to break up the 

 soil -crumbs into their constituent soil- particles, which 



94. A single soil -crumb magni- 

 fied to show the soil-particles 

 of which it is composed ; the 

 particles are held together by 

 the water (w), just as are the 

 hairs of a brush when wetted. 

 The white spaces between the 

 particles represent air. 



