362 GRASSES AND HOW TO GROW THEM. 



pacted. that the air does not readily penetrate them 

 sufficiently, to the great detriment of the crops whicli 

 grow upon them. Grass roots growing in the same 

 render them more friable bj separating the particles, 

 and, consequently, more easily tilled. The labor of 

 pulverizing these soils is lessened in proportion as these 

 are present. The aid thus rendered to aeration will 

 be readily apparent, since the humus substances that 

 have been distributed between the earth particles are 

 easily penetrated by the air. The extent to which such 

 vegetable substances increase the power of soils to hold 

 moisture is very great. They act like a sponge in tlie 

 soil, arresting moisture going downward or upward and 

 holding it for the growing crops. The service thus 

 rendered in leechy soils is even more valuable than in 

 those that are retentive ; hence, interjecting these crops 

 frequently into the rotation is one of the surest safe- 

 guards against drought. The way in which they trans- 

 form inert plant life in the soil in their decay into 

 active or valuable plant food cannot be discussed here. 

 The fact only can be stated, and it is one of much 

 significance to the farmer. The influence of certain 

 grass plants in preventing washing and gullying is fur- 

 ther discussed in Chapter XVI. (see page 399). The 

 extent to which the lifting of light soils by wind is les- 

 sened is largely dependent on the kind or kinds of 

 grasses grown and the frequency with which they are 

 grown. Those grasses with a strong and matted root 

 system, as quack grass, best serve this end. The influ- 

 ence of some of these grasses in thus preventing soils 



