Healthy Host and Its Requirements 69 



surface fvuTOw or check methods. A portable 

 spray equipment meets these conditions well. The 

 disadvantage of this system Hke that of rain is the 

 baking of the surface soil, thereby necessitating more 

 frequent cultivations. Moreover, when spray ir- 

 rigation is overdone it is hkely to encourage the 

 development of mildews and various leaf spots. 



Need of Sanitary Environments 



Science has shown us the reasons for sanitary liv- 

 ing for men and animals and equally for plants. 

 Since the soil is the home of the plant, we must keep 

 that soil as clean as we do otir houses, or the stalls in 

 stables. The soil organisms give off numerous poison- 

 ous excreta, which become harmful to them and to 

 the crops. Through their own activities, the roots of 

 plants, too, throw off certain poisonous excreta. If 

 they are allowed to accumulate in the soil through 

 growing the same crop too long in the same soil, a 

 point is reached where that crop will refuse to grow 

 there any longer, even if there is no evidence of soil 

 exhaustion. The best purifier of soils is organic 

 matter appHed as manure or green vegetable matter 

 which is converted into humus. 



Clean Culture. There are two other means by 

 .which we can keep soils in a sanitary condition. 

 Rotation of crops is discussed on page 372. Clean 

 culture, too, is an essential means of safeguarding 

 the health of our economic plants. Not only do 

 weeds help to carry fungous diseases which are also 



