102 Elementary Principles of Agriculture 



tribution, facilitating Teitilizing, rotation (which is 

 closely related to diversification) affords other ad- 

 vantages: 



(a) Tends to free the land from noxious weeds, as where 

 oat stubble is planted to June corn, the late cultivation 

 of the corn prevents the seeding of the weeds, such as 

 cockle burs or Johnson grass. 



(b) Exterminates insect and fungous diseases. Insect 

 and fungous pests usually attack only particular kinds of 

 crops. If the same crop is grown on the same land year 

 after year, the larvae of insects and spores of the fungi 

 lodging in the ground during the fallow season will 

 find their food ready when the season is ready for them 

 to multiply. (See 1 217 and 1) 228.) 



(c) Avoids the injurious effects of growing the same 

 crop continuously on the same land. Recent investiga- 

 tions have shown that the decreased yields resulting 

 from growing the same crop on the same land from sea- 

 son to season is due not only to the loss of mineral nutri- 

 ents, but also to the formation of toxic substances (| 134) 

 in the soil. These toxic substances are not usually inju- 

 rious to other crops, though there are cases known where 

 one crop will leave substances in the soil poisonous to 

 some other crop. 



147. Distributes the Labor. Rotation and diversifi- 

 cation make it possible for the work to be more evenly 

 distributed through the year. Not all the crops will need 

 to be planted, cultivated or harvested at the same time. 

 The farmer will thus be able to keep busy, and not have 

 to pay out so much for help during rush seasons that 

 come with a one-crop system of farming. 



