PLANTING 85 



potatoes were planted year after year in the same corner of 

 a garden, the land would very likely become infected, and in 

 time scarcely any potatoes in a whole harvest would be free 

 from disease. If, oh the other hand, when the first trace of 

 scab appears, the potato patch is transferred to another spot, 

 the fungus, faithful to its choice, is starved out. 



Insects, to be sure, allow themselves a larger range of food 

 supply than fungi do, not remaining constant to one plant. 

 But still the plan of shifting a group of plants from one part of 

 a garden to another is, for the reasons already given, strongly 

 advised. The hard-pushed gardener grimly enjoys giving 

 young insects whose birthplace has been nicely selected by 

 the mother the surprise of a lifetime in a total change of crop. 



Anybody who lives near a truck farm hears technical ex- 

 pressions with which he becomes familiar. Gardeners talk, 

 for example, about catch crops, cover crops, and green manure. 

 By catch crop they mean a crop that is planted between two 

 money-making crops. A cover crop means some crop planted 

 late in the season, chiefly for the purpose of holding the sol- 

 uble food which would otherwise drain away. Clover is per- 

 haps the best, but winter wheat and rye and turnips also 

 make good cover crops. These are usually plowed under in 

 the spring ; they act in this way as a form of green manure. 

 Green manuring means the planting of certain herbaceous 

 plants for the sole purpose of enriching the soil. Some plant 

 organisms are constituted so that they can successfully play 

 this role of benefactor to the land. Those that stand pre- 

 eminent as great soil renovators are the leguminous plants. 

 It is a fact that three representatives of this family, the clover 

 in the north, and the cowpea and the alfalfa in the south, have 

 rejuvenated miles of worn-out farm land. 



These few hints will at least serve to show how certain defi- 

 nite changes in crops are planned by the farmer according 



