CROP PRODUCTION 71 



ever, reference to accurate studies of these effects will be 

 found in Chapter XVI. 



Methods of cultivation, or intertillage, are essentially the 

 same for all crops. They consist in stirring the soil surface 

 by means of some farm implement. The most effective 

 implement is a one- or two-row cultivator supplied with a 

 number of small shovels. 



The amount of cultivation depends somewhat upon the 

 nature of tlie crop. In general, cultivation should be re- 

 peated often enough to keep down the weeds and to preserve 

 a good mulch until the ground is well shaded by the growing 

 crop. For example, four cultivations seem to be enough for 

 corn. The yield is rarely improved by more than four. 



Depth of cultivation has already been referred to; the 

 conclusion being made that shallow rather than deep culti- 

 vation was to be preferred. There are at least four advantages 

 in shallow cultivation: it gives the most effective depth for 

 a mulch; it does not destroy the roots of the growing plants; 

 it leaves the surface nearly level; it requires less energy to 

 pull the cultivator. 



Protection. The growth of crops is interfered with by 

 weeds, plant diseases, and insects. The effect and control 

 of weeds will be referred to in a subsequent chapter on weeds. 

 One of the problems of successful crop growing has to do 

 with the control of plant diseases and insects. Each crop 

 has its own difficulties with these destructive agencies. For 

 example, wheat rust is a disease which attacks wheat, and 

 sometimes timothy and other grasses; the Hessian fly 

 confines its injuries chiefly to wheat. Some of the most 

 important general facts relating to the control of plant dis- 

 eases and insects will be found in Chapters XV, XVII. In the 

 discussion of each particular crop, reference will also be made 



