THE TILLAGE OF STUBBLE LAND 147 



119. The Causes of Low Yields. — The causes of low 

 yields on stubble fields are usually few in number. The 

 most common ones are: — 



1. The low moisture content of the soil. 



2. The presence of grass, shrubs and weeds. 



3. A poor seed bed. 



4. Insufficient "available" plant food. 

 " 5. The stubble itself. 



6 : Unavailable soil moisture. 

 The first is the most general, but any one or all of the 

 others may be contributing causes of poor crops. A few 



Tig. 59. — ^Harvesting Wheat near High Eiver, Alberta. 

 On the left may be observed a fairly typical surface covering of wheat 



stubble. 



of these cannot be controlled absolutely, but all can be- 

 materially influenced by man and some are entirely 

 within his control. Each is affected by tillage, but, un- 

 fortunately, the tillage operation which produces a favor- 

 able condition in one often produces an unfavorable 

 condition with respect to another. Hence no fixed pro- 



