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 



Fig. 59. Harvesting Wheat near High River, 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- 



