48 



DRY FARMING 



tent and the amount required for a 30-bushel crop, we 

 can calculate the numbers of 30-bushel crops that are 

 theoretically possible (column 3). 



TABLE VI. "Supply and Demand" of Essential Elements in Av- 

 erage Saskatchewan Soil. 



While such an average analysis of soils does not repre- 

 sent the true average for the province and while 16 

 samples are not enough upon which to base final judg- 

 ment, the results do have instructive value. It is ap- 

 parent that nitrogen and phosphorus, of the essential 

 elements, will first limit crop yields. Undoubtedly the 

 deficiencies are already felt in the poorer, longer-cropped 

 soils. This method of computation is subject to several 

 further qualifications however. 



First, the nitrogen supply is subject to additions and 

 deductions other than cropping (exclusive also of 

 fertilization and green manuring). Certain free-living 

 aerobic bacteria (Azotabacter) living in the soil have 

 the power of extracting nitrogen from the air; certain 

 amounts of nitrogen are brought down in the rain, 

 chiefly as ammonia and nitric acid. On the other hand 

 certain bacteria may cause the loss of soil nitrogen into 

 the air (denitrification) and what is most important, 

 great losses of soil nitrogen occur through surface 

 erosion, and through leaching. The latter is relatively 



