THE SOIL 



49 



more important in humid regions. But in general even 

 under semi-arid conditions the losses from all sources 

 other than cropping greatly exceed the gains. Thus 150 

 crops of thirty bushels each (4,500 bus.) is by far too 

 liberal an estimate from the nitrogen standpoint. 



Second, with phosphorus, and this applies equally to 

 nitrogen, even under otherwise ideal conditions, we 

 could not' continue to remove 30-bushel crops of wheat 

 successively until the phosphorus was exhausted. Under 

 humid conditions phosphorus becomes a serious limit- 

 ing factor when the supply is below 1,000 pounds. The 

 yield is determined by the total supply of plant food, 

 and, as will be discussed later, its "availability". In gen- 

 eral the yields will become decreasingly smaller as the 

 phosphorus or nitrogen (or both) is exhausted. 

 [ 36. Average Saskatchewan Soil vs. Other Soils. — In the 

 following table the "average" Saskatchewan soil is com- 

 pared with some well known fertile soils : 



Table VII. — Average Saskatchewan Soil versus Other Soils in 

 pounds per acre 6 2-3 inches deep — approximately 2,000,000 

 pounds. 



* Hopkins' "Soil Fertility and Permanent Agriculture", 

 t Doneghue, North Dakota Agricultural Experimental Station 

 Bulletin 126, 



