358 DRY FARMING 



the air in the process of forming one pound of dry 

 organic matter in the plants' tissues. In Western Can- 

 ada we receive from 12 to 22 inches of water from the 

 clouds annually. The precipitation in most agricultural 

 countries ranges between 10 inches and 150 inches per 

 year. Our supply is small and our need is great. Man 

 must make the best use of what nature supplies or 

 furnish artificially the shortage, otherwise he must be 

 content with low yields. 



In humid regions the supply of moisture from the 

 clouds is generally sufficient to produce large returns. 

 In arid and semi-arid climates there is insufficient rain- 

 fall to produce large crops every year with the result 

 that the supply of water must be increased by irrigation 

 or steps must be taken to store a portion of one season's 

 moisture in the soil for the use of the next season's crop, 

 as by summerfallowing, or the use of intertilled crops. 



The efficient utilization of our precipitation for the 

 development of the latent wealth that is in our soil is the 

 biggest material problem Western Canada has to face. 

 She has made some progress towards its solution, but 

 much work still remains to be done. 



315. Plant Food Materials, the Limiting Factor on Poor 

 Soils. — Of the eighty odd chemical elements known to 

 science, fifteen may be used by plants, but only ten are 

 essential to growth, and of these ten, all but four — nitro- 

 gen, phosphorus, potassium and calcium — are provided 

 in liberal quantity in the soil. The available supply of 

 one or more of these essential elements of plant food is 

 sometimes so small in humid climates that large yields 

 on poor soils are an impossibility. Under these condi- 

 tions the terms "'manure" and "fertilizer" are therefore 

 almost synonymjous with soil fertility, and the chief 



