. PROBLEM OF CROP PRODUCTION 357 



Nature supplies plenty of air above ground and in all 

 except humid climates, low lying soils, or very heavy 

 lands sufficient in the soil as well. It is our business only 

 to regulate the supply so that the soil will contain 

 neither too much nor too little, but just the right amount 

 for good tilth. 



313. Frost Limits the Yield in Northern Climates. The 

 average number of days between spring and fall frosts 

 for a period of twelve years in the Central West varies 

 from 73 in the north to 133 in parts. of the south. The 

 shortest period between frosts during the same length of 

 time was 33 days for the north and 101 for the south 

 while the longest frost-free period in each of these dis- 

 tricts was 112 and 163 days respectively. These figures 

 probably represent the extremes in length of frost-free 

 period for most of the present settled area of Western 

 Canada. 



It is apparent from these data as well as from actual 

 experience that in northern climates nature sometimes 

 fails to provide enough heat to mature crops without in- 

 jury from low temperature, and that, therefore, man 

 must either add to the supply or take such steps as will 

 offset the danger from frost. This is one of the two 

 chief difficulties facing the crop grower in Western Can- 

 ada. There is need for developing a system of "North- 

 ern Farming" for northern regions just as there is a 

 need for a system of "Dry Farming" for dry regions or 

 one of "Humid Farming" for wet ones. 



314. Water in the Soil Determines the Yield in Dry 

 Climates. Water is generally the limiting factor in crop 

 production here. In other words, it is the chief cause of 

 low yields. From 250 to 1,000 pounds of moisture are 

 extracted from the soil and transpired by the plant into 



