92 SOIL WATER AND ITS RELATION TO SOIL FERTILITY 



during the earlier stages of growth. .The oat yield is less dependent 

 on the character of the rainfall than is the hay crop. An ideal oat 

 year is fairly dry and warm during the time of sowing. During the 

 growing season, however, oats should have from three and one-half 

 to four inches of rainfall per month about two inches every fifteen 

 days. If the rainfall should be less than this the grain would not 

 fill properly; if more, the oats generally grow too rank and lodge 

 easily, especially on manured lands. Temperature also affects 

 the yield. If the temperature should average rather warm while 

 the oat crop is just heading out, the yield may be reduced from 

 three to ten bushels per acre even though the moisture conditions 

 be favorable. 



The yield of winter wheat is determined largely by the rainfall 

 during September and October, and by the snowfall and tempera- 

 ture during the winter. 



Water the Limiting Factor in Dry-farming. In dry climates 

 the water requirement of plants is greater than in a humid climate; 

 and, since the rainfall is so much less, water is the all important 

 problem and the limiting factor of production in dry-land farming. 

 Dry-land farming means the profitable production of useful crops 

 without irrigation in districts receiving an annual rainfall of from 

 ten to twenty inches. Every effort is made, therefore, in these dis- 

 tricts to conserve the rainfall for use by crops. 



Moisture Often a Limiting Factor in Humid Climates. In a 

 climate having an annual rainfall of more than thirty inches it 

 would seem that moisture can never be a factor limiting soil fer- 

 tility. The fact is, few seasons pass in which some important crop 

 is not reduced from one-fourth to two-thirds in yield because of 

 the lack of sufficient moisture, owing to the character and distribu- 

 tion of the rainfall. Sometimes the rain comes in torrential showers, 

 and much of the water is lost as surface run-off; sometimes the 

 snow melts rapidly while the ground is still frozen, and again much 

 water runs away; at times the rain comes in light showers and is 

 soon lost from the soil by evaporation; and frequently rains come 

 at the wrong time or do not come at all when most needed. 



Water Problem in Semi-arid vs. Humid Climates. The real 

 problem that confronts the dry-land farmer is, "Is it possible to 

 conserve and use the rainfall so as to make it available for the 

 production of profitable crops?" 



In humid climates, because of water losses and irregularities in 

 rainfall which materially affect crop yields, the question arises, 



