30 DEY FARMING IN RELATION TO RAINFALL AND EVAPORATION. 



recorded in April and May, which explains the equality in yield of 

 the spring-plowed and summer-fallowed plats. In 1909, April and 

 May were very dry, while good rains fell in June. Here again we 

 find the conditions under which conservation methods become espe- 

 cially effective, namely, a dry spring followed by timely summer rains. 

 The wheat sown on spring-plowed land at Amarillo during this season 

 was a failure, while the wheat on summer-tilled land gave a yield of 

 10.5 bushels on a seasonal rainfall of 8.5 inches. 



In comparing the seasonal rainfall with the yields for all the stations 

 it will be seen that when the rainfall from April to July, inclusive, is 

 less than 8 inches the yields are hardly sufficient to cover the expense 

 of producing the crop. With a seasonal rainfall of 8 to 10 inches 

 the yield of wheat grown on summer-tilled land is generally consid- 

 erably higher than wheat grown on spring-plowed land. When, how- 

 ever, the seasonal precipitation is above 10 or 12 inches, the increased 

 returns from the method of alternate cropping and summer tillage 

 do not exceed the returns from wheat grown in rotation or on spring 

 plowing by an amount sufficient to offset the increased cost of sum- 

 mer tillage. 



YIELD IN RELATION TO RAINFALL IN SOUTHERN TEXAS. 

 THE THREE YEARS ' RECORD AT SAiJ ANTONIO. 



Field experiments have also been conducted by the Office of 

 Western Agricultural Extension of the Bureau of Plant Industry, 

 during the past three years, at San Antonio, Tex. The normal rain- 

 fall of this region is about 26 inches, but the evaporation is equal to 

 that on the high plains of Texas during the summer months, which, 

 combined with the irregularity of the rainfall, often results in drought. 

 The following table shows the yields of some of the standard crops 

 of this region during three years in which there was a marked varia- 

 tion in the seasonal rainfall. 



Table IV. — Relation of yield to rainfall at San Antonio, Tex. 



Crop and year. 



Yield per acre. 



Seasonal 

 rainfall. 



Annual 

 rainfall. 



Corn: 

 \ ■ 1907. 



1908. 

 Q 1909. 

 Cotton: 

 1907. 

 1908. 

 Q . 1909. 

 Sorghum 

 • 1907. 

 "1908. 

 .. 1909. 



17.5 bushels. 

 27.4 bushels. 

 9.9 bushels. . 



377 pounds. . 

 1,090 pounds 

 640 pounds. . 



3.1 tons 



13 tons 



1.9 tons 



Inches. 

 9.3 

 10 

 4.4 



13 

 17.7 

 9.3 



11.3 

 16.4 

 8.2 



Inches. 



Inferring to Table IV, it will be seen that the total rainfall for 

 the year 1009 was only one-half that of either of the two preceding 



188 



