28 DRY FARMING IN RELATION TO RAINFALL AND EVAPORATION. 



obtained at this station. The rainfall during this year was 5 inches 

 above the normal, but it will be seen from the table that the rainfall 

 during the growing season was only 1.4 inches above that in 1907. 

 The lower yield at Highmore in 1908 is probably to be explained by 

 the fact that the last three weeks of July and the first week of August 

 were very dry, less than 1 inch of rain falling during this time. These 

 results show the necessity of knowing the seasonal rainfall when study- 

 ing the relation of yield to rainfall in the Great Plains. Frequently, 

 as in tins case, a knowledge of the monthly rainfall is not even suffi- 

 cient, but the daily distribution must be known to determine the 

 influence of the precipitation upon the yield. 



At Belief ourche, in western South Dakota, in 1907, a yield of 24 

 bushels was obtained on spring-plowed land as compared with 32 

 bushels on summer-tilled land and 29 bushels as the average yield on 

 the rotation plats. These yields were obtained. with a rainfall of about 

 8 inches during the growing season. It will be noted that while sum- 

 mer tillage gave the highest yields, the yield from the rotation plats 

 was nearly as high. Therefore, on the heavy gumbo soils repre- 

 sented by the Bellefourche district, with a seasonal rainfall of 9 

 inches, the results indicate that crop rotations will give much better 

 returns than summer tillage, considering the fact that a crop on 

 summer-tilled land is obtained only once in two years. 



At North Platte, Nebr., the seasonal precipitation has been very 

 uniform for the three years during which experiments have been con- 

 ducted, varying from 12 to 14 inches. The yield of wheat during 

 these three years, based upon an average of 10 plats, varied from 20 

 to 29 bushels. We will consider briefly the cause of this fluctuation 

 in yield. 



The soil at North Platte is a silt loam of loess formation and absorbs 

 water readily. The season of 1906 was very wet, so that the season 

 of 1907 opened with a good supply of moisture in the soil. Conse- 

 quently, good yields were obtained in 1907, although the influence 

 of a rather dry spring is shown in the lower yields of the spring- 

 plowed plat. 



The fall of 1907 and the spring of 1908 at North Platte were very 

 dry, only 3.5 inches of precipitation occurring from October 1 to May 

 1. In May, June, and July there was abundant rain. It is under 

 such conditions that moisture-conservation methods become most 

 effective. The moisture stored in the soil by conservation methods 

 is sufficient to carry the crop through a dry spring, and when supple- 

 mented by abundant summer rains is sufficient for the production 

 of large crops. Thus, during 1908 a yield of 40.5 bushels was obtained 

 from summer-tilled land as compared with 22.7 bushels from wheat 

 sown on spring-plowed land. 



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