12 BULLETIN 546, U. S. DEPARTMENT OF AGRICULTURE. 



dition of the soil near the surface. After the layer of easily pene- 

 trated soil becomes wet, it becomes so swollen and compact that it 

 is nearly impervious, and further water movement is very slow." 



The results secured in the fall-irrigation experiments at Belle 

 Fourche are in accord with these observations. The application of 

 irrigation water in the autumn filled the surface soil to its moisture- 

 carrying capacity, but apparently had no effect beyond the third 

 foot. Irrigation water applied uniformly to all plats during the 

 growing season, together with the natural precipitation, equalized 

 the moisture content of the soil in these plats, irrespective of their 

 treatment the previous fall. It is important to note also that an 

 abundance of moisture was present in all the plats at the time of 

 sampling in the spring and early summer and that after this time 

 moisture was supplied by irrigation. 



These facts account for the absence of significant differences be- 

 tween the average yields of crops on the fall-irrigated plats and 

 those secured on the check plats. It is possible that if an adequate 

 supply of moisture had not been maintained by irrigation during 

 the growing season the soil-moisture content of the fall-irrigated 

 land would have been higher in the spring than that of the check 

 plats. It seems certain, however, that where crops are properly 

 irrigated during the growing season fall irrigation on this soil will 

 have no material effect. 



SUMMARY. 



The light precipitation received during the winter months in the 

 Great Plains area commonly causes soil to remain dry from the time 

 crops are harvested until the rainy season the following year. In 

 some soils this deficiency of moisture may have an unfavorable in- 

 fluence on the growth of crops, both by hindering the germination 

 of spring-sown seed and by retarding or preventing the desired 

 movement of the water received as precipitation or applied in irri- 

 gation during the growing season. 



Fall irrigation has been advocated as a corrective of this condi- 

 tion in irrigated regions. It has been found efficacious on sandy 

 loam soil in western Nebraska, where it resulted in increased soil 

 moisture in the spring and in greater moisture absorption by the 

 soil throughout the irrigation season. 



In order to test the practice of fall irrigation on a heavy clay soil, 

 experiments were conducted at the Belle Fourche Experiment 

 Farm, in western South Dakota, in 1914, 1915, and 1916. These 

 experiments included wheat, oats, barley, flax, potatoes, sugar 

 beets, and corn, each crop being grown each year in duplicate tenth- 

 acre plats both on fall-irrigated land and on land which received no 

 fall irrigation. 



