2i CIECULAK 5 6 3, U. S. DEPARTMENT OF AGRICULTURE 



responsible for the widespread acceptance of the guides to practice 

 afforded by it. 



In a later publication Mathews and Barnes (7) show that at a rep- 

 resentative station (Dalhart, Tex.) the available water in the soil at 

 seeding time is a close determinant of the crop of milo that will be 

 harvested. 



The authors, studying the daily rate of use of water by spring 

 wheat on the Great Plains (3), showed that there was a high corre- 

 lation between the total use of water and the yield. In that study 

 the difference between the water in the soil at seeding time and that 

 at harvest time, added to the total precipitation during the interval, 

 constituted the total water used. They showed further that the 

 total quantity is seldom more than sufficient for the needs of the crop. 



Thysell (9) has made an important contribution to knowledge of the 

 effects of different tillage methods and crops on the quantity of water 

 in the soil at seeding time, and to its exhaustion by crops. He gave 

 yields in pounds of total crop (grain plus straw harvested with a 

 binder). 



The senior author (#), using data from 19 field stations in the 

 Great Plains for a total of 387 crop years, demonstrated a high degree 

 of correlation between precipitation for the crop year and three indexes 

 of yield of spring wheat. It was shown in this study that fallowing, 

 w r hich stores water in the soil in advance of the crop year, reduced 

 the dependence of the crop on precipitation during the crop year and 

 increased yields in the northern Plains an average of about 7 bushels 

 per acre above yields obtained by continuous cropping. 



MATERIAL FOR AND METHODS OF STUDY 



The Division of Dry Land Agriculture and cooperating State agri- 

 cultural experiment stations 2 have accumulated in continuous records 

 covering more than 30 years a large volume of data on the moisture 

 content of the soil and the yields of crops in which the control exer- 

 cised by the initial (springtime) water content of the soil over the 

 yield of spring wheat may be studied. 



The present study is confined to spring wheat on 3 methods of soil 

 preparation at 15 field stations (considering for this purpose the 2 

 fields at Mandan, N. Dak., as separate stations) for those years 

 from 1907 to 1938 in which the moisture content of the soil was deter- 

 mined at a suitable time in the spring and when the crop was not 

 destroyed or the yield seriously impaired by hail or rust. The locations 

 of these and other stations in the Great Plains at which the Division 

 has conducted experiments are shown in figure 1. So far as affected 

 by variety or cultural treatment the yields are highly comparable 

 between years and stations. This is more fully discussed in United 

 States Department of Agriculture Technical Bulletin 636 (2). Soil 

 moisture was determined in units of foot sections of depth, each deter- 

 mination being based on 4 cores of soil taken with a King soil tube 

 having a cutting edge 20 mm. in diameter. The standard depth of 

 sampling was 6 feet, but some samples were taken to greater depth, 

 and at some stations the character of the soil either prevented going 

 to that depth or the water content made it unnecessary. 



2 The data from Montana, North Dakota, Wyoming, Nebraska, and Colorado were obtained in coopera- 

 tion with the agricultural experiment stations of the respective States. 



