12 CIRCULAR 5 63, U. S. DEPARTMENT OF AGRICULTURE 



condition exists. The advent of rains may make conditions favorable 

 for crops that may be seeded later, such as corn, sorghums, Sudan 

 grass, millet, or small grain to be harvested for hay. Frequently, 

 however, summer fallowing is likely to be indicated. A crop can then 

 be sown on the fallowed land the following year. The yield of this 

 crop may be compared with the sum of the two yields obtained if the 

 land were seeded each year. The data in hand afford opportunity 

 for 74 comparisons of 2 crops on spring-plowed land and 82 on fall- 

 plowed land when the first crop had only 1 foot or less of wet soil with 

 1 crop on fallowed land the second year. 



The average yield of the one crop on fallowed land was 93 and 94 

 percent, respectively, of the averages of the sum of two successive 

 crops on spring-plowed and on fall-plowed land. In considerably 

 more than half of the comparisons the single crop on fallowed land 

 exceeded the total of the two crops on continuously cropped land. 

 The slight loss in total production is much more than offset by the 

 lesser cost of growing and harvesting one crop instead of two. 



Other crops may enter into the rotation, the effects of which on the 

 following crop generally lie somewhere in the field between those of 

 continuous cropping to small grain and alternate fallowing and crop- 

 ping. Thysell (9) has shown that corn may leave in the soil a residue 

 of water that was unavailable to or unused by it but which may be 

 available to a crop of wheat that follows. Among other crops having 

 somewhat similar effect may be mentioned potatoes, beans, and small 

 grains harvested in an immature state for hay. The effect of sorghums 

 in the northern Plains or other short-season localities may be much 

 the same as that of corn, but farther south where they have ample 

 time to mature before frost they are not likely to leave in the soil any 

 water available to wheat. Much of the residue of water left by such 

 crops is apt to be in the lower depths of the feeding zone of wheat and 

 consequently adds something to the effectiveness of the depth of soil 

 that is wet to its field-carrying capacity by precipitation between 

 harvest and seeding time. 



There are occasional years when the precipitation, temperature, and 

 other controlling factors are such that good yields are produced 

 regardless of the cultural practices or the initial water content of the 

 soil. On the other hand, there are years when conditions during the 

 growing season are so unfavorable that only failures or low yields are 

 realized, regardless of the initial condition. The first-named condi- 

 tion is of altogether too infrequent occurrence to base any expectation 

 of continued success upon its appearance overcoming an unfavorable 

 spring condition of soil water. Examination of the year-by-year 

 results given in the appendix will show that only about once in 12 to 

 20 years at any of the stations studied will a good yield be obtained 

 from wheat seeded on soil that is not wet more than 1 foot deep. 

 Such an examination will also show the relative infrequency of failure 

 from crops sown with the most favorable conditions of soil moisture. 

 Both of the conditions named together are not frequent enough to 

 destroy the practical value of the initial soil-water content as an 

 indicator of production. 



It seems self-evident that the success or failure of an agricultural 

 system or practice must depend upon the average results that flow 

 from it over a considerable period of years. Although temporary 

 success may be achieved under abnormal conditions in spite of the 



