LEGUMES AND GRASSES 



51 



Table 35. — Changes in nitrogen and organic-carbon content in surface and 

 subsurface soils compared with virgin sod in rotations with and without 

 grass or alfalfa at several locations in the northern and central Great Plains, 

 1906-47 l 2 





Gain or loss of nitrogen at — 





0- to 6-inch depth 



6- to 12-inch depth 



Location 



Rota- 

 tion 

 without 

 grass or 

 alfalfa 



Grass 



and 



grain 



rotation 



Alfalfa 



and 



grain 



rotation 



Rota- 

 tion 

 without 

 grass or 



alfalfa 



Grass 



and 



grain 



rotation 



Alfalfa 



and 



grain 



rotation 



Mandan, N. Dak 



Percent 

 -31 

 -49 

 -27 

 -34 



Percent 

 -25 

 ________ 



-30 



Percent 

 -18 

 -44 

 -18 

 -35 



Percent 

 -13 

 -30 



3 -10 

 -6 



Percent 

 -6 



-3 



Percent 



+ 3 



Dickinson, N. Dak 



Sheridan, Wyo 



-31 

 3 -5 



Archer, Wyo 



— 9 







Mean 



-35 



-23 



-29 



-15 



-6 



— 10 









Gain or loss of organic carbon 



Mandan, N. Dak 



-36 



-31 

 -41 



-29 

 -20 

 -38 



-24 

 -26 

 -44 



-13 

 2 -10 



-6 



-8 



+ 3 



Sheridan, Wyo 





Archer, Wyo 



-4 



-8 







Mean 



-36 



-29 



-31 



-10 



-6 



-2 







i Data from Haas and coworkers (27). 



J Periods of cropping varied from 30 to 40 years. 



* Sampling depth was 6 to 20 inches. 



low-nitrogen soils. The high- 

 nitrogen soils produced a greater 

 quantity of nitrate-nitrogen and 

 were able to maintain production 

 for a longer time. The results 

 indicate that not only was the total 

 nitrogen content of the soils reduced 

 an average of 36 percent but the 

 quantity of nitrate-nitrogen pro- 

 duced by these soils was also 

 reduced by almost the same per- 

 centage. 



The effects of number of years of 

 cropping on crop yields were studied 

 by means of multiple regression 

 analyses of the data from Mandan, 

 Dickinson, Havre, Sheridan, Archer, 

 and Hays (27). There were two 

 independent variables — precipita- 

 tion and the years of cropping. 



The dependent variable was yield. 

 Results showed that corn and kafir 

 yields were significantly reduced 

 by cropping at some locations but 

 that wheat yields were not. 



When the percentage changes in 

 soil nitrogen were compared with 

 the percentage change in yield, 

 there was little relationship between 

 the two (27). In some instances, 

 yield changes were greater than the 

 nitrogen changes; while in others, 

 the reverse was true. 



Summary 



The effects of cropping systems 

 on the carbon and nitrogen content 

 of the soils at 10 locations in the 

 northern and central Great Plains 

 were studied. Nitrogen losses av- 



