58 MISCELLANEOUS PUBLICATION 952, U.S. DEPT. OF AGRICULTURE 



SUMMARY AND CONCLUSIONS 



In the drier areas of the Cher- 

 nozem and in all the Chestnut and 

 Brown soil zones of the United 

 States and Canada, legumes and 

 grasses, used as green-manure, hay, 

 or pasture crops, usually depress 

 the yields of the crops immediately 

 following. Yield reductions are 

 commonly associated with depleted 

 soil-moisture reserves. Even if the 

 land is fallowed before planting the 

 following crop, yields are frequently 

 not increased over those with or- 

 dinary fallow. 



Along the eastern edge of the 

 northern and central Great Plains, 

 where annual precipitation is 

 greater, legumes in rotation in- 

 crease yields of corn and small 

 grain in years of average or above- 

 average precipitation. In dry years, 

 grain yields may be depressed by 

 dry soil conditions after legumes. 



The increased crop yields in areas 

 of higher rainfall are attributed to 

 the addition of nitrogen from the 

 legume rotation. Limited data in- 

 dicate that inorganic nitrogen is 

 usually more efficient than legume 

 nitrogen in increasing crop yields. 

 More research data are needed to 

 compare legume and inorganic nitro- 

 gen sources. Based on yields of 

 following crops, the average legume 

 inorganic nitrogen equivalent is 

 estimated to be 30 to 40 pounds 

 per acre per year. Values vary 

 according to legume-growing condi- 

 tions. This amount of nitrogen is 

 probably not enough to meet the 

 nitrogen requirements of grain crops 

 in 3- and 4-year rotations. Supple- 

 mental nitrogen will almost cer- 

 tainly be needed for optimum crop 

 production. 



From the standpoint of increasing 

 yields of following crops, available 

 data indicate little justification for 

 growing legumes and grasses in 

 short rotation on class I, II, and III 

 land 8 in' the northern and central 



Great Plains. 



Available research data were ex- 

 amined to determine whether or not 

 water was being lost through deep 

 percolation with existing cropping 

 systems. If water was being lost, 

 deeper rooted crops could be grown 

 to use this water. In 1939 {12) data 

 from Havre, Mont., Mandan, N. Dak., 

 North Platte, Nebr., Hays and Colby, 

 Kans., showed that the annual crop- 

 land cycle of water charge and dis- 

 charge was, generally, confined to 

 the root zone and that no water was 

 moving to the underlying strata. 



At both Lincoln, Nebr., and 

 Manhattan, Kans., alfalfa used soil 

 moisture to at least 30 feet when 

 moisture was available from the 

 surface downward. When deep soil 

 moisture reserves were depleted at 

 Lincoln, this moisture was not re- 

 stored on medium- to heavy-tex- 

 tured soils under continuous cereal 

 cropping. Five years of continuous 

 fallow rewet the soil to only 1 1 feet. 



At Manhattan, 2 years of fallow 

 restored deep subsoil moisture re- 

 serves that had been depleted by 

 alfalfa. Cereal cropping restored 

 deep soil moist ure reserves after 12 

 years (1938 to 1949). During one 

 4-year period, total precipitation 

 exceeded average precipitation by 

 36.6 inches. Soil-moisture samples 

 were not taken during this period, 

 but it seems probable that much of 

 the deep moisture restoration may 

 have been during this period. Since 

 precipitation at Lincoln and Man- 

 hattan is greater than in the Plains, 

 it seems probable that water is not 

 bein£ lost to the water table on 



8 Land-capability classes I, II, and III 

 are explained by Steele (76) as follows: 

 Class I land is good land with few manage- 

 ment limitations. Class II land has 

 moderate limitations. Some easily ap- 

 plied conservation practices are necessary. 

 Class III land has severe limitations. 

 Continuous cultivation is possible if these 

 limitations are handled properly. 



