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



At Hays, green-manure rotations 

 caused slightly greater nitrogen and 

 carbon losses than continuous small 

 grain or alternate small grain and 

 fallow. Alternate row crop and 

 fallow caused the greatest nitrogen 

 and carbon losses. 



At Colby, green-manure rota- 

 tions caused large losses of soil 

 nitrogen and carbon. These rota- 

 tions were as destructive as con- 

 tinuous row crop and alternate row 

 crop and fallow. 



At Garden City, green-manure 

 rotations were intermediate be- 

 tween continuous small grain and 

 continuous row crop in soil nitrogen 

 and carbon losses. 



Reasons for the differential effects 

 of green manures are not known. 

 Soil erosion may have been a 

 factor. 



Hobbs and Brown (35) studied 

 the effects of manure and straw 

 applications on nitrogen and carbon 

 losses at Hays. Manure applica- 

 tions reduced both nitrogen and 

 carbon losses (fig. 4). However, 3 

 tons of straw was as effective as 3 

 tons of manure in reducing these 

 losses (35). 



Long extrapolations are question- 

 able, but extrapolating the curves 

 to zero nitrogen and carbon losses 

 indicated that 24 to 30 tons of 

 manure applied every 3 years may 

 be necessary to maintain soil nitro- 

 gen and carbon. This is a fan- 

 tastic figure for dryland areas where 

 little manure is available. 



Metzger (56) studied the nitrogen 

 and organic-carbon contents of soils 

 as influenced by cropping systems 

 and soil treatments. This study 

 was on the soil-fertility field plots 

 at Manhattan for the period 1915 

 to 1934. The soil was Geary silty 

 clay loam. The cropping systems 

 studied were a 16-year rotation 

 (alfalfa, 4 years, and corn, wheat, 

 wheat rotated for 12 years), a 3- 

 year rotation (corn, soybeans, 



wheat), continuous wheat, and con- 

 tinuous alfalfa. 



The 16-year rotation led in the 

 rate of loss of nitrogen, but the 

 3-year rotation led in rate of loss 

 of carbon. Corn dissipated nitro- 

 gen and carbon two to three times 

 faster than rotation or continuous 

 wheat. Alfalfa grown continuously 

 increased the supply of nitrogen 

 and carbon in the soil at the rate 

 of 0.71 percent and 0.43 percent 

 per year, respectively. Alfalfa, in 

 continuous culture, continued to 

 add to the supply of nitrogen in the 

 soil for 19 years, although all top- 

 growth was regularly removed as 

 hay. 



Results from studies (56) of the 

 nitrate content of the wheat plots 

 previously cropped to alfalfa for 

 periods up to 9 years indicate a 

 rather sharp decline in nitrate pro- 

 duction 6 years after plowing the 

 alfalfa sod. Residual effects in 

 nitrate accumulating capacity were 

 evident for 8 years or more on all 

 plots cropped to alfalfa for 2 years 

 or longer. 



Manured and green-manured 

 plots maintained higher nitrogen 

 and carbon levels than did control 

 plots (56). Commercial fertilizer 

 had a similar but less marked effect. 



Fritschen and Hobbs (22) re- 

 ported further studies on the soil- 

 fertility field plots at Manhattan in 

 1956. They determined the aver- 

 age nitrogen and carbon contents 

 under three cropping systems for 

 1915-56 (fig. 5). ^All cropping 

 systems showed losses of both nitro- 

 gen and organic carbon for the 

 period of study. Greatest total 

 losses of both nitrogen and organic 

 carbon occurred in the 3 -year rota- 

 tion. Total losses of nitrogen from 

 plots under the 16-year rotation 

 (alfalfa for 4 years, and corn, wheat, 

 wheat for 12 years) were signifi- 

 cantly greater than those from 

 plots in continuous wheat. Initial 

 nitrogen level was higher for the 



