174 BACTERIA AND SOIL FERTILITY 



to supply nitrogen to his soils as commercial fertilizers cannot 

 be economically used for the production of many crops. This 

 may be seen by the following: 



The nitrogen in a 50-bushel wheat crop would cost $14.40, 

 twenty tons of sugar beets $15, and one ton of alfalfa hay $7.50, 

 if bought as a commercial fertilizer. We know legumes can 

 use nitrogen from the atmosphere, but will they draw on this 

 nitrogen while there is a supply in the soil? Or will they follow 

 the line of least resistance and turn to the atmosphere only when 

 nitrogen is lacking in the soil? If they do, they must first drain 

 the soil of its valuable nitrogen and thus leave it no richer than 

 it was before the legume was grown upon the soil. Let us see. 



Rothamsted Rotation. — Crop rotations in which the legumes 

 enter have been practiced for centuries, but the oldest system on 

 which we have accurate information is the one on Agdell Field at 

 the Rothamsted Experiment Station. This system was inaugurated 

 in 1848 and is still being carefully followed. It consists of a 4- 

 year rotation as follows: 



First year — Swedish turnips (rutabagas) 



Second year — barley 



Third year — clover or beans 



Fourth year — wheat 



Still another system has been running parallel and similar to 

 this, except that fallow cultivation is practiced in the third year 

 instead of growing a legume. The average yields for 20-year 

 periods are given on page 175. These systems are of special in- 

 terest to western farmers, for if we substitute sugar-beets for 

 turnips and alfalfa or peas for the clover and beans we have 

 nearly an ideal rotation for western soils. 



Even where the legume was used in the system there had been 

 a decline in the yield. The yield of the turnips during the first 

 twenty years was 5,264 pounds, the second 1,723, and the third 

 only 967 pounds, thus showing a decrease of about five-sixths the 

 original in sixty years. 



