Nitrogen Loss 239 



numbers and kinds of bacteria present in it. It has been 

 shown in the study of our northwestern wheat soils that 

 the continuous cultivation of wheat on the same land 

 involves very serious losses of nitrogen and of humus. 

 The losses are due apparently not only to the leaching 

 out of the soluble nitrates in the late summer and early 

 fall, but, also, to the destructive decomposition of the 

 humus by the soil bacteria and the liberation of free 

 nitrogen gas. On the other hand, when crops are grown 

 in regular rotation with the inclusion of clover, the losses 

 are greatly reduced. In fact, gains of both humus and 

 of nitrogen may be shown under favorable conditions. 

 These facts indicate that the bacteria that are so destruc- 

 tive to the nitrogen compounds of the soil, are largely 

 suppressed when crop-rotations are practiced. 



In systems of green-manuring, the use of legumes 

 after cereal crops has the additional advantage, there- 

 fore, of diminishing the losses of humus-nitrogen. 

 Hence, leguminous green-manures are doubly valuable 

 in adding nitrogen to the soil and in decreasing the losses 

 of humus-nitrogen by suppressing the activities of the 

 injurious bacteria. It is easy to understand why legumi- 

 nous cover-crops have gained in favor to so great an 

 extent within the last two or three decades. 



Historical sketch. The practice of turning under 

 crops for soil-improvement is not new. To the farmers 

 of ancient Rome, lupins, vetches and beans were known 

 as soil-renovating crops, and their use as green-manures, 

 at least in the case of lupins, was more or less extensive. 

 Even when these crops were not turned under, but rather 

 removed from the land, the stubble and roots were fre- 



