LEGUMINOUS CROPS. 101 



It so happens that both the scientific interest and the Legumin. 

 practical value of these crops, whether as elements in rota- °^ d p ^ ts m 

 tion, or as grown in the mixed herbage of grass-land, depend gen. 

 very largely on the amount of nitrogen which they contain, 

 and on the sources of their nitrogen ; and especially on the 

 great differences in these respects between them and the 

 representatives of the other Orders with which they are 

 grown, either in alternation in our rotations, or in association 

 in our meadows and pastures. 



So much is this the case, that it is essential to a proper 

 understanding and appreciation of the characteristics of 

 growth of these crops, and for the illustration of their value 

 and importance as depending on those characteristics, to 

 compare and to contrast the conditions and results of their 

 growth with those of the crops of other Orders. 



"We will, therefore, first briefly call attention to the differ- 

 ence in the amounts of nitrogen assimilated over a given 

 area by different crops when each is grown for many years 

 in succession on the same land without any nitrogenous 

 manure — that is to say, under conditions in which the soil 

 is to a great extent exhausted of accumulations of nitrogen 

 due to recent supplies by manure, and when, therefore, the 

 plants have to rely largely on what may be called the natural 

 resources of the soil, and on those of the atmosphere. 



Yield of Nitrogen per acre in different Crops. 



Table 32 (p. 102) shows the yield of nitrogen per acre per Yield of 

 annum, with mineral, but without any nitrogenous manure — n iifferent m 

 in wheat and in barley as gramineous crops, in turnips as crops. 

 representatives of the Cruciferae, in sugar-beet and mangel- 

 wurzel of the Chenopodiacese, and in beans and clover as 

 leguminous crops, when each is grown for many years in 

 succession on the same land. 



Incidentally it is to be noticed that in the case of each of Gradual 

 the crops — wheat, barley, and beans — thus grown year after 3^^^ 

 year on the same land for many years in succession without 

 nitrogenous manure, there was a reduction in the yield of 

 nitrogen per acre per annum over the second period com- 

 pared with the first ; that is, as the previous accumulations 

 within the soil became reduced. 



Disregarding this tendency to reduced yield, it is seen Yield of # 

 that over the same period of 24 years, with full mineral but J]£jf 2J$* 

 without nitrogenous manure, the wheat yielded an average of barley. 

 22.1 lb., and the barley 22.4 lb. of nitrogen per acre per 

 annum ; the two allied crops, therefore, yielding almost 

 identical amounts in their above-ground produce without 



