LEGUMINOUS FORAGE CROPS 137 



been grown. Grass crops also leave a considerable quantity 

 of vegetation behind them, sometimes a greater quantity than 

 red clover, for example, but usually it is not so rich in nitrogen, 

 phosphorus, and potassium. 



The deeper root habits of most leguminous plants not only 

 make it possible for them to make use of fertility at lower 

 depths but thus, possibly by leaving these elements near the 

 surface, to make them available to shallower rooted plants. 

 This, however, has not been demonstrated experimentally, and 

 a critical study of the root habits of leguminous plants leads 

 to the belief that this quality in them has been overestimated. 



(M7) 



4. The so-called virgin fertility of a soil is largely due to 

 the nitrogen combined in organic matter. While this fertility 

 has, doubtless in many instances, been collecting through count- 

 less ages, yet due to denitrification and the leaching of nitrates, 

 the balance of available nitrogen at any time is not large, and 

 by certain systems of cropping it can be easily brought to such 

 a point as to make the soil unproductive. On the other hand, 

 the necessary available nitrogen can be readily, although not 

 necessarily, economically restored. 1 Leguminous forage crops 

 furnish the agency through which the necessary available 

 nitrogen may be in a measure maintained, and frequently are 

 the most economical means of doing it, since they are capable 

 of securing their nitrogen supply from the free nitrogen which 

 constitutes four-fifths of the atmosphere. What proportion of 

 the nitrogen found in the leguminous plants comes ordinarily 

 from nitrates already in the soil, and what proportion comes 

 from the free nitrogen of the air through the aid of tubercle- 

 bacteria, has not been fully determined. The indications are 



1 The algebraic sum of the gains and losses of nitrogen which occur in 

 all soils is a plus quantity in untilled soils and a minus quantity in soils 

 constantly cultivated. In the latter case, however, the minus quantity may 

 be converted into a plus quantity by proper methods of fertilization and 

 crop rotation. 



