78 THE CHEMISTRY OF THE FARM 



Leguminous Crops. — Some of these are grain crops, 

 as beans and peas ; others are fodder crops, as red 

 clover, sainfoin and lucerne. A striking characteristic 

 of all these crops is the large amount of nitrogen 

 which they contain, the quantity being about twice as 

 great as that found in cereal crops of the same weight. 

 The quantity of potash and lime in leguminous crops 

 is also very large. The relative proportion of these 

 two bases varies much in crops grown on different 

 soils ; upon a calcareous soil lime will preponderate in 

 the crop, but on a clay soil potash. The lime is found 

 chiefly in the leaf. Silica is nearly absent in legu- 

 minous crops. 



The amount of nitrogen collected by leguminous 

 crops is very remarkable. A good crop of clover, when 

 cut for hay, removes a large quantity of nitrogen from 

 the land, but it nevertheless leaves the surface soil 

 actually richer in nitrogen than it was before, from 

 the residue of roots and stubble left in the soil. From 

 whence is this large quantity of nitrogen obtained? 

 It must be procured either from the subsoil or the 

 atmosphere. The question is made more puzzling by 

 the fact that nitrogenous manures generally produce 

 but little effect upon leguminous crops. It seems now 

 quite certain that leguminous crops possess in their 

 root-tubercles an apparatus capable of bringing the 

 nitrogen of the atmosphere into combination. The 

 special agent residing in these tubercles is a micro- 

 organism derived from the soil (see p. 11). When 

 attempting to grow a leguminous crop for the first 

 time on a hitherto unfertile peat or sand, it has been 

 found advantageous to scatter on the land a little soil 

 which has already grown the leguminous crop in gueg- 



