LEGUMINOUS CROPS. 53 



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 leguminous crops. 



The amount of nitrogen collected by leguminous 

 crops is very remarkable. A good crop of red 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 atmo- 

 sphere. The question is made more puzzling by the fact 

 that nitrogenous manures generally produce but little 

 effect upon leguminous crops. It seems now quite cer- 

 tain that leguminous crops possess in their root tubercles 

 an apparatus capable of bringing the nitrogen of the 

 atmosphere into combination (see p. 10). The special 

 agent residing in these tubercles is a micro-organism 

 derived from the soil. In the case of a poor soil the pre- 

 sence or absence of the necessary organism in the soil 

 may determine its fertility or barrenness for a leguminous 

 plant. It must, however, be recollected that, apart from 

 the tubercles, leguminous plants are nourished in an 

 ordinary way through their root-hairs, and that a deeply- 

 rooted plant, like red clover or lucerne, obtains consider- 

 able food supplies from the subsoil. 



Except in the case of extraordinary rich soils, land 

 loses the power of growing most leguminous plants by 



