THE ROOT 103 



elements necessary to the plant. This remarkable 

 property of the soil is of great importance in the 

 economy of nature. Substances necessary to the 

 plant, i.e. those that exist in the soil in very limited 

 quantities, are thereby prevented from being washed 

 away by the rain, and are kept in the soil, which only 

 gradually gives them up to the water circulating between 

 its hard particles. 



Nitric acid (as saltpetre) forms an exception to this 

 rule, as it is rather easily washed from the soil ; yet, 

 as we have seen, it supplies the plant with nitrogen, 

 the most important of nutrient elements. The investi- 

 gations of scientific agriculturists are drawing the 

 attention of farmers more and more urgently to the 

 necessity of utilising this substance as fully as possible 

 by means of cultivated plants. It is in this very rela- 

 tion of the plant to the nitric acid in the soil that an 

 explanation has been sought for the part played by 

 leguminous plants in the rotation of crops. Until quite 

 lately their role seemed very mysterious. Leguminous 

 plants contain more nitrogen than cereals, and yet 

 nitrogenous manures affect them less than cereals. 

 Moreover, when leguminous plants are cultivated in 

 alternation with cereals in an unmanured soil, cereal 

 crops are gathered as heavy as any succeeding the bare 

 fallow. This seemed to show that leguminous plants, 

 instead of exhausting the soil, even enrich it, an opinion 

 which would have been strictly justified could it only 

 have been proved that leguminous plants absorb 

 nitrogen from the air instead of from the soil. Yet 

 this was for a long time contradicted by exact experi- 

 ments. 



The only other possible explanation of the relation 

 of the leguminous plants to nitrogen lay in the fact 

 that, developing a network of roots which sank very 

 deeply into the soil, and growing in the soil for a longer 

 time, leguminous plants absorbed more completely the 



