PLANT FOOD AND MANURES 125 



nitrates, it follows that the supply of nitrogen in the 

 soil is peculiarly liable to become diminished. This is 

 found to occur in practice, for nitrogen is usually the 

 first item of plant food which becomes deficient in the 

 soil, and most of the efforts of the cultivator, in the way 

 of keeping up the stock of plant food, are directed 

 towards supplying nitrogen. 



Leguminous Plants and Nitrogen. 



It has just been said that plants are unable to use 

 the nitrogen of the air ; that they must have a supply of 

 nitrogen-containing bodies in the soil ; and that nitrifica- 

 tion makes these useful to plants. There is, however, a 

 remarkable exception inasmuch as plants belonging to 

 the pea and bean order (Leguminosa) are able to thrive 

 in soils containing no nitrogen. It has been found that 

 this interesting and important property is due to the 

 presence of great numbers of bacteria (microbes or 

 germs) which inhabit small nodules or swellings to be 

 found on the roots of plants of this order. The bacteria 

 living in these swellings are able to feed on the nitrogen 

 of the air and pass on this nitrogen to the plants in 

 connexion with which they live. They thus enable 

 these plants to use the nitrogen of the air. Now, as 

 nitrogen is the most expensive constituent of plant food, 

 this property, possessed by leguminous plants, of using 

 and building up into their own structures nitrogen from 

 the air, is of great value to the cultivator. He is able 

 to grow crops of beans and peas upon soils which are 

 too poor in nitrogen to produce remunerative crops of 

 other plants. When the bean crop is reaped the roots 



