THALLOPHYTES 63 



nitrogen for protein manufacture. This is what is often 

 meant when a soil is said to be " impoverished." 



There are at least two general ways by which fresh supplies 

 of available nitrogen are being added to the soil continuously, 

 and the agents in both cases are bacteria. One way is to 

 obtain nitrates (nitrogen in its available form for plants) 

 from the decay of organic matter, and whole series of bac- 

 teria are active in the various steps of this process. There- 

 fore, as bodies of plants and animals die, or leaves and 

 branches fall, or manure is spread on the soil, relays of bac- 

 teria lay hold of this material, and among the products of 

 their activity are the nitrates. 



The other way is to use the free nitrogen of the air in the 

 manufacture of nitrates, and certain bacteria are the agents 

 in this remarkable process. It is an interesting fact that 

 although the air is about four-fifths free nitrogen, and that 

 plants are therefore living submerged in an ocean of nitrogen, 

 they cannot use it in this free form, that is, it is not available. 

 But the soil bacteria referred to are very exceptional among 

 plants in the power to use free nitrogen (soil water contains 

 free nitrogen in solution), probably in the manufacture of 

 their proteins. This work results in more organic material 

 containing nitrogen to become the source of nitrates. These 

 " nitrogen-fixing " bacteria^ as they are called, work conspicu- 

 ously in connection with the clovers, alfalfa, etc., on whose 

 roots they form little swellings (" tubercles," Fig. 49), and 

 from which they obtain carbohydrate food. The clovers in 

 turn consume the bacteria, and if the clover crop is plowed 

 under (" green manuring "), a large amount of nitrogen- 

 containing material is added to the soil, whose nitrogen came 

 from the air by way of the nitrogen-fixing bacteria. 



These two kinds of work partially explain why the soil 

 improves when a field " lies fallow " (is not used), and why it 

 is of advantage to alternate a clover or alfalfa crop with 

 other kinds of crops (" rotation of crops "). In the former 



