STUDIES OF SPORE-PLANTS 287 



pounds readily available for absorption by roots of plants. 

 Nitrate of soda from Chile was probably formed by bacterial 

 action which united the nitrogen from organic matter (sea- 

 weed, or guano) with sodium derived from sea-water. Simi- 

 lar bacterial changes take place in all manures used on farms. 

 The odor of ammonia around stables is evidence that the 

 decomposing bacteria are at work, and that valuable nitro- 

 gen in the ammonia is escaping into the air, because other 

 kinds of bacteria are not working properly and fixing the 

 nitrogen in nitrites and nitrates. Agricultural books give 

 rules for handling manures so as to avoid such wasteful 

 decomposition ; that is, they teach how to cause both kinds of 

 bacteria to work together so that the products of decompo- 

 sition will be built into available crude plant foods with little 

 loss of nitrogen (e.g., in ammonia) to the air. This is a 

 problem of such great financial value that every farmer 

 should study it carefully in the special books on soil fer- 

 tilizers. 



In still another way certain bacteria are useful in soils. 

 The free nitrogen in the air is not usable by animals and 

 plants, with the sole exception of certain bacteria known as 

 the nitrogen-fixing bacteria, which live in the root-tubercles 

 on certain plants ( 152). These bacteria unite the nitrogen 

 absorbed from the air into nitrogen compounds which can 

 be used by the plants that have the tubercles. In recent 

 years, the importance of this method of increasing the amount 

 of nitrogen fertilizers in the soil has been recognized, and 

 now every scientific farmer gives special attention to raising 

 clovers, alfalfa, peas, vetches and similar plants whose roots 

 are favorable to the nitrogen-fixing bacteria. By growing 

 such plants soils can be much improved in fertility at an 

 expense much lower than by use of nitrate of soda and other 

 commercial fertilizers. The only other known way of pre- 

 paring nitrogen from the air for use by plants is an expen- 

 sive method of combining the nitrogen with sodium by 



