496 MISCELLANEOUS FARM SUBJECTS 



ous fertilizers can be avoided by the growing of green-manure crops, 

 especially legumes. 



There are two principal ways in which nitrogen is naturally 

 added to the soil. The first and much more important way is by 

 means of legumes. This takes place only when the roots of the leg- 

 uminous plant bear bacterial nodules or tubercles. The amount of 

 nitrogen that any particular legume can add is directly proportional 

 to the number and size of the tubercles on its roots. Little is known 

 regarding the relative abilities of different legumes to fix atmos- 

 pheric nitrogen, though they differ widely. It has sometimes been 

 assumed that those are most efficient whose herbage is richest in this 

 substance. This is at least questionable, as these same species may 

 be able to utilize more of the soil nitrogen than others. 



The second way in which nitrogen is added to the soil is through 

 the activity of certain soil bacteria. These, especially if provided 

 with decaying vegetable or animal matter, are able to use the nitro- 

 gen of the air, forming nitrogen compounds. The amount of the 

 nitrogen thus fixed depends on a number of factors. A supply of 

 decaying material must be available as food for the bacteria, the soil 

 must be well aerated and must not be sour, and there must be lime 

 or other substance in the soil with which the nitrogen can be com- 

 bined. With these conditions fulfilled the bacteria are most active 

 when the soil is warm that is, in summer. It is now believed that 

 the principal benefit achieved by summer fallowing is the increase in 

 the amount of nitrogen by soil bacteria. It may here be pointed out 

 that in experiments green-manure crops have yielded the best re- 

 sults when turned under shallow. This result is perhaps due to the 

 fact that the best conditions are thus provided for the activity of the 

 soil bacteria. 



Loss of Soil Nitrogen. It is important to know that under cer- 

 tain conditions nitrogen may escape from the soil as a gas. This 

 result is brought about principally by certain organisms, called 

 denitrifying bacteria, that break up nitrogen compounds and permit 

 the nitrogen to escape, usually in the form of ammonia. Little is 

 yet known of this process, but it is believed to occur under various 

 different conditions. There are probably different bacteria which 

 under different conditions cause these losses. It is known, however, 

 that large loss is likely to occur when there is present abundant de- 

 caying matter, high temperature, and lack of air. Such losses occur 

 particularly in rich water-logged lands and in manure heaps. For 

 these reasons it is not good practice to endeavor to obtain a large 

 store of nitrogen in the soil beyond the needs of the crops. For the 

 same reason green-manure crops should not be used on lands that 

 are decidedly wet. 



Composition of Green-Manure Crops. The essential fertilizing 

 substances added to the soil as manures are nitrogen, phosphorus, 

 and potash. The accompanying table gives the amount of these 

 substances in both the tops and roots of several legumes, as shown 

 by a number of analyses of each. 



