130 ECOLOGICAL RELATIONS 



portant of all the elements is the nitrogen which is essential for 

 all plants and animals. Nitrogen is very abnndant in the air, 

 but the plants cannot make use of it in this free form. However, 

 the air penetrates the soil and its nitrogen is seized upon by 

 certain bacteria and fungi and fixed in the form of nitrates which 

 can be used by the plants. (Chapter X.) This gives us one 

 good reason for keeping the soil in which we are growing plants 

 in loose condition. Nitrogen is also carried down and into the 

 soil by rainfall. The bacteria found in the tubercles of the 

 leguminous plants are the most important organisms in this 

 work. Nitrogen is also obtained from decaying organic mater- 

 ials and thi^ is one reason that humus is so important in agri- 

 cultural lands. 



Use of Fertilizers. — If the soil does not contain these food 

 elements in the proper proportions or if any of them are unavail- 

 able, the farmer endeavors to overcome its deficiencies by the 

 application of fertilizers, such as stable manure, bone meal, ni- 

 trate of soda, rock phosphate, potash, lime, etc., or by the grow- 

 ing of the leguminous plants or by both. In order to do this to 

 the best advantage it is important that he understand the crop 

 plants to be grown and their food requirements. The elements 

 most likely to be lacking in farm soils are nitrogen (N), phos- 

 phorus (P), potash (K), and sometimes calcium (Ca). 



Temperature. — This is another factor of plant growth which 

 we very readily recognize and we know that certain fruits such 

 as oranges and bananas grow only in tropical and subtropical 

 countries, and that many other plants are limited in their 

 range by the temperature. In traveling across the country we 

 easily see that forest and prairie sections can be readily divided 

 into smaller areas, such as the pine forest, oak forest, beech 

 forest, etc. We also recognize great belts or areas of agricultural 

 crops, such as the corn belt, cotton belt, wheat belt^ peach belt, 

 etc. On the boundaries of these belts we frequently find these 



