202 Bureau of Farmers' [nstitutes. 



nitrogen of the air i- not large. In general, those plants which 

 are called leguminous, such as the bean, pea, clover, alfalfa, etc., 

 can take uncombined nitrogen from the air. 



(2) Nitrogen of Ammonia Used by Plants. — The leaves 

 of some plants have the power of absorbing ammonia directly from 

 the air and obtain nitrogen in this way, but only in very small 

 quantities. Some plants obtain nitrogen from ammonium salts 

 through the soil, but in general, the compounds of ammonia arc 

 changed into nitrates in the soil before being used by plants. 



(3) Nitrogen of Nitrates Used by Plants. — The largest 

 part of the nitrogen obtained by most plants is taken up by their 

 roots from the soil in the form of nitrates; that is, nitric acid com- 

 bined with some metal, as sodium or potassium. As already stated, 

 most of the nitrates used by plants are formed by changing into 

 nitrates ammonia compounds and organic substances in the soil by 

 the process called nitrification. Hence, nitrogen, in the form of 

 nil rules, is the most available form for most plants; that is, it can 

 be most readily taken up and used by plants. 



(c) Relation to Fertilizers. — Experiments have shown that 

 nitrogen is essential to the growth of plants ; that the quantities of 

 nitrogen available as plant food are very small; that nitrogen is 

 one of the first elements in the soil to be used up; that, of all fer- 

 tilizing elements, nitrogen is and always has been the most expen- 

 sive. 



(d) Specific Functions in Plant Growth. — The influence of 

 nitrogen in its various forms upon plant growth is shown by at 

 least three striking effects. 



1. The growth of stems and leaves is greatly promoted, while 

 that of buds and flowers is retarded. Ordinarily, most plants, 

 at a certain period of growth, cease to produce new branches and 

 foliage, or to increase those already formed, and commence to pro- 

 duce flowers and fruits, whereby the species may be perpetuated. 

 If a plant is provided with as much available nitrogen as it can 

 use just at the time it begins to flower, the formation of flowers 

 may be checked while the activity of growth is transferred back 

 to and renewed in stems and leaves, which take on a new vigor and 

 multiply with remarkable luxuriance. Should flowers be pro- 

 duced under these circumstances, they arc sterile and produce no 



seed. 



2. The next effect of nitrogen upon plants is to deepen the 

 color of the foliage, which is a sign oH increased vegetative activity 

 and health. 



