ASSIMILATION OF NITROGEN 



71 



contained 2 g. of nitrogen as nitrates, and still others each contained about 2 g. 

 of nitrogen as ammonium salts. In a second series calcareous marl was added 

 throughout, in addition to the fertilizers mentioned above. The results of this 

 experiment are brought together in the following table (see also Fig. 42). 



Fertilizer 



Dry Yield 



Increase in Yield, 

 Compared with Cul- 

 ture without 

 Nitrogen 



Without lime 



With lime 



grams 



Without addition of nitrogen .... 6.3 



2 g. of nitrogen as nitrate 94-4 



2 g. of nitrogen as ammonium salt. 29.4 



Without addition of nitrogen 9.6 



\ 2 g. of nitrogen as nitrate 92 .0 



I 2 g. of nitrogen as ammonium salt . . 86 . 7 



grams 



88.1 

 23.1 



82.4 



77.1 



Thus, ammonium salts have but little value as fertilizers for soils poor in lime. 

 But soils rich in lime show almost as good yields with ammonium salts as with 

 nitrates (Fig. 43). These experiments show that nitrate fertilizer is suitable 



Pig. 43. — Comparison of the effect of nitrate and ammonium salts on growth of plants in 

 soil rich in lime. O, no fertilizer; NO3, nitrate added; NHi, ammonium salts added. (After 

 P. Wagner.) 



for many different kinds of soils whereas ammonia fertilizer is suitable for only 

 a limited number. There are two reasons for this: first, if we suppose that 

 the ammonia is all oxidized to nitric acid before assimilation, then free nitric 

 acid may be produced in the soil that lacks calcium (as in the first series of ex- 

 periments just described), and this acid retards the growth of the plants as well 

 as the nitrification process. Secondly, if we suppose that a part of the ammonia 

 is assimilated unchanged, then free acid may again accumulate in the soil lack- 

 ing calcium; for ammonium salts are physiologically acid, their basic radicals 

 being absorbed by the plants to a greater extent than are their acid radicals. 1 



1 This is more fully considered in Chapter IV. 



