ASSIMILATION OF NITROGEN 



67 



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 



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

 soil rich in lime. 0, no fertilizer; NOs, nitrate added; NHz, ammonium salts added. (,After 

 P. Wagner.) 



for many different kinds of soils whereas ammonia-fertiUzer 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.^ 

 The presence of calcium carbonate prevents the accumulation of free acid in 

 both cases. 



Such experiments with natural soils cannot answer the question regarding 

 the direct assimilation of ammonia. Sterilized soils must be used, in which the 



1 This is more fully considered in Chapter IV. 



