146 SOIL CONDITIONS AND PLANT GROWTH 



amount of nitrogen present as free or combined ammonia is 

 about "oooi per cent. {i.e. i part per 1,000,000) in arable soils 

 not rich in organic matter,^ and some ten times this quantity 

 in pasture or heavily dunged arable soils ; much larger quan- 

 tities occasionally occur in special cases, as in heated soil, but 

 abnormal growth effects are then produced.^ There is con- 

 siderable variation in the amount of nitrogen present as 

 nitrate ; rich garden soils may contain 60 or more parts per 

 1,000,000 ("006 per cent.), arable soils 2 to 20 parts (-0002 to 

 •002 per cent), pasture soils rather less and woodland soils 

 still less.^ No soil constituent fluctuates more in amount 

 than nitrates ; plants and rain rapidly remove them and 

 bacterial action rapidly forms them. The producing agencies 

 are active in spring, and work throughout summer and autumn, 

 while the removal agencies are active in summer and winter. 

 Thus the amount of nitrate actually present in arable soil is 

 usually highest in spring, falls in summer, often rises some- 

 what in autumn, and falls again in winter as shown in Fig. 18 

 (p. 145) (see also Table LXVL, p. 249).* 



The nitrate and ammonia together rarely account for more 

 than I per cent, of the nitrogen in the soil : the remainder is 

 in more complex forms. The part soluble in alkalis ^ prob- 

 ably includes the same type of compound as was obtained 

 by Maillard in the condensation of sugars and amino acids 

 (p. 142). The part insoluble in alkalis may be of protein 

 nature. On boiling with strong HCl soluble nitrogen com- 

 pounds are formed, and it has been supposed^ that the re- 

 action qualitatively and quantitatively resembles protein 

 hydrolysis. This may be true, but exact proof is difficult. 



1 Andr^ (Compt. Rend., 1903, 136, 820) obtained higher results in early spring 

 which he attributed to the cessation of nitrification, but not of ammonification, 

 during winter. 



2 See E. J. Russell and F. R. Petherbridge, jfourn. Ag. Sci., 1913, 5, 248. 



* It is sometimes stated that woodland soils do not contain nitrates and are , 

 unsuited for nitrification, but Weis (301) has shown this to be incorrect. 



*For American data see R. Stewart and J. E. Greaves, Centr. Bakt. Par., 

 1917. 34. "5- 



' After preliminary treatment with dilute acid (see p. 139). 



'E.g. Jodidi, Iowa Research Bull., No. i, 191 1, 



