8 INTRODUCTORY [chap. 



constitute more than 95 per cent, of the dry matter of 

 the plant, are derived by the plant from the atmosphere, 

 and that if the plant be supplied with the 2 per cent, 

 or so of mineral constituents which are found in its ash, 

 it will then draw upon the atmosphere for all the other 

 materials the crop ultimately contains. 



Coming at a time when much intellectual interest 

 was directed towards agriculture, and backed by his great 

 scientific reputation and his commanding personality, 

 Liebig's views aroused instant and general attention ; 

 they became the foundation both of practical experi- 

 ment and scientific research, and were the starting-point 

 of a controversy the echoes of which are but now dying 

 down. In several respects Liebig's views required 

 modification ; he seemed to consider all the ash con- 

 stituents were of equal importance, and even that one 

 base like soda could replace any similar one like potash ; 

 he regarded the composition of the plant's ash as deter- 

 mining its proper fertiliser — a point which will be con- 

 sidered later ; and he misapprehended the part played 

 by nitrogen compounds in manuring. At that time, 

 owing to imperfections in the methods of analysis, very 

 exaggerated ideas were current as to the amount of 

 ammonia naturally present in the atmosphere : the 

 rain was believed to bring down 30 or 40 lb. per acre 

 per annum of combined nitrogen instead of the 3 or 4 lb., 

 which we now know to be contributed to the soil, and 

 to this source Liebig was disposed to look for the 

 nitrogen in the plant. He stated that manures contain- 

 ing nitrogen certainly stimulated growth, because they 

 fermented and increased the proportion of ammonia in 

 the air round the plant ; but in the main nitrogenous 

 manures were unnecessary, for full crops could be grown 

 if only the constituents removed in the ash were annually 

 returned to the soil. 



