AND OF PRODUCTS OF METABOLISM. 283 



tions of season, and other causes, are practically elimi- 

 nated. 



From this table we see that the average yield from 

 land left entirely without manure was 16.5 bushels of 



u 



grain. On adding various manures, all of which con- 

 tained about the same weight of combined nitrogen, the 

 yield of grain was doubled, or, in the case of rape-cake 

 manure, increased to two and a half times the amount. 

 The addition of various inorganic salts to the soil also 

 had a favourable effect, though to nothing like the same 

 degree as that of the nitrogenous manures. Thus we see 

 that, when no nitrogenous manure whatever is present, 

 the addition of superphosphates increases the yield by 

 32 per cent. ; of potassium, sodium and magnesium sul- 

 phates by 9 per cent. ; and of both superphosphates and 

 these sulphates, by 36 per cent. When the nitrogen is 

 added as ammonium salts or nitrates, then combinations 

 of nitrogenous and mineral manures give a very much 

 better yield than the nitrogenous manure alone, but 

 when it is added as rape cake, the growth of the crop 

 has already been so much increased that the further 

 addition of mineral salts effects but little. Yet even the 

 highest of the numbers in this table does not represent 

 the maximum amount of growth of which the barley is 

 capable, for a soil treated with farm-house manure, and 

 no additional mineral salts, yielded on an average 48.6 

 bushels per acre. In all these experiments the yield of 

 straw was increased in more or less similar proportions 

 to the yield of grain, and hence we may conclude that 

 the growth of a plant in normal soil can be very nearly 

 trebled if only favourable enough conditions are 

 afforded it. 



