238 JOSEPH HENRY GILBERT 



the view that each crop requires certain mineral elements from 

 the soil, and that crops will not flourish where the appropriate 

 elements are lacking. Every soil contains some element in 

 the minimum. Whatever element this minimum may be it 

 determines the abundance and continuity of the crop. The 

 only fertiliser which acts favourably is that which supplies a 

 deficiency of one or more of the food elements in the soil. 

 The atmosphere, according to Liebig, supplies in sufficient 

 quantity both the carbon and nitrogen required by crops, and 

 the function of manure is to supply the ash constituents of 

 the soil. The exhaustion of soils is to be ascribed to their de- 

 creased content of mineral ingredients rather than to decrease 

 in nitrogen. 



When careful study of the composition of the atmosphere 

 proved that the amount of ammonia brought down to the earth 

 by rain scarcely exceeds a few pounds per acre annually, Liebig 

 maintained that plants are capable of directly absorbing ammo- 

 nia by means of their leaves. He pointed out that the beneficial 

 effects of nitrogenous manures are most apparent in the case of 

 cereal crops with a comparatively short vegetation period, and 

 least apparent in the case of leafy crops with a long vegetation 

 period. The long vegetation period of crops like clover allowed 

 time for the utilisation of the ammonia of the air and no artificial 

 supply was necessary. On the other hand, crops with a short 

 vegetation period had a limited time for accumulating ammonia 

 from the air, and responded readily to applications of nitrogenous 

 manures. 



Gilbert, early in his work at Rothamsted, noticed that the 

 results of his field experiments were at variance with this 

 " mineral theory," as it was called, of Liebig, and soon found 

 himself involved in a controversy with the great German 

 chemist which was not always free from bitterness. He found 

 that the nitrogen compounds of the atmosphere were sufficient 

 only for a very meagre vegetation. Cereals treated with ammo- 

 nium salts and other nitrogenous manures showed a far greater 

 increase of produce than when phosphates, potash or other ash 

 constituents only were supplied. "As more nitrogen was assi- 

 milated a greater amount of the fixed bases were found in 



