28o THEORIES OF FERTILISER ACTION [chap. 



apprehended from their respective compositions, may 

 be correlated with the habits of growth of the two 

 plants : wheat is sown in the autumn after but a slight 

 preparation of the ground, nitrification is thus restricted, 

 especially as the chief development of the plant takes 

 place in the winter and early spring before the soil has 

 warmed up ; as a consequence, the crop is particularly 

 responsive to an external supply of some active form of 

 nitrogen. On the other hand, the wheat plant possesses 

 a very extensive root system and a long period of 

 growth, hence it is specially well fitted to obtain 

 whatever mineral constituents may be available in the 

 soil. In ordinary farming the only fertiliser used for 

 the wheat crop will be a spring top-dressing of i cwt. 

 per acre or so of nitrate of soda, or an equivalent 

 amount of sulphate of ammonia or soot. 



Barley is a spring-sown crop, for which the soil 

 generally receives a more thorough cultivation ; in 

 consequence the nitrates produced with the rising 

 temperature will be sufficient for the needs of the 

 crop ; often more than enough when the barley follows 

 a root crop that has been liberally manured and perhaps 

 consumed on the ground by sheep. But being shallow- 

 rooted, and having only a short growing season, the 

 barley plant experiences a difficulty in satisfying its 

 requirements for phosphoric acid, hence the necessary 

 fertiliser consists, in the main, of this constituent. Only 

 on sandy and gravelly soils, exceptionally deficient in 

 potash and subject to drought, is any benefit derived 

 from a supply of potash to the barley crop. 



A still more noteworthy example is provided by the 

 Swede turnip crop ; an analysis of a representative 

 yield would show it to withdraw from the soil about 

 150 lb. per acre of nitrogen, 30 lb. of phosphoric acid, 

 and 120 lb. of potash. Yet the ordinary fertiliser for 



