54 THE WORK OF THE ROOTS [chap. 



shows that the extra growth thus produced above that 

 on the plot wholly without nitrogen was small, and is 

 closely proportional to the nitrogen supply, when com- 

 pared with the increase of crop produced by a much 

 larger amount of nitrogen. 



Indeed, the Rothamsted experimental results, taken 

 collectively, are only consistent with the supposition that 

 the ordinary farm crop obtains the whole of the nitrogen 

 it requires from the soil ; on all the plots receiving no 

 nitrogen whatever, the crop (leguminous plants alone 

 excepted) has been reduced to a very low level ; indeed, 

 it is only due to the large stock of nitrogen originally 

 present in the soil and to certain recuperative actions 

 at work there that it is maintained at all. The difficulty 

 about this conclusion — that plants utilise only the com- 

 bined nitrogen in the soil — is to understand how many of 

 the rich virgin soils, black with organic matter down to 

 a depth of nine or ten feet, can have accumulated the 

 nitrogen they contain. If the plant contains only the 

 nitrogen it has taken from the ground, there can be no 

 gain of nitrogen when the plant falls back to the ground, 

 however numerous the generations in which such a 

 vegetative cycle may be repeated. Some other agencies 

 than the mere growth and decay of plants must have 

 been at work fixing nitrogen, and these will be discussed 

 later. 



While we are dealing with the nutrition of the plant 

 by means of the root, one other point requires considera- 

 tion : the analysis of the ash of any given plant shows 

 a very similar composition, wherever or on whatever 

 kind of soil the plant has been grown. For example, the 

 ash of wheat straw (see Table V.) is characterised by a 

 high percentage of silica and a low one of lime ; this 

 will be the case whether the wheat is grown on a sandy 

 or a chalky soil, and is in sharp contrast to the ash of 



