NITROGEN : WHY AND WHERE CROPS MUST GET IT. 199 



The conditions of Prof. Atwater's experiments were such that 

 there would seem to be no source from which this nitrogen could 

 have been derived other than the atmosphere. But still he does 

 not venture to claim for his results that they necessarily give sup- 

 port to the doctrine that free nitrogen is assimilable. We are, 

 then, naturally led to consider the atmosphere as a source of com- 

 bined nitrogen. There is alwa^'s a minute quantity of ammonia 

 there, in a gaseous form, and a minute quantity of nitrate. Ac- 

 cording to the theory of an eminent French agricultural chemist, 

 the millions of pounds of nitrates carried into the sea every year 

 by all the great rivers, which is almost entirely the product of the 

 wastage of the human and other animal inhabitants of the land — 

 are converted, through the agency of the vegetable and animal life 

 of the sea, into ammonia. This is given off into the air above the 

 water, and wafted b}' the winds over the land. Thus, if this not 

 unreasonable theorj' be true, there is provision for the constant 

 replenishment of the supply of combined nitrogen in the atmosphere. 



Although the quantity appears to be small — only one part in 

 530,000 parts of air — yet in the laj-er of air, four miles high, over 

 each acre of land there are about 140 pounds of this plant food, 

 very precious to the crops if they can get hold of it. When we 

 consider that the air is in constant motion, not only in horizontal 

 directions but also upwards and downwards, it is not difficult to 

 understand how a large portion of this ammonia may be brought 

 in contact with vegetation and the soil, during the growing season. 

 But, when it comes within reach, how can vegetation take posses- 

 sion of it ? 



First. To some extent — no one can say how much — through 

 the foliage. It has been proven by experiment that a plant can 

 supply itself with nitrogen in this way, if its leaves are exposed to 

 air containing ammonia gas. It is very natural that clover, with 

 its great abundance of foliage, should be supposed to have the 

 power to help itself to nitrogen compounds from the air more lib- 

 erally than wheat with its much smaller leaf surface ; and this 

 v.iew appears to be supported by the well known fact that clover is 

 a good crop to come before wheat, which needs nitrogen in the 

 soil. But such very few experiments as have been performed, to 

 test this supposed special feeding power of clover on nitrogen 

 compound's in the air, do not support this view ; and the beneficial 

 effect of clover preceding wheat can be accounted for, at least to a 



