In pastures flocks are feeding all the summer and furnish us with 

 nitrogen in the form of milk, cheese and meat ; yet the soil of these 

 natural pastures still contains quantities of nitrogen greater than ary 

 to be found in ploughed and copiously manured land. Crops of all 

 kinds remove from the soil more nitrogen than the manure supplies. 



It is evident that there must be some recuperating agency, or the 

 stock of nitrogen would long ago have disappeared from the old 

 countries. Experiments show that soil gains nitrogen if it is 

 allowed to remain undisturbed or in natural conditions. 



Whence comes this nitrogen ? It can only come in the last place 

 from the inexhaustible reservoir of the atmosphere. 



The great German chemist Liebig, who contributed so much to our 

 knowledge of the chemistry of plants, supposed that they obtain 

 their supply of nitrogen directly and in its elemental form irom the 

 air. but about the same time a French chemist, Boussingault, 

 showed by experiments conducted during some twenty years that 

 plants are quite unable to do this, but must obtain it from some 

 mineral in tne soil. The discovery of this fact was a step of great 

 importance in plant physiology. 



How, then, is this nitrogen fixed ? In what form does it 

 occur in the soil? And how is it made available for the plant? 

 In the farmer's manure heap are found countless minute 

 organisms, fungi or moulds and bacteria, which are feeding 

 on the vegetable substance which makes up the manure. By the 

 action of these a fermentation is set up ; in other words, the manure 

 becomes rotted. It owes its warmth to this chemical action, and 

 as we know the gas ammonia is given off in abundance. This 

 ammonia is one of the results of the decomposition of the vegetable 

 matter, and is wholly the work of these minute bacteria, and all 

 nitrogenous manure except nitrate of soda, all such substances 

 as guano, dung, wool-waste, bone-meal, sulphate of ammonia, 

 have to undergo the action of bacteria before the products 

 are of any use to the plants. The result of the action, then, 

 is the formation of ammonia. But in this form the nitrogen 

 is still useless. The gas enters the soil, and is here converted 

 by chemical combination into ammonium carbonate. But still 

 further changes have to take place before it is of use to the plant. 

 Now scientific agriculturists are generally agreed that the most im- 

 portant food materials of the plant are nitrates ; that is, nitric acid 

 in combination with mineral bases, and for the moment we may speak 

 of nitric acid as the important thing. "If the soil were utterly 

 devoid of this substance it would be incapable of yielding the barest 

 pretence of a crop of any sort, even if the soil be in other respects of 

 the most superfine texture, however favourably it might be situated, 

 however well drained, tilled and supplied with the purely mineral 

 ingredients of plant food."t Yet nitric acid occurs in only very 

 minute quantities in the soil. 



tFrankland, P. F., "Our Secret Friends and Foes." (Romance of Science 

 Series, S.P.C.K.). 



