76 THE farmers' handbook. 



.By means of its roots, however, tlie plant absorbs nitrogen, in the form 

 of either nitrates, nitrites, or ammonium salts dissolved in the water of 

 the soil. 



It appears probable that the nitrogenous material taken up by the plant 

 from the soil by means of its roots enters the plant in the form of nitrates. 

 In the case of leguminous plants, as we shall see later on, the free nitrogen 

 contained in the air imprisoned in the soil can be made use of under certain 

 conditions by the roots of these plants. 



Formation of Nitrates in the Soil. 



It has long been known that the addition to a sterile soil of a quantity of 

 a more fertile one rendered the sterile one fertile; and the operation of top- 

 dressing poor soils in this manner was a common one amongst the- nations 

 of antiquity. 



When the subject was approached in the light of more recent advance in 

 scientific knowledge, it was shown that this increase in fertility was accom- 

 panied by an increase in the quantity of nitrates in the soil. How it was 

 that the addition of so small a proportion of a soil containing nitrates could 

 bring about the large increase of nitrates observed was a problem which 

 remained unsolved until the discovery by Pasteur of organisms capable of 

 inducing fermentation and of producing certain chemical substances as by- 

 products, paved the way for a rational explanation of the process. Pasteur 

 himself surmised that this gain in nitrates was due to bacterial activity. 



(a). — From Nitrogenous Organic Matter. 



In 1877, it was shown by Schloessing and Muntz that the formation of 

 nitrates (nitrification) only took place within certain temperatures, and that 

 it could be entirely stopped by antiseptics — such as chloroform vapour — 

 showing that nitrification was brought about by the action of organisms. 



We know now that quite a number of different organisms take part in 

 the conversion of organic nitrogenous material into nitrates. In the first 

 stage of the process, the nitrogenous matter in vegetable or animal refuse 

 is converted by putrefaction into humus, carbonic acid being evolved and 

 the nitrogen converted into simpler forms such as amides (asparagin, urea) 

 and ammonium carbonate; the familiar odour of stables and dung-heaps 

 being due to the ammonia evolved. Quite a number of bacteria and moulds 

 possess this power of converting albuminoid matter into ammonium salts. 

 In ordinary manure heaps moulds produce this decomposition; in arable 

 soils it is brought about by the action of bacteria (particularly by Bacillus 

 mycoides, which is widely distributed in the surface-soils, air, and water). 

 The further nitrification of ammonium salts is the work of yet other bacteria. 



Warington first showed that there were two distinct stages in the conver- 

 sion of ammonium salts into nitrates, and that nitrites were formed as an 

 intermediate oxidation. Consequently, there must be two distinct organisms 

 involved. 



In 1890 these were isolated by Winogradsky, who found two distinct 

 organisms capable of converting ammonium salts into nitrites — namely, 

 Nitrosomonas europea, occurring in soils of the Old World, and a nitro- 

 coccus, which he found in American and Australian soils. 



The further conversion of nitrites into nitrates is the work of another 

 organism, of the genus nitro-bacter. 



