July 2, 1900.] 



KNOWLEDGE. 



161 



the sources whence the vegetable world obtains its 

 Nitrogen. 



It has already been noticed that protoplasm, the 

 ■■ living substance " of all organic beings, is composed of 

 the complex Niti'ogen-coutainiug bodies which we 

 call " proteids." Nitrogen, then, is a constituent 

 of the protoplasm itself, to say notliiug of other sub- 

 stances found in plants which also contain it; hence we 

 must regard it as a food element of primary importance. 

 In considering the assimilation of Carbon by plants, 

 we saw that an inijjortant r6le played by vegetable life 

 in the world's economy is the formation of complex 

 Carbon compounds from Carbon dioxide, in other words 

 the raising of Carbon from an inorganic to an organic 

 state in which alone the animal is aljlc to assimilate 

 it. With regard to Nitrogen we find that plants play 

 an equally important part. The animal c;<n only assimi 

 lat€ Nitrogen when it is in the form of complex organic 

 substances such as proteids. Plants make use of simpler 

 organic compounds, inorganic substances such iis Nitrates 

 or even (the lower forms of vegetable life) of free Nitro- 

 gen itself. By the plant, the element or simple com 

 pounds containing it are built up into organic forms such 

 as can convey Nitrogen into the animal system. The 

 waste products of the animal body — living or dead — 

 upon their decomposition, yield up the Nitrogen iu 

 simpler forms which are again ready to be assimilated 

 by the plant. 



A few plants — such, for example, as our British insect- 

 eating Sundews and Butt«rworts — obtain part of then 

 Nitrogen from the complex proteids and peptones of the 

 bodies of their prey. In this respect, these and other 

 insectivorous plants live after the manner of the animal, 

 that is, they do not build up complex nitrogenous 

 substances from simpler compounds, but make use of 

 those which have been produced by other plants. They, 

 however, form a very small proportion of the vege- 

 table kingdom. Most higher plants, by which we mean 

 those bearing green leaves and conspicuous flowers, 

 obtain their Nitrogen from nitrates and compounds of 

 Ammonia present in the soil, and of these the majority 

 grow best when they are supplied with nitrates. It 

 has been pointed out above that nitrates are very readily 

 washed out of the soil by percolating water. In some 

 experiments at Rothamsted it was found that in each 

 of the four yeai's between 1877 and 1881 an average of 

 41.81 lbs. of Nitrogen** per acre in the form of nitrates 

 was washed out of the soil and escaped with the 

 drainage water. If we consider that this amount of 

 nitrogen (42 lbs. per acre per annum) would be sufficient 

 to supply an ordinary crop of wheat or barley we shall 

 realise how serious is the loss of nitrate from the soil 

 which occurs with every fall of rain. It is clear, on 

 the other hand, that water-plants growing partially or 

 entirely immersed in natural waters — the accumulations 

 of water which has drained over or through the land 

 — must be well supplied with nitrates. 



Since the loss of nitrate from the soil in any but a 

 very dry climate is continuous, we must enquire by 

 what means Nature meets the difficulty of supplying 

 Nitrogen to that large portion of the vegetable kingdom 

 which seeks it in this particular form. It ha,s long been 

 known that the passage of a lightning flash through the 

 atmosphere is accompanied by a combination of Nitiogen 

 and Oxygen of the air and a consequent production 

 of small quantities — very small quantities — of nitrates. 

 These are washed down into the soil and, until they 



•* Equal to 254 lbs. of nitrate uf eoila (KaNOJ 



are washed out again, are available to plants. The 

 amount thus produced is, however, infinitesimal com- 

 pared witli the requirements of the world's green vege- 

 tation. Atmospheric electricity is, however, responsible 

 for a much hu'ger production of oxidised Nitrogen 

 (nitrate) than results from the spasmodic discharges of 

 thunderstorms. Wherever earth and air are in contact 

 I he air is at a slightly diflerent electrical potential from 

 the earth, and in consequence there is a continuous silent 

 electric discharge between them. This discharge is 

 accompanied by the chemical union of Nitrogen and 

 Oxygen and the production of Nitrates. The amount 

 formed in this way at any one place is of course ex- 

 ceedingly small, but the process occurs continuously over 

 wide areas, and it is therefore not difiicult to belie ire 

 with M. Berthelot that an important contribution is 

 thus made to the nitrate-demands of the vegetable world. 



Probably, however, the most important of the natural 

 agents iu the formation of Nitrates are the numerous but 

 little known microbes which inhabit the upper 9 to IS 

 inches of clay soils in prodigious numbers. It is pro- 

 bable that the power of oxidising atmospheric Nitrogen 

 is not rare among these lowly organisms, but at 

 present we know of only one which has actually 

 been proved to possess it.ft The organism, which 

 resides in the roots of Leguminous and some other 

 plants, and enables them to make use of the free Nitrogen 

 of the atmosphere, will be noticed when we ai"e con- 

 sidering the functions of roots. Others of these soil 

 bacteria are concerned in the production of Nitrates, 

 not from the free element, but from those vast stores of 

 combined Nitrogen which exist in the roil in the form 

 of humus. It has been believed that the higher plants 

 arc unable to absorb and assimilate the complex organic 

 Carbon — and Nitrogen — containing substances which are 

 present in humus. As to this vexed question we ar? 

 unable to speak definitely. Certain it is, however, that 

 a vast ai'my of minute workers are constantly employed 

 in transfering cei'tain of the products of the decom- 

 position of humus into Nitrates. These bacterial 

 labourers are divided into at least two classes. There 

 are in the first place those which seize upon the com- 

 pounds of Ammonia which result from the decay of 

 humus, and oxidise them, producing in this way salts 

 called Nitrites which, as their name implies, are related 

 to Nitrates, differing from them in containing less Oxy- 

 gen. These receive attention from another section of 

 the bacterial inhabitants of the soil, which oxidise them 

 further, the final product being Nitrates which contain 

 Nitrogen in the condition required by the majority of 

 the higher classes of the vegetable world. 



We have noticed the principal known methods by 

 which Nature jsrovides for the Nitrate wants of th-^ 

 vegetable kingdom. Under natural conditions thesj 

 agencies — and perhaps others also, as yet undiscovered- 

 are sufiicient to replenish the soil with oxidised Nitrogeu 

 — a replenishment which must be continuous to be 

 efi'ective owing to the rapidity with which Nitrates are 

 removed from the soil by rain-water. But under the 

 artificial conditions of cultivation the equilibrium of 

 these relations is upset. In removing from the land 

 his annual crop, the farmer carries off the greater part 

 of the year's supply of potential humus whence th^ 

 soil looks to be provided with Nitrates — by the action 

 of the soil-bacteria — for the coming season. Hence 

 arises the necessity for the application of manures con- 

 taining Nitrates. The supply of these manures becomes 



1t "vVinograd6k,r. Compies Reyidus. Ib'Jl, T. CWill., p. 353. 



