238 



KNO^VLEDGE. 



} 



[Deckmber 1, 1893. 



the roots — it has to be fixed in some way— before the plant 

 can feed upon it. Recent researches seem to have 

 proved that on leguminous or pod-bearing plants tliere 

 lives a class of bacteria which have the power of feeding 

 directly upon the free nitrogen of the air. They " fix " 

 the nitrogen, which afterwards becomes available for plant 

 food. Perhaps this only happens after the death of the 

 bacteria, when their substance has undergone decomposition 

 and the nitrogen compounds have been converted into a 

 soluble form so that they can enter the sap in the same 

 way as the rest of the food derived by plants from the 

 soil. 



The soluble form in which it appears that nitrogen is 

 mostly taken up by the i-oots of plants is known as the nitrate 

 " form " or state of combination, in which nitrogen is 

 combined with oxygen as it is in nitre and in nitric acid. 

 Besides bacteria associated with leguminous and possibly 

 other orders of plants, there is another agency which 

 brings the nitrogen gas of the atmosphere into combina- 

 tion with oxygen ; this agent is electricity. Electric 

 discharges in the atmosphere cause the combination of 

 relatively small quantities of nitrogen, and the soluble 

 oxides of nitrogen thus formed are carried down in 

 solution by the rain, thus adding to the quantities of 

 nitrates in the soil. As far as it is at present known, 

 these two agencies, electricity and bacterial life, are the 

 only can-iers of nitrogen from the air to the soil. 



Most of the nitrogen at any moment present in the 

 world's soil is, however, derived from the substance of tlie 

 preceding generations of plants. The nitrogen required to 

 be supplied to the world's soil for the crop of any one year 

 is only the difference between what is abstracted from the 

 soil by the plant crop and what is restored to the soil by the 

 death and decay in the vegetable and the animal kingdom. 



The difference between the two amounts may be 

 relatively small for the whole of the earth's surface, and 

 might perhaps be 7iil if large quantities of nitrogen as 

 nitrate were not being constantly carried into the sea by 

 rivers. 



The nitrogen in vegetable and animal substances is 

 combined with carbon, and nitrogen in this state of com- 

 bination may be termed oiyanic nitrotjen. Nitrogen when 

 in this form is not directly available as a food for plants. 

 By the process termed decay in the case of plants, and 

 putrefaction or decomposition in the case of animals, 

 the nitrogen is set free from its combination with 

 carbon, and ammonia or a compound of ammonia is 

 produced. In ammonia, nitrogen is in combination with 

 hydrogen ; and nitrogen, in this state of combination, we 

 will call iimmonidcal nitrogen. Free ammonia gas, formed 

 in the decomposition of materials containing organic 

 nitrogen, is ultimately brought into the soil in a state 

 of solution by means of rain or dew, owing to the fact 

 that ammonia gas is extremely soluble in water. Much 

 of the ammonia produced by the decomposition of organic 

 matter meets at once either with water or with some 

 material with which it can combine, and thus from 

 the first is retained in the soil. Plants are able to feed 

 directly upon ammoniacal nitrogen ; but in the greater part 

 of the nitrogenous food of plants the nitrogen is in com- 

 bination with oxygen, i.e., in what we have termed the 

 "nitrate" condition. The decomposition of the organic 

 matter of the soil (which is commonly called Inmms) is 

 effected through the agency of the nitrifi/in;/ Ijacterin. 

 They first spht up the organic matter into water, carbonic 

 acid, and ammonia, and then further assist the oxidation 

 of ammonia to nitric acid. The conditions favourable to 

 nitrification are that the oxygen of the air should have 

 free access, that the soil should be sufficiently moist, but 



not water-logged, and that the temperature should be fairly 

 high. In the presence of an alkali, or alkaline earth in the 

 soil, the nitric acid forms a salt (termed a nitrate). Where 

 lime or carbonate of lime is present, soluble calcium nitrate 

 is produced, and in this form plants obtain much of their 

 nitrogen. Where potash is present, nitre or saltpetre is 

 formed, as, e.ii., in dry districts of India, where nitre is 

 found as an efflorescence on the surface of the soil. Chili 

 nitre, or Chili saltpetre, is the nitrate of sodium, and 

 occurs in large deposits in the rainless districts west of the 

 Andes, in C'hili and Peru. In a wet climate such deposits 

 would be speedily washed away, and carried into the rivers 

 and the sea. We have referred before to the loss of tlie 

 nitrogen which the soil of continents undergoes on account 

 of soluble nitrates finding their way into the sea. The great 

 quantities of nitrates from Chili and Peru which have of late 

 years been applied to the fertilization of the land are a 

 contribution won back from the ocean ; the nitrates of the 

 nitrate beds have been formed by the oxidation of guano, 

 the dejecta of fish-feeding sea birds. Recently "artificial 

 guano " has been manufactured to a considerable extent 

 from the carcases of coarse fish, caught for the purpose by 

 fishing from ships specially employed in connection with this 

 manufacture. 



Where crops are cut and carried from the place where 

 they were grown, it is necessary to provide for the 

 restoration to the soil of certain materials, particularly 

 nitrogen, which are thus removed instead of being allowed 

 to return to the soil through the death and decay of the 

 plants, as would be the case in a state of nature. This is 

 partially effected by returning to the soil the manure from 

 the animals of the farm which fed upon the crops. When 

 the beasts themselves, however, are sent into the towns, 

 they carry away large quantitiL'S of the particularly 

 valuable constituents of a fertile soil, such as nitrogen and 

 phosphorus. In a more primitive condition of agriculture, 

 the beasts would be eaten on the farm itself, and the 

 greater part of the nitrogen, &c., would find its way back 

 to the soil whence it came. Facilities for communication 

 and transport, however, and the concentration of the 

 population in towns all tend to make farming, more 

 particularly manuring or soil-feeding, a more complicated 

 matter. This is especially the case where nitrogenous stuff, 

 ejj., hay, is sold off the farm, instead of being consumed 

 upon it, and oil cake, or other artificial feeding-stuff is pur- 

 chased in its place. The oil cakes are rich in fat as well 

 as in nitrogen, but the albuminoid ratio is always high, in 

 some cake as high as 5 : 7. The method of calculating the 

 nutrient or albuminoid ratio was explained in a former 

 article [ride Knowledge, July, 1893). Account has to be 

 taken of this ratio in deciding upon combinations of 

 artificial foods for the use of stock, just as the authorities 

 of prisons, &c., have to consider the nutrient ratio of the 

 diet they supply. Young growing animals retain a larger 

 proportion of the nitrogen supplied to them than the full- 

 grown beasts which are being fatted. In the former case 

 much of the nitrogen goes to build up the muscular flesh, 

 and the manure given by young growing animals is pro- 

 portionately poorer in nitrogen. On the other hand, full- 

 grown animals which are being fatted for market store up 

 chiefly fatty tissue, which contains no nitrogen, and con- 

 sequently these animals return a larger proportion of 

 nitrogen to the ground. In feeding different species of 

 animals account has to be taken not only of the albuminoid 

 ratio but of what is called the diijestUm ca-e/ficient, or propor- 

 tion which the food-stuff' taken bears to the amount digested. 



This proportion is different in the case of diff'erent species 

 of animals. In the case of ruminants a large proportion 

 of iudige3tible fibre in the food is actually necessary. 



