ASSIMILATION OF NITROGEN. 



35 



It is quite evident, therefore, that the 

 common view concerning the influence of 

 certain salts upon the growth of plants 

 evinces only ignorance of its cause. 



The action of gypsum or chloride of cal- 

 cium really consists in their giving a fixed 

 condition to the nitrogen or ammonia 

 which is brought into the soil, and which is 

 indispensable for the nutrition of plants. 



In order to form a conception of the effect 

 of gypsum, it may be sufficient to remark 

 that 110 Ibs. of burned gypsum fixes as 

 much ammonia in the soil as 6880 Ibs. of 

 horse's urine* would yield to it, even on the 

 supposition that all the nitrogen of the urea 

 and hippuric acid were absorbed by the 

 plants without the smallest loss, in the form 

 of carbonate of ammonia. If we admit with 

 Boussingaultf that the nitrogen in grass 

 amounts to y^ of its weight, then every 

 pound of nitrogen which we add increases 

 the produce of the meadow 100 Ibs., and 

 this increased produce of 100 Ibs. is effected 

 by the aid of a little more than 4 Ibs. of 

 gypsum. 



Water is absolutely necessary to effect the 

 decomposition of the gypsum, on account 

 of its difficult solubility, (1 part of gypsum 

 requires 400 parts of water for solution) and 

 also to assist in the absorption of the sul- 

 phate of ammonia by the plants : hence it 

 happens, that the influence of gypsum is 

 not observable on dry fields and meadows. 

 In such it would be advisable to employ a 

 salt of more easy solubility, such as chloride 

 of calcium. 



The decomposition of gypsum by carbo- 

 nate of ammonia does not take place instan- 

 taneously ; on the contrary, it proceeds very 

 gradually, and this explains why the action 

 of the gypsum lasts for several years. 



The advantage of manuring 'fields with 

 burned clay, and the fertility of ferruginous 

 soils, which have been considered as facts 

 so incomprehensible, may be explained in 

 an equally simple manner. They have been 

 ascribed to the great attraction for water, 

 exerted by dry clay and ferruginous earth j 

 but common dry arable land possesses this 

 property in as great a degree : and besides, 

 what influence can be ascribed to a hundred 

 pounds of water spread over an acre of 

 land, in a condition in which it cannot be 

 serviceable either by the roots or leaves ? 

 The true case is this : 



The oxides of iron and alumina are dis- 

 tinguished from all other metallic oxides by 

 their power of forming solid compounds 

 with ammonia. The precipitates obtained 

 by the addition of ammonia to salts of alu- 



* The urine of the horse contains, according to 

 Fourcroy and Vauquelin, in 1000 parts, 



Urea 7 parts. 



Hippurate of soda . . 24 " 

 Salts and water . . 979 " 



1000 parts. 



t Bonssingault, Ann. de Ch. et de Phys. t. Ixiii. 

 page 243. 



mina or iron are true salts, in wnich the 

 ammonia is contained as a base. Minerals 

 containing alumina or oxide of iron also 

 possess, in an eminent degree, the remark 

 able property of attracting ammonia from 

 the atmosphere and of retaining it. Vau- 

 quelin, whilst engaged in the trial of a crimi- 

 nal case, discovered that all rust of iron 

 contains a certain quantity of ammonia. 

 Chevalier afterwards found that ammonia 

 is a constituent of all minerals containing 

 iron ; that even hematite, a mineral which 

 is Dot at all porous, contains one per cent, 

 of it. Bouis showed also, that the peculiar 

 odour observed on moistening minerals con- 

 taining alumina, is partly owing to their ex- 

 haling ammonia. Indeed, gypsum and 

 some varieties of alumina, pipe-clay for ex- 

 ample, emit so much ammonia, when mois- 

 tened with caustic potash, that even after 

 they had been exposed for two days, red- 

 dened litmus paper held over them becomes 

 blue. Soils, therefore, which contain ox- 

 ides of iron, and burned clay, must absorb 

 ammonia, an action which is favoured by 

 their porous condition ; they further prevent 

 the escape of the ammonia once absorbed 

 by their chemical properties. Such soils, 

 in fact, act precisely as a mineral acid would 

 do, if extensively spread over their surface ; 

 with this difference, that the acid would pe- 

 netrate the ground, enter into combination 

 with lime, alumina, and other bases, and 

 thus lose, in a few hours, its property of 

 absorbing ammonia from the atmosphere. 

 The addition of burned clay to soils has also 

 a secondary influence; it renders the soil 

 porous, and, therefore, more permeable to 

 air and moisture. 



The ammonia absorbed by the clay or fer- 

 ruginous oxides is separated by every shower 

 of rain, and conveyed in solution to the soil. 



Powdered charcoal possesses a similar ac- 

 tion, but surpasses all other substances in 

 the power which it possesses of condensing 

 ammonia within its pores, particularly when 

 it has been previously heated to redness. 

 Charcoal absorbs 90 times its volume of am- 

 moniacal gas, which may be again separated 

 by simply moistening it with water. (De 

 Saussure.) Decayed wood approaches very 

 nearly to charcoal in this power ; decayed 

 oak wood absorbs 72 times its volume, after 

 having been completely dried under the air- 

 pump. We have here an easy and satisfac- 

 tory means of explaining still further the pro- 

 perties of humus, or wood in a decaying 

 state. It is not only a slow and constant 

 source of carbonic acid, but it is also a 

 means by which the necessary nitrogen is 

 convey eel to plants. 



Nitrogen is found in lichens, which grow 

 on basaltic rocks. Our fields produce more 

 of it than we have given them as manure, 

 and it exists in all kinds of soils and mine- 

 rals which were never in contact with or- 

 ganic substances. The nitrogen in these 

 cases could only have been extracted from 

 the atmosphere. 



