102 SOURCE AND ASSIMILATION OF NITROGEN. 



gypsum, it may be sufficient to remark, that 110 lbs. 

 of burned gypsum fixes as much ammonia in the 

 soil as 6887 lbs. 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 ^^ of its weight, 

 then every pound of nitrogen which we add in- 

 creases the produce of the meadow 110 lbs., and 

 this increased produce of 110 lbs. is effected by the 

 aid of a little more than 4 lbs. of gypsum. 



Water is absolutely necessary to effect the decom- 

 position 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 absorp- 

 tion of the sulphate 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 carbonate of 

 ammonia does not take place instantaneously; 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; 

 but common dry arable land possesses this property 



* The urine of the horse contains, according to Fourcroy and Vau- 

 quelin, in 1000 parts, 



Urea ... 7 parts. 



Hippurate of soda . . 24 " 

 Salts and water . . 979 " 



1000 parts, 

 t Boussingault, Ann. de Ch. et de Phys., t. Ixiii. page 243. 



