DECOMPOSES GYPSUM. 53 



and brought to the earth again by the rains and dews ; so that at the 

 same time the air is purified for the use of animals, and the ammo- 

 nia brought down for the use of plants. And second, whatever salts of 

 ammonia are contained in the soil, being dissolved by the rain, are in 

 a condition to be taken up, when wholesome, by the roots of plants; or 

 to be carried off by the drains when injurious to vegetation. 



4°. I have already alluded to the fact of this gas being absorbed by 

 porous substances, and to its presence, in consequence, in porous soils, 

 and in burned bricks and clay. With the purer kinds of unburned 

 clay, however, and with the oxide of iron contained in red (or ferrugi- 

 nous)* soils, ammonia is supposed to form a chemical compound of a 

 weak nature. In consequence of its affinity or feeble tendency to com- 

 bine with these substances, they attract it from the air, and from decay- 

 ing animal or vegetable matters, and retain it more strongly than many 

 porous substances can, — yet with a sufficiently feeble hold to yield it 

 up, readily as is supposed, to the roots of plants, when their extremities 

 are pushed forth in search of food. In this case the carbonic, acetic, 

 and other acids given oflT, or supposed to be given off by the roots, exer- 

 cise an influence to which more particular allusion will be made here- 

 after. 



6°. In the state of carbonate it decomposes gypsum, forming carbon- 

 ate of lime (chalk) and sulphate of ammonia.f The action of gypsum 

 on grass lands, so undoubtedly beneficial in many parts of the world, 

 lias been ascribed to this single property; it being supposed that the 

 sulphate of ammonia formed, is peculiarly favourable to vegetation. 

 This question will come properly under review hereafter. I may here, 

 however, remark that if this be the sole reason for the efficiency of gyp- 

 sum, its application ought to be beneficial on all lands not already 

 abounding either in gypsum or in sulphate of ammonia.J But if the 



* Soils reddened by the presence of oxide of iron. 



t Gypsum is sulphate of lime— consisting of sulphuric acid (oil of vitriol) and quicklime. 

 Carbonate of ammonia consists of carbonic acid and ammonia. W^hen the two substances 

 act upon each oth^r in a moist state— the two acids change places— the sulphuric acid, as it 

 vf ere, preferring the ammonia, tlie carbonic acid the lime. 



t Liebig says — "tlie sinking fertility of a meadow on which gypsum is strewed depends 

 only on its'fixing in the .soil the ammonia of the atmosphere, which would otherwise be vola- 

 tilized with the water which evaporates."— O^an/c Chemistrtj applied to Agriculture, p. 86. 

 [^y fixing is meant the formina ni sulphate with the ammonia. Rain water is supposed to 

 bring down with it carbonate of ammonia (common smelling salts), which acts upon the sul- 

 pfiateoflimei'iypimn) in such a way that sulphate of ammo7iia ami carbonate of lime axe 

 jiroduced. Ttie carbonate of ammonia readily volatilizes or rises again into the air, the sul- 

 phate does not — hence the use of the word fix.] 



When we come to consider the subject of mineral manures in general, we shall study 

 more in detail the specific action of gypsum in promoting vegetation— a very simple calcula- 

 tion, however, will serve to shew that the above theory of Liebig is far from affording a satis- 

 factory explanation of all the phenomena. 



Supposing the gypsum to meet with a sufficient supply of nmmonia in the soil, and that it 

 exercises its full influence, 100 lbs. of common unburned gypsnm will flx or form sulphate 

 with nearly 20 lbs. of ammonia containing IBJlbs. of nitrogen. One hundred weight, there- 

 fore, (112 lbs.) will form as much sulphate as will contain 22| lbs. of ammonia, and if intro- 

 duced without loss into the interior of plants will furnish them with \Sh lbs. of nitrogen. 



1°. In the first volume of British Husbandry, pp. 322, 323, the following experiment is 

 recorded : ^ ^ , , , , , 



Mr. Smith, of Tunstal, near Sittingbourne, top-dressed one portion of a field of red clover 

 with powdered gypsum at the rate of five bushels (or four hundred weight') per acre, and 

 compared the produce with another portion of the same field, to which no manure had been 



[• A ton of pure gypaum, when crushed, will yield 25 bushels. It should, however, al- 

 ways be applied by weight. ] 



3* 



