6OO AGRICULTURAL ANALYSIS 



pends upon its percentage of hydrated calcium sulfate. The quan- 

 tity of gypsum of all kinds mined in the United States in 1906 was 

 1,540,585. short tons. Of this amount only 62,671 short tons 

 were sold as land plaster. 76 In the same time there were imported 

 into the United States 440,586 short tons. If the same proportion- 

 ate part of this were used for fertilizing purposes, it may be said 

 that the annual consumption of land piaster in the United States 

 at the present time for agricultural uses is 80,294 short tons. 



Gypsum, being a very soft mineral, is easily ground and should 

 be in the state of a fine powder when used for fertilizing purposes. 

 It is soluble in about 500 parts of rain water, so that when ap- 

 plied as a top dressing it is carried into the soil by rain. Its 

 favorable action is both as a plant food and mechanically in modi- 

 fying, in an advantageous, way, the physical constituents of the 

 soil. It is also valuable for composting and for use in stables 

 by reason of its power of fixing ammonia by the formation of 

 lime carbonate and ammonium sulfate : 



( H 4 N ) ,CO 3 +CaSO 4 = ( H 4 N ) 2 SO 4 +CaCO 8 . 



506. Analysis of Gypsum. For agricultural purposes it will 

 be sufficient to determine the quantity of sulfuric acid, and to 

 calculate therefrom the amount of calcium sulfate in the sample : 

 Or the lime may be determined and the quantity of sulfate cal- 

 culated therefrom. 



(1) Insoluble Matter. In the conduct of the work the sam- 

 ple of gypsum is rubbed to an impalpable powder in an agate 

 mortar. The sample, about one gram, is dissolved in a large 

 excess of dilute hydrochloric acid, the digestion being contin- 

 ued at near the boiling-point, with frequent stirring, for at least 

 two hours. The solution is made alkaline, filtered, and the resi- 

 due washed and dried to constant weight. 



(2) Sulfuric Acid. The washings and filtrate from the above 

 determination are made up to a definite volume with water and 

 divided into two equal parts. The sulfuric acid is estimated in 

 one part by adding to it sodium carbonate until the acidity is 

 nearly neutralized. The sulfuric acid is then thrown down at 

 near boiling temperature by the gradual addition of barium 



78 Burchard, Mineral Resources of the United States, 1906 : 1073. 



