414 SMITH'S INTERMEDIATE CHEMISTRY 



The proteins are changed by air bacteria into ammonia. The 

 formation of nitrates seldom happens in manure piles, but scat- 

 tered manure, with the help of soil bacteria, develops nitrates. 



The potassium compounds form potassium hydrogen carbonate 

 KHC0 3 . The phosphorus and sulphur compounds become sol- 

 uble phosphates and sulphates. 



Indirect Fertilizers. Not all substances which are added 

 to the soil are employed with the direct object of their assimilation 

 for plant growth. Often indirect effects induced by their pres- 

 ence are of greater importance. A few cases where calcium salts 

 are of service as indirect fertilizers may be briefly presented as 

 illustrations of this point. 



(1) Gypsum CaS0 4 ,2H 2 is added to manure at the rate 

 of 100 pounds per ton. This slightly soluble salt reacts in the 

 soil solution with the ammonium carbonate produced by the hy- 

 drolysis of urea, precipitating the much less soluble calcium car- 

 bonate and leaving ammonium sulphate in solution: 



(NH 4 ) 2 C0 3 + CaS0 4 -> (NH 4 ) 2 SO 4 + CaCO 3 J . 



Now ammonium sulphate, being a salt of a strong acid with a 

 weak base, is only very slightly hydrolyzed in solution (see p. 

 369). Ammonium carbonate, however, is a salt of a weak acid 

 and a weak base, and is extensively hydrolyzed. This hydrolysis, 

 unless a large excess of water is present, would lead to rapid loss 

 of ammonia from the manure. The smell of free ammonia, in- 

 deed, is often very noticeable in manure piles. The addition of 

 gypsum fixes this valuable constituent in the fertilizer for plant 

 use. Lime, on the other hand, would assist in the liberation of 

 ammonia. 



(2) Gypsum is often added, also, to clay soils with the object 

 of converting insoluble compounds of potassium into more sol- 

 uble compounds. Lirne and calcium carbonate are employed to 



