244 IIOAV^ CROPS FEED. 



pounds with ammonia are freely soluble in water; hence 

 strong solution of ammonia dissolves them from the soil. 

 But when ammonia salts of these acids are put in contact 

 with lime, magnesia, oxide of iron, oxiile of manganese, 

 and alumina, the latter being in prej^onderating quantity, 

 there are formed double compounds which are insoluble 

 or slightly soluble. Since the humic, idmic, crenic, am, 

 apocrenic acids always exist in soils which contain organic 

 remains, there can be no question that these double salts are 

 a chemical cause of the retention of ammonia in the soil. 



2d. Certain phosphates and silicates hereafter to be no- 

 ticed have the power of forming difficultly soluble com- 

 pounds with ammonia. 



Reserving for a subsequent cliapter a further discussion 

 of the causes of the chemical retention of ammonia in the 

 soil, we may now appropriately recoimt the observations 

 that have been made regarding the condition of the am- 

 monia of the soil as regards its volatility, solubility, etc. 



Volatility of the Ammonia of the Soil. — We have 

 seen that ammonia may esca])e from the soil as gaseous 

 carbonate. The fact is not only true of this substance as 

 physically absorbed, but also under certain conditicms of 

 that chemically combined. When we mingle together 

 equal bulks of suli)hale of lime (gypsum) and carbonate 

 of ammonia, both in the state of fine powder, the mixture 

 begins and continues to smell strongly of ammonia, owing 

 to the volatility of the carbonate. If now the mixtuie be 

 drenched with water, the odor of ammonia at once ceases 

 to be perceptible, and if, after some time, the mixtuie be 

 thrown on a filter and washed with water, we shall find 

 that what remains undissolved contains a large proportion 

 of carbonate of lime, as may be shown by its dissolving 

 in an acid with effervescence; Avhile the liquid that has 

 pnssed the filter contains sulphate of ammonia, as may be 

 learned by the appropriate chemical tests or by evaporat- 

 ing to dryness, when it will remain as a colorless, oiloi less. 



