SOILS AND THEIR PROPERTIES 75 



most important ingredient in soils. Probably phosphorus 

 and nitrogen are the two most commonly lacking soil 

 ingredients. Ferric hydrate in the soil is capable of combining 

 with phosphoric acid and forming insoluble phosphates, 

 which undoubtedly react to a limited extent with calcium 

 salts, so that in the soil phosphorus will occur as phosphates 

 of all the bases, and will also be found in the organic matter. 

 Water containing carbonic acid is a better solvent of the 

 complex phosphates than water itself, and the amount that 

 will enter into solution will depend partly upon the 

 concentration of carbonic acid in the water of the soil, which 

 will in turn depend on the percentage of carbon dioxide in 

 the soil atmosphere. I^arge amounts of iron in the soil 

 hinder the solution of the phosphoric acid by carbonic acid. 



Sulphuric Acid. — Sulphuric acid in the form of 

 calcium sulphate is common in all soils, and is probably the 

 chief source of the sulphur that is necessary for the formation 

 of plant proteins. It is being incessantly regenerated in 

 the soil itself by the oxidation of organic sulphur compounds 

 acting upon lime, also present in the soil. In the vicinity 

 of large towns the sulphur thrown into the atmosphere by 

 the combustion of coal comes down with the rain, washes 

 into the soil, combines with lime, and produces calcium 

 sulphate. Where the amount of lime is insufficient, the soil 

 becomes acid, and less fertile. Whenever super-phosphate 

 or sulphate of ammonia are used, considerable quantities 

 of sulphuric acid are added to the soil, so that modern 

 conditions of agriculture near big industrial districts do not 

 usually require the addition of sulphate to the soil, but 

 agricultural districts far removed from industrial scenes may 

 show a deficiency of this element. 



Carbonic Acid. — Carbonic acid occurs in the soil 

 both in the free and combined condition. Wlien carbon 

 dioxide in the air dissolves in water a certain amount of 

 the true carbonic acid exists in solution, and acting upon 

 any base present, produces bi-carbonate. When such soil is 

 dried, and removed to the laboratory, an ordinary carbonate 

 is formed. The amount of calcium carbonate in the soil 



