SOIL AMENDMENTS 585 



that point is reached. One way in which this occurs is 

 by curtailing the quantity of calcium carbonate in the 

 soil. An easily available base to combine with the 

 organic acids affords the most favorable condition for the 

 decomposition processes due to bacterial action, and 

 hence the best results cannot be obtained where carbonate 

 of lime is not present. Its action in improving tilth also 

 facilitates desirable forms of bacteriological activity by 

 increasing the permeability of the soil for air. 



455. Liberation of plant-food materials. — It has been 

 stated that the alkalies and the alkaline earths are more 

 or less interchangeable in certain compounds in the soil. 

 The addition of lime may in this way liberate potassium, 

 when otherwise it would be difficult for crops to obtain 

 a sufficient supply from a particular soil. The substitu- 

 tion of bases has been discussed (par. 251) and the 

 liberation of potassium is in accord with these phenomena. 

 Magnesium, although rarely deficient, may also be made 

 available in this way. The use of calcium salts may, 

 under some soil conditions, render phosphorus more use- 

 ful, probably by supplying a base more soluble than iron 

 or alumina, with which, in soils deficient in calcium, the 

 phosphorus might otherwise be combined. Experiments 

 by Prianischnikov,^ in which plants were grown in washed 

 sand containing HellriegeFs nutrient solution to which 

 mono-, di-, and tri-calcium phosphate respectively were 

 added, both with and without calcium carbonate, showed 

 a decreased availability of the tricalcium phosphate due 

 to the presence of the carbonate, but neither a reduced 

 nor an increased availability of the other forms of phos- 



1 Prianisclinikov, D. Ueber den Einfluss von Kohlensauren 

 Kalk BAX£ die Wirkung von VerscMedenen Phosphaten. Landw. 

 Vers. Stat., Band 75, Seite 357-376. 1911. 



