SOIL AMENDMENTS 539 



as 500 CaO to 1 MgO by weight. On the other hand, 

 excessive applications of magnesium compounds have 

 been found to be injurious on some soils. Even on a 

 very heavy clay soil, at Cornell University, an applica- 

 tion of 1333 pounds to the acre of magnesite markedly 

 decreased the yields of sorghum and oats. The soil 

 originally contained about equal parts of calcium and 

 magnesium. 



459. Forms of calcium. Calcium is used on the 

 soil in the form of calcium oxide, or quicklime (CaO), 

 water-slaked lime (Ca(OH) 2 ), air-slaked lime (CaCO 3 ), 

 ground limestone, marl (also a carbonate), and calcium 

 sulfate, or gypsum (CaSO4 . 2 H 2 0). The application of 

 any of these is usually called liming the soil, although 

 gypsum does not serve exactly the same purpose as do 

 the other forms. Owing to differences in the molecular 

 weights of these compounds of calcium, it requires more 

 of some forms than of others to furnish the same amount 

 of calcium. Approximately equivalent quantities of some 

 of the common forms when fairly pure are : 



Quicklime 56 pounds 



Water-slaked lime 74 pounds 



Air-slaked lime, marl, and ground limestone 100 pounds 



Quicklime, and the hydrate, when added to the soil, even- 

 tually assume some of the more insoluble forms of com- 

 bination or remain as the carbonate, never being present 

 as the oxide. It is always desirable to have present in the 

 soil at least a small amount of calcium carbonate. 



460. Caustic limes. Quicklime and water-slaked lime 

 have a markedly alkaline reaction, and hence neutralize 

 quickly any active acidity that may exist in the soil. 

 They act quickly also in liberating plant-food, particularly 



