206 Bureau of Farmers' Institutes. 



bined with oxygen to form a compound known as calcium oxide, 

 was called Lime, and this calcium oxide or lime was supposed to be 

 present in calcium compounds. 



Calcium combined with oxygen and carbon forms a compound 

 which is known as calcium carbonate or carbonate of lime. Lime- 

 stone, marble, chalk, eggshells and coral consist of calcium car- 

 bonate or carbonate of lime. Calcium and sulphuric acid form a 

 compound known as calcium sulphate or sulphate of lime; gypsum 

 and plaster of paris are familiar forms of calcium sulphate; it 

 is often called simply " plaster." 



Calcium combined with oxygen, as already noted, forms calcium 

 oxide, which is commonly known as lime or quicklime. This is 

 made by burning some form of calcium carbonate, as limestone, 

 oyster shells, coral rock, etc.; the carbon dioxide (carbonic acid) is 

 driven off by the heat, and calcium oxide or quicklime remains. 



When quicklime is exposed to the air, it slowly absorbs moisture 

 and carbon dioxide and is changed back into calcium carbonate. 

 When quicklime is changed into calcium carbonate (carbonate of 

 lime), the lime is said to be air-slacked. 



(b) Relation to Fertilizers. — All plants require calcium, or 

 as it is more commonly called, lime. While many soils appear to 

 contain an inexhaustible supply of lime compounds, there is quite 

 often need of special attention to this element of plant food; rather 

 more, in fact, than was formerly believed necessary. 



(c) Specific Functions in Plant Growth. — Calcium forms a 

 part of several compounds found in plants. Its chief function 

 appenrs to be that of strengthening cell walls. It is often found 

 united with different acids forming calcium (or lime) salts. Thus, 

 in beet leaves, we find the salt calcium oxalate. In what specific 

 ways it otherwise affects the growth of plants, we do not yet know 

 definitely. 



Magnesium. 



(a) Source. — Mngnesium is found in several common com- 

 pounds present in the soil. Magnesium appears to form an essen- 

 tial constituent of plants. 



(b) ]'< bil 'inn to Fertilizers. — While magnesium is regarded 

 as a necessary constituent of plants, it does not often need to be 

 added to the soil artificially so far as our present knowledge goes. 

 There are some magnesium compounds existing, as impurities 

 in the German potash salts, and when these latter are used, some 

 magnesium is incidentally added to the soil. 



