112 THE CULTURE OF FARM CROPS. 



and even in the particles of atmospheric dust. But although 

 their presence is universal, they possess a less marked im- 

 portance in vegetable growth than the potash compounds ; 

 appearing in much less quantity in the ashes of plants. 

 With the exception of salt, none of these compounds are 

 used in agriculture, excepting incidentally as impurities in 

 the more costly potash fertilizers. These consist of sulphate 

 of soda and chloride of sodium chiefly, and are mingled to 

 a considerable extent with magnesia salts in the so called 

 German potash salts from the Strassfurth salt mines. 



The universal diffusion of these compounds in nature sup- 

 plies all the needs of the farmer for the growth of his crops, 

 and if any one is thought necessary, salt will serve every pur- 

 pose. This will be considered at greater length when the 

 subject of manures is under consideration. 



CALCIUM AND ITS COMPOUNDS. 



CALCIUM, like the preceding two metals is silver white in 

 color, and by its union with oxygen forms lime. It is not 

 known to exist in an uncombined state in nature and there- 

 fore has no direct action upon vegetation. 



LIME, is the oxide of calcium, and has so very great an 

 affinity for water and for carbonic acid that it only remains- 

 in its pure state a short time. It is prepared from the com- 

 mon limestone, the crystallized form of which is known a& 

 marble, by burning it in a kiln. The carbonic acid is driv- 

 en off in the combustion, leaving the lime in a caustic con- 

 dition, or as it is termed quick lime, and loses 44 per cent* 

 of its weight in the burning. 



LIME, is by far the most important mineral constituent 

 of plants and forms the greater part of the ash of the major- 

 ity of them. Its relation to plant growth, and its action in 

 many ways upon the soil, gives it a high position in the es- 

 timation of farmers, both as a direct fertilizer, and an indi- 

 rect aid in the preparation of the soil for the growth of crops. 

 It has an exceedingly destructive action upon all organic 

 matter, quickly decomposing it and reducing it to its origi- 

 nal elements, and preparing it for plant food. It has also 



