314 IMPROVEMENT OF THE SOIL 



nures. There is, moreover, no danger of adding them in too 

 large quantities. They are nourishers, and their action, as 

 salts, does not produce insoluble compounds, but tends to 

 render inert bodies active and useful. The nitrates are all 

 exceedingly mild, although very active, and useful in their in- 

 fluence upon vegetation. 



2. Phosphates. This family includes substances already 

 considered, such as bone, earth, horn, hair, hoofs, etc. The 

 only mineral phosphates, which may be used as manures, are 

 phosphate of lime (apatite) and phosphate of magnesia ; but 

 these substances are not found in sufficient quantities to ren- 

 der their application practicable. Phosphates act very much 

 like nitrates, the acid is food, and exists in vegetables Jn 

 combination with magnesia. It also acts upon silicates, and 

 eliminates their alkali. 



Bone dust is principally phosphate of lime, and is a highly 

 concentrated manure. 



3. CMonates, This family includes common limestone, 

 marl and air-slacked lime ; potash, ashes and white-ash or 

 barilla. Carbonate of lime is known under the names of 

 chalk, shells, marble, marl, limestone, etc. The most com- 

 mon forms in which it is used in agriculture, are shells, 

 marl and air-slacked lime; although ground limestone has 

 sometimes been applied to fertilize the soil. Salts of lime 

 have long been used for agricultural purposes. Their bene- 

 ficial effects were known to the ancients. They have been 

 used in England, France and Germany for the last 100 years, 

 with the very best results, and yet practical farmers are not 

 all agreed whether lime is useful or hurtful in its effects. 

 Experience shows that it is sometimes injurious and at others 

 highly beneficial. Any theory, therefore, which shall enable 

 us to decide the quantity which may be safely used (for it ap- 

 pears that the bad or good effects depend mostly upon the 

 quantity employed), must be of the highest benefit to the 

 practical farmer. 



