HOW PLANTS GROW. 603 



South Carolina. Large deposits of phosphatic guano are also 

 found in Navassa, and other islands in the Caribbean Sea. 



The South Carolina rock is now largely pulverized, or beaten 

 into a very fine powder, and sold under the name of floats. 

 This becomes more quickly of use to crops than coarse ground 

 bones, but a still greater solubility is attained by making the 

 rock into superphosphate, by dissolving it in sulphuric acid. 

 This is known as acid phosphate in the markets. Superphos- 

 phate was formerly largely made from bones, but now it is 

 almost altogether made from phosphate rock and phosphatic 

 guano. All cultivated soils contain more or less phosphoric 

 acid, but it is more generally deficient than most other forms 

 of plant food. Hence the great importance attached to phos- 

 phatic fertilizers. 



Nitrogen. Nitrogen is found largely in Chili, in the shape 

 of nitrate of soda, which is now largely used as a top-dressing 

 during the growing season. Nitrate of soda dissolves very 

 rapidly when scattered on the soil, and is quickly taken up by 

 growing plants. It absorbs water so rapidly that it is hard to 

 keep without losing value, and is little used in mixing with other 

 fertilizing substances that are to be kept for any length of time. 



Nitrate of potash (saltpetre), so largely used in making gun- 

 powder, is found in the soil and is artificially formed by suitable 

 materials. It is too costly to use as a farm fertilizer. 



Nitrogen is also used in various forms of ammonia compounds. 

 Ammonia (hydrogen and nitrogen) has been found of more 

 benefit to some plants than nitrogen in the shape of nitrates. 

 It has been found, however, that salts of ammonia in the soil 

 rapidly change to nitrates. It has been found that, with the 

 potato crop especially, while ammonia did no good, nitrates of 

 soda had a powerful effect on the crop, showing that there is a 

 great difference in the liking of plants for different forms of 

 nitrogen. Some suppose that plants will absorb ammonia from 

 the air by their leaves, but this is by no means certain. The 

 best and cheapest form in which ammonia is to be had is as 

 sulphate of ammonia. This is prepared from the ammonia 

 water, or gas liquor, produced in making gas for lighting cities 

 and dwellings. Years ago the best shape in which nitrogen 

 could be had was in Peruvian guano from the Chincha Islands, 



