82 GENERAL CONSIDERATIONS 



it should only be applied in small doses, and just at 

 the time when it will be most useful in the growth 

 of the plant. It is much used by market gardeners, 

 and sometimes as a top dressing for lawns and small 

 grains. Sulphate of ammonia is another mineral 

 source of nitrogen for fertilizers. It is a by-product 

 of certain manufacturers. It, too, is very soluble, 

 but it acts more slowly than nitrate of soda, since, 

 before becoming available, it must be acted on by 

 the soil bacteria, and be converted into the form 

 of a nitrate. It is a valuable source of nitrogen, 

 especially for heavy clay soils. If used in large 

 quantities on sandy lands that are deficient in lime, 

 an acid condition may ultimately be induced that 

 will be harmful. Dried blood, tankage, and other 

 slaughterhouse refuse, and fish scrap, the waste 

 material from the manufacture of fish oil, are the 

 usual animal sources of nitrogen. They are all 

 extensively used, and give excellent results with 

 most crops. All have to undergo fermentation and 

 nitrification in the soil before becoming available. 

 Dried blood undergoes the changes very quickly, 

 but the tankage acts more slowly. This is some- 

 times an advantage for crops that need to be nour- 

 ished during a long growing season. Guano, the 

 dried excrement of sea birds found in certain desert 

 islands off the Pacific coast of South America, should 

 also be mentioned in this connection. It has been 

 much used as a fertilizer and contains considerable 

 nitrogen. The bat guano, found so abundantly in 

 caves in many regions, has usually been subject to 

 leaching by water, and so has lost most of its nitro- 



