FERTILIZERS 139 



at once harrowed in. The ground should be stirred again be- 

 fore seeding. The application of one ton per acre every 4 to 6 

 years is advised for Illinois uplands. 



Marl. This consists essentially of carbonate of lime. Often 

 considerable amounts of organic matter, sand and clay are also 

 present. It originated in the breaking down of fresh water- 

 shells. Its action is more slow and "mild" than that of lime. 

 It has been "regarded rather as an amendment than a ferti- 

 lizer." Its chief functions are in improving the tilth, neutral- 

 izing acidity, and promoting nitrification, besides supplying 

 lime. The marls of New Jersey also contain potash and phos- 

 phoric acid. 



Nitrate of Soda. Trade in this as a fertilizer began between 

 1830 and 1840. The supply is limited. In 1860 all estimates 

 showed that it would last nearly 1,500 years. By 1900 these 

 estimates had fallen to less than 50 years, and the world's 

 markets were annually consuming nearly 1,500,0,00 tons, the 

 United States requiring about 15 per cent of this amount. It 

 is by far the most expensive fertilizer in use, and it is the best 

 carrier of nitrogen. 



Potash. The main sources are wood ashes, and, since 1860, 

 the products furnished by the potash industry at Stassfurt, 

 Germany. Several forms are imported from Germany, each 

 containing a different but correctly warranted quantity of this 

 fertilizer. Nearly 500,000 tons are imported annually by the 

 United States. This is over half the product. Since 1860, the 

 Stassfurt salts have been almost the only source of concentrated 

 potash. Good wood ashes contain perhaps 10 per cent of pot- 

 ash. They were long used without the real reason of their value 

 being known. Besides potash, ashes often contain consider- 

 able lime. Hartlib gave 23 fertilizers and means of fertilizing 

 the ground, and among them were included lime, marl, ashes 

 and chalk. 



Gypsum or Land Plaster. Those who are experimenting with 

 this material report varied results. It has long been used, 

 however, and the most reliable conclusions seem to be: (1) 

 That gypsum has undoubted fertilizing value on many soils; 

 (2) that its chief value depends on three processes: (a) Pres- 

 ervation of ammonia and perhaps other nitrogenous forms; 



