138 THE BOOK OP WHEAT 



sources of nitrogen. Most of the guano beds are now complete- 

 ly exhausted. 



Phosphoric Acid. The four main sources of this are: 

 (1) Bones; () phosphatic deposits like those of South Caro- 

 lina, Florida, Tennessee, North Carolina and Virginia in the 

 United States, or the keys of the Carribean sea; (3) accumu- 

 lation of fossil and excrementitious material; and (4) Thomas 

 slag, a by-product of the smelting of iron ores. Bones were 

 used to a limited extent in England before 1810. They were 

 ground until Liebig made the discovery of preparing super- 

 phosphate of lime by dissolving bones in sulphuric acid. One 

 bushel of bone dust dissolved by one-third its weight of the 

 acid is superior as a manure to four bushels of bone dust. The 

 lime of the bones is converted into gypsum, and the phosphoric 

 acid is reduced to a state more easily soluble and assimilable. 

 Formerly bones were also often burned. 



In 1817 superphosphates were first manufactured in Eng- 

 land, and the first phosphate mined commercially in South 

 Carolina was in 1867, six tons. The earliest form of mineral 

 phosphate used for fertilizer was apatite. In Canada 10 years' 

 experience has shown that finely ground, untreated mineral 

 phosphate has no value as a fertilizer. The Bessemer process 

 of manufacturing steel gives a by-product, rich in phosphoric 

 acid, "produced by the union of the phosphorus of the iron 

 with the lime of the flux employed." This is reduced to a 

 fine powder and applied, without treatment, to the soil. It 

 contains from 15 to 20 per cent of the acid. The yield of 

 wheat seems to be little affected by the carrier or source of 

 phosphoric acid if the material used is finely ground. Some 

 30 to 60 pounds of the acid should be applied to an acre of 

 land. 



Lime. It has been claimed that this is one of the first 

 mineral elements to show depletion. Sourness of soil often 

 results. To correct this and supply lime, 20 to 40 bushels of 

 lime per acre may be used. Lime has a tendency to work 

 down, and should never be deeply plowed under. It can be 

 applied to the soil most advantageously prior to the planting 

 of maize in the rotation, for wheat does not seem to be directly 

 benefited by it. If it is applied directly to wheat, it should be 

 scattered over the plowed field a few days before seeding and 



