No. 6. DEPARTMENT OF AGRICULTURE. 817 



tliat the salts of phosphoric acid, or phosphates, as they are called, 

 are the only sources from which phosphorus of plants can be de- 

 rived. Phosphoric acid is a combination of the element Phosphorus 

 (P) with Oxygen gas (O). In phosphates, the phosphorus and 

 oxygon unite in the proportion of two parts of the former to five 

 of the latter, forming what is commonly designated as /^Ao^^AoWc 

 acid^ and this union is expressed by the sign or symbol Pg O5. Phos- 

 phorus, when uncombined with other elements, is a yellowish, 

 waxy looking, solid substance. It is soft and can be cut as easily 

 as ordinary bees-wax. It is very poisonous. It ignites easily, and, 

 therefore, has to be kept under water. When phosphorus burns, 

 it simply unites with the oxygen of the air, forming phosphoric acid 



Phosphoric acid usuaHy occurs in the soil in combination with 

 lime, magnesia, alumina and iron. These phosphates are all prac- 

 tically insoluble in water; that is, they are dissolved by pure water 

 so slowly and to so slight an extent that they sustain no appreciable 

 loss in the soil by drainage water. Hence, the quantity in the soil 

 is diminished almost wholly through the agency of crops. The 

 amount of phosphoric acid, even in a fertile soil, is comparatively 

 small. A ton of good soil will contain about three pounds; many 

 will contain less, and some considerablv more. On this basis an 

 acre of average soil would contain to the depth of 9 inches, about 

 4,500 pounds of phosphoric acid. 



The character of the soil affects very considerably, the available 

 condition of the plant food. One of the problems that confronts the 

 farmer is to use such methods in soil management as will convert the 

 plant foods which the soil contains into forms available for crops. 



FORMS OF PHOSPHORIC ACID. 



As has already been stated, phosphoric acid exists in soils in com 

 bination with lime, magnesia, alumina and iron, and it is in these 

 same combinations in which it is found in the various sources, from 

 which phosphates are manufactured. For the manufacture of fer- 

 tilizer and in agriculture, the phosphate of lime is most highly prized 

 and preferred. 



IRON AND ALUMNIA PHOSPHATES. 



This represents a large class of natural phosphates. They are in- 

 soluble in water, the same as the natural lime phosphate. They con- 

 tain almost the same percentage of phosphoric acid as lime phos- 

 I)hates. They are not well adapted for the manufacture of soluble 

 jjhosphates as the treatment with acid produces a sticky mass which 

 is hard to dry and keep in a good condition. 

 52—6^1902 



