194 



f oluble,'" " citrate soluble," and " acid soluble" phosphoric acids. In some- 

 States the soluble and reverted are classed together as "available" phospho- 

 ric acid, a term which has been adopted by the manufacturer rather than- 

 the agriculturist, since the question of availability in the soil is yet an 

 undetermined one and varies greatly with the character of the soil. It 

 is true jthat the soluble phosphoric acid, when applied to the soil, after 

 a while reverts, from contact with basic principles in the soil, but before 

 doing so it diffuses itself throughout the soil, and by this initial velocity 

 becomes thoroughly incorporated with it, and when precipitated in the 

 reverted form, each particle is so minute as to be readily available to 

 the acid secreting roots of plants. The reverted phosphates being in- 

 soluble in water. Lack this initial diffusion, and therefore remain in the 

 exact position in which they were placed by the planter. The particle* 

 are more segregated and less diffused, and therefore less available. 



In buying phosphates, therefore, due regard should be given to ob- 

 taining the highest possible content of soluble phosphoric acid. 



Slag Meal is a by-product in the manufacture of steel or wrought 

 iron by the "Thomas Gilchrist" process, from pig iron rich in phosphoric 

 acid. The pig iron is melted in converters lined with lime and when 

 thoroughly melted, the lime unites with the phosphoric acid present, 

 and forms a slag which floats on the surface of the melted iron. This 

 slag is removed, cooled and ground into a powder, and sold as " phos- 

 phoric slag," or " slag meal." Besides phosphate of lime, it usually 

 contains goodly quantities of lime, and therefore cannot be mixed with- 

 out injury with fertilisers containing either ammonia or soluble phos- 

 phates. It usually contains 15 to 20 per cent, of phosphoric acid, and 

 upon some soils, with certain crops, is highly esteemed. It has not 

 pro^ven as valuable under sugar cane as the soluble forms. There is a 

 promise of a large supply of this slag in the near future by the manu- 

 facturers who work iron by this process. 



Natubal Phosphates. 



Besides the mineral phosphates already described, there are found on 

 the smaller islands oi the ocean, immense deposits of phosphates, which 

 require little or no manipulation to prepare them for use. They are be- 

 lieved to be the residues from the deposits of fish- eating birds. In a 

 rainless climate, like Peru, such deposits give us the celebrated Peru- 

 vian guano, rich in both ammonia and phosphates. In a rainy climate, 

 such as prevails in the Caribbean Sea, the ammonia and other soluable 

 matters have been washed out, leaving only the insoluble phosphates. 

 In such a manner has originated the natural guanos, known as Grand 

 Cayman, Bedonda, Orchilla, Carib, etc , all coming from islands in the 

 Caribbean Sea. These natural guanos should be used with great care, 

 since they are simply phosphates, and not what their name imports 

 guanos." 



In selecting phosphates for use, intelligent farmers, both in Europe 

 and America, give preference to "super," or " acid phosphates" a fact 

 evidenced by the ever-increasing demand for these goods. 



