150 



As a result of these investigations the union of German experiment stations at its 

 meeting at Kiel, in September, 1896, adopted the method of determining the rela- 

 tive value of the slag according to its phosphoric acid solubility in a 2 per cent citric 

 acid solution and did away with the previous standard of total phosphoric acid and 

 fineness. 



Wagner, as well as Maercker, repeatedly called attention to the fact that experi- 

 ments both in the laboratory and with plants gave positive evidence that those slags 

 of like phosphoric acid content which were richest in silicic acid gave the best results. 

 G. Hoyermann, working independently, came to similar conclusions. At the present 

 time, according to Wagner, practically all of the iron works treat the molten slag as it 

 flows from the converter with hot quartz sand, with the result that the availability of 

 the phosphoric acid is increased from 10 to 30 per cent.& 



CHEMICAL COMBINATION OF PHOSPHORIC ACID IN SLAG. 



The form in which the phosphoric acid exists in the slag has never been fully 

 explained. It was formerly supposed that it was combined with lime as a tetracal- 

 cium phosphate and the latter being less stable than tricalcium phosphate became 

 easily available to the plants by being decomposed, under the influence of dilute 

 acids, into the calcium salt of the dissolving acid and dicalcium phosphate. The 

 tetralime phosphate, however, has never been made artificially, c although it has 

 been recognized by the aid of the microscope in the slag and exists as a mineral under 

 the name of isoklas. 



M >re recent investigations having shown, as already indicated, that those slags richest 

 in silicic acid of like phosphoric acid content gave the best results, the conclusion 

 followed that a part of the lime must be in the form of lime silicate. It is now gen- 

 erally held, especially by Wagner, d that the phosphoric acid is combined in the slag 

 as a double salt of tricalcium phosphate and calcium silicate and that in this form 

 the roots are able to utilize it. It is also believed probable that some of the phosphoric 

 acid is more or less united with iron as a basic iron phosphate. 



THE USE OF PHOSPHATIC SLAG. 



Basic slag has been shown to work especially well upon sour marsh and meadow 

 lands, upon porous, well-aired soils rich in humus, and upon sandy soils deficient 

 in lime. 



When a rapid development of the crop is not desired, the slag may be used exclu- 

 sively in place of acid phosphate. On the other hand, in cases when it is feared that 

 the crop will not mature early enough upon heavy, cold land and in high altitudes 

 where the season is short, acid phosphate should be given the preference. 



The phosphoric acid in slag is comparable in its quickness of action to nitrogen in 

 barnyard manure, tankage, and green crops; and the phosphoric acid in acid phosphate 

 to the action of nitrogen in nitrate of soda. A combination of slag and sulphate of 

 potash (500 pounds of slag and 100 pounds of potash) has been found to work especially 

 well upon grass land and to be very favorable to the development of clover. 



a Method slightly modified from the original. Present method described in Konig's 

 Untersuchung landwirtschaftlich und gewerblich wichtiger Stoffe, 3d ed., pp. 173-174. 



& Loc. cit.; also Wagner, Anwendung kiinstlicher Dimgemittel, pp. 74-75. 



c Hilgenstock, Jahresber. chem. Technologic, 1887, p. 282, after Adolf Mayer, loc. 

 cit. 



cit. 



