so EXPERIMENT STATION. [Jan. 



ability of the i^hospboric acid is improved 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 Avith lime as a tetra-calciura phosphate, and that 

 this latter compound, being less stable than tri-calcium phos- 

 phate, under the influence of dilute acids became easily available 

 to the plants by being decomposed into the calcium salt of the 

 dissolving acid and bi-calciuin phosphate. The tetra-lime phos- 

 l)hate, however, has never been made artificially,^ although it 

 has been recognized by the aid of the microscope in the slag, 

 and exists as a mineral under the name of isoklas. 



More recent investigations having sho^^'n, 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 

 generally held, especially by Wagner,'^ that the phosphoric acid 

 is combined in the slag as a double salt of tri-calcium 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 j)hos- 

 phoric acid is more or less united with iron as a basic iron 

 I)hosphate. 



The Use of Phosphatic Slacj. 



Basic slag has been shown to work especially well upon sour 

 marsh and meadow lands, upon porous, well-aired soils rich in 

 humus, as well as upon sandy soils deficient in lime. 



When a rapid development of the crop is not desired, the slag 

 nuiy ])e M^^cd exclusively in jjlace of acid phosphate. On the 

 othei- hand, in easels Avhen it is feared that thf cmp will not 

 mature early enough, upon heavy, cold land and in high alti- 

 tudes, where the season is short, acid phosphate should be given 

 the preference. 



» Already citefl, p. 28; alf=io, Anwendung Kiinstlicher Diingemittel, vlerte Auflage von 

 Wnsrner, pp. 74, 1'i. 



2 nn-mstock ; .TaliroBhcr. Clicm. Technologle, 1887, p. 2S2, after Addf Mayer, already 

 cited. 



3 Wagner, already cited. 



