STOEING OF SUPERPHOSPHATE. 139 



•soluble phosphoric acid, and the water soluble phosphoric acid, 

 enter into solution simultaneously. It results that when the 

 •analysis is made by the citrate method (without a separate estima- 

 tion of the phosphoric acid soluble in water) there are confused 

 together both the phosphoric acid soluble in water, of great fertiliz- 

 ing value, and the phosphoric acid combined with iron, which has 

 but very little. The citrate method is, therefore, uncertain. The 

 results may represent bodies which are very good for plants if they 

 -consist of monocalcic phosphate or free phosphoric acid which are 

 ■dissolved in this reagent. They may, on the other hand, represent 

 substances which only have a very mediocre action, if they are 

 phosphates of iron, called retrograde phosphates, which have passed 

 into solution. Analysts ought, therefore, to give preference to 

 estimating by the solubility in water as in Great Britain, Germany, 

 and the United States. They ought, at least, to estimate separately 

 the soluble in water and likewise separately the citrate soluble." 



It is now admitted, without going further, that there is no great 

 difference from a fertilizing point of view between the water soluble 

 phosphoric acid and the citrate soluble. There is, however, a sharp 

 line of demarcation between the two products ; the first is im- 

 mediately assimilable by plants, the second is rendered soluble and 

 assimilated in the soil to the same extent as the phosphoric acid 

 in basic slag. But then there is no occasion for the difference in 

 price between the selling price of superphosphates (supposing them 

 to be partially retrograded since they are sold' as such) and basic 

 slag. Now this difference is considerable. On 1 January, 1909, 

 for basic slag at Paris stations the price of the unit of phosphoric 

 acid in basic slag and superphosphates was exactly in the ratio of 

 3 to 4, which makes the phosphoric acid in retrograde phosphates 

 25 per cent dearer than the citrate soluble phosphoric acid of basic 

 ■slag. It is asserted, it is true, that if retrogradation has not already 

 occurred in the superphosphate before spreading it occurs rapidly 

 in the soil. The validity of this assertion will be examined later 

 on in giving Dr. P. Wagner's opinion on the matter. In Germany 

 superphosphates are sold by their content per cent of water 

 soluble phosphoric acid, the citrate soluble not being taken into 

 consideration. Now, in superphosphates containing 1 per cent of 

 insoluble phosphoric acid this latter is two-thirds soluble in water ; 

 the manufacturer is thus prejudiced. In Spain and in the Medi- 

 terannean regions the customs are the same as in France. In 

 Austria-Hungary customs vary with the localities ; in some super- 

 phosphates are sold according to their content of water soluble 

 phosphoric acid, in others according to their content of citrate 

 soluble. It is the same in Eussia. In America sales are also 

 Teased on the citrate soluble which enables manufacturers to work 

 low grade phosphates w4th a high percentage of sesquioxides At 



