124 JOURNAL OF THE 



limits of variation in analytical work. With soil 3, which con- 

 tains the largest percentage of organic matter, we notice a 

 decrease of insoluble phosphoric acid which, though slight, is 

 decided. This important result is due undoubted 1 v to the presence 

 of vegetable matter. It has been seen that organic matter pic- 

 vents the decomposition of the soluble phosphoric acid. Now 

 we see that another result is reached which is much more im- 

 portant; for the organic acids when formed actually act on the 

 insoluble phosphoric acid present in the fertilizer, effect its 

 decomposition, thus rendering it available to the plant, or it may 

 be that by so preventing the precipitation of soluble phosphoric 

 acid it furnishes the soluble with the means of acting on the 

 undecomposed tricalcic phosphate to render it soluble in ammo- 

 nium citrate. I am inclined to the former supposition ; in either 

 case the effect is the same. 



On the other hand, in regard to soil 2 with the large percentage 

 of iron and alumina the effect is just the reverse, for the presence 

 of much ferric oxide and alumina renders some of the phosphoric 

 acid originally available to the plants useless for such purposes, 

 because some of the soluble phosphoric acid is changed to the 

 insoluble. This reversion has indeed gone on till at the end of 

 the 27th week it amounts to 2.15 per cent., or, in other words, 

 16.90 per cent, of the whole soluble phosphoric acid originally 

 present has been converted into the insoluble form. This is due, 

 I think, to the formation, with the large excess of ferric oxide 

 and alumina in the soil, of basic phosphate of iron and alumina, 

 which is, as stated by A. Millot,* not soluble iu ammonium 

 citrate at H5° C, as is the case with the normal phosphate. 



The reverted phosphoric acid in the above series, embodying 

 as it does the phosphoric acid soluble only in ammonium ci- 

 trate, combines the change which has taken place in the soluble 

 phosphoric acid by which a part is rendered insoluble in water, 

 and also the change, if any, by which the phosphoric acid insolu- 

 ble in ammonium citrate has become soluble in that liquid; 

 consequently it is of much interest, and attention is called to it. 



*Bnll. de la Soc. Chim., 1880, p. 98. 



