153 

 PART IV. COMPARISON OF EXTRACTION METHODS. 



Although there have been, during these investigations, but very few 

 opportunities of comparing the relative solvent powers of different extrac- 

 tion media upon one and the same soil, it will still be possible to draw 

 some conclusions from the analyses of soils which may well be assumed to 

 be of similar chemical composition. 



Of the different methods applied in the course of the investigations, 

 two, i.e., Methods III. and IV., may be expected to result in a greater sol- 

 vent action than that adopted as the standard method, and one, Method 

 v., would obviously exert a lesser dissolving power. For these reasons' it 

 seemed needless to seek by experiment for a confirmation of what was so 

 latent; at the same time the question still remained open in what ratio 

 the solvent action of the one exceeded that of the other, and whether, in 

 fact, there could be said to be a definite ratio at all. Less evident, how- 

 ever, did it appear whether there would be any appreciable difference be- 

 tween Methods I. and II., and, although here again no comparative tests 

 have been made on any particular soil, yet soils of such similar type have 

 been treated, some by the one and some by the other method, as to 

 afford a fairly safe means of comparing the two methods. 



The only instance of comparison between Method IV. (Hilgard's) and 

 any other, is that afforded by the four soils 21, 22, 23, and 24 from the 

 experiment station at Robertson. These soils were extracted by Methods 

 I., IV. and V., and in one case also a determination of lime in an aqueous 

 extract of the soil was made. Comparing Methods I. and IV. we find 

 that, with regard to lime, Method IV. extracted practically 110 more than 

 what was extracted by Method I., in fact, it may be said that in the case 

 of No. 21 and also of No. 24, which was a patch in the area represented 

 by No. 22 the proportions extracted were really identical. Apparently 

 in such cases Method I. extracted the entire reserve stock of lime, using 

 the word " reserve " as Prof. Hilgard uses it. The lime, it must be noted, 

 occura here largely as carbonate. As far as concerns potash, the quan- 

 tities extracted by Method IV. were, in soils 21 and 22 about five times 

 those removed by Method I., and in No. 23 nearly four times as much, 

 while from the brack soil, No. 24, Method IV. took out nearly seven 

 times as much potash as Method I. removed. From the results with re- 

 gard to potash obtained by Method IV. one might be disposed to infer 

 that the soil was very well supplied with potash in an available form, 

 but practical experiments with cereals on the farm have shown that 

 to supply the soil with phosphates was unavailing unless potash was also 

 given, indicating that although phosphatic fertilisers may increase the 

 stock of phosphates up to the limits of adequacy, yet the law of the 

 minimum still operated in respect to potash. It also illustrates the re- 

 mark already made, that in this Colony results arrived at by Method IV. 

 rather overstate the stock of available plant food in the soil.* 



Turning now to Method III. we may expect to find an even wider 

 divergence from the results of Method I., inasmuch as the solvent action 

 on the soil is of necessity still more energetic. No comparative tests 

 between these two methods on absolutely identical samples of soil have 

 been made, but Nos. 1, 2, and 3 of the George Division soils, are, for 

 all practical purposes, the same as Nos. 20, 21 and 22 in the same list: 

 the former three were extracted by Method III., and, as pointed out when 

 dealing with the soils of that Division, yielded much higher figures than 

 the latter three, which were treated by Method I.f The soils of George, 



See page 12. f See page E9. 



