N. M. Comber 377 



{d) It is well known that ignition ultimately depresses the solubihty 

 of iron and other soil constituents. Clearly therefore the rise in solubility 

 — or rather in the amount dissolved — takes place only in the early stages 

 of ignition. By means of the colorimetric test the relative amounts of 

 iron extracted from soils and clays after ignition for various times was 

 measured. In nearly all the soils and clays examined the solubility of 

 iron in acid was on the dechne before 10 minutes of ignition over a bunsen 

 flame. Only in one experiment with a very fat clay did the increase 

 continue for as long as 10 minutes. The following arbitrary figures are 

 typical. 



Table II. 



Relative amounts o/Fe extracted by N/HClfrom soils ignited for 



different times. 



(e) One of two equal portions of a clay subsoil was partially ignited. 

 Each portion was then extracted with A^/5 HCl. Equal volumes of the 

 filtrates were boiled, cooled and titrated with N/5 NaOH using first 

 methyl orange and then phenolphthalein. The difference between the 

 methyl orange reading and the phenolphthalein reading was then taken 

 as a measure of the acid-salt-forming bases (FcgOg and AI2O3) in solution, 

 and the difference between the phenolphthalein reading and the titre of 

 the original acid was taken as a measure of the neutral-salt- forming bases 

 which had gone into solution. 



The experiment was also carried out with the same subsoil at its 

 maximum base absorbing power (that is, after extraction with acid as 

 previously described). 



The following table shows, in respect of two subsoils, the actual 

 differences of burette readings for 100 c.c. filtrate and the ratios calcu- 

 lated therefrom. 



Table III. ■ ■ 



Relative amounts of neutral- salt-forming bases {n) and acid-salt-forming 

 bases (a) extracted from tivo subsoils, before and after ignition, by iV/5 HCl. 



Before ignition After ignition 



A A 



r f ^ I — ■ ^ 



Burette readings Burette readings 



, '^- , Ratio , * ^ Ratio 



n a n : a « a n : a 



A Original subsoil 19-2 3-6 1:019 10-8 l.i-S 1:1-43 



E.xtraoted „ 3-2 7-2 1 : 2-25 5-6 32-0 1 : 5-70 



B Original subsoil 30-8 9-2 1 : 0-30 21-2 40-4 1 : 1-91 



Extracted „ 92 160 1:1-74 11-2 47-2 1:4-21 



