•288 Note on the Mechanical Ajiali/sis of Humus Soils 



Attempts were therefore made to effect the destruction of organic 

 matter without attacking clay or fine mineral particles. This requires of 

 course a neutral reagent. Th; first oxidising agent tried was ammonium 

 persulphate. Considerable oxidation of the organic matter can be 

 effected by the use of this reagent in aqueous solution but the sulphuric 

 acid liberated dissolves mineral material and it is necessary to maintain 

 neutrality by repeated additions of alkali. There is the further dis- 

 advantage of the large quantities of ammonium sulphate introduced 

 whicii has to be removed before the soil can be subjected to mechanical 

 analysis. With this somewhat inconvenient method, however, it was 

 found that considerably higher figures were obtained for the clay fraction 

 than by the ordinary method. Some results obtained on a typical Welsh 

 soil using different methods of dispersion may now be given. 



I'rcliiniiiary treatment Clay % 



l)r<Unarv method 6-9 



One hour digestion with HCl (d. 112) 13-5 



Digestion «ith NaBiO (Atleiherg) 10-.")* 

 Ammonium persuljihate oxidation (10 : 20). 



No neutralisation 1304t 

 Ammonium persulphate oxidation {'> : 20). 



Maintained approx. neutral 160 



* 7-58 "{, of material dissolved. f ''"^S °o of material dissolved. 



Microscopical examination of the fractions obtained showed striking 

 differences. The fine silt and silt obtained by the ordinary method con- 

 tained considerable quantities of amorphous material. The same fractions 

 obtained after preliminary oxidation were seen to consist entirely of 

 crystaUine mineral matter. Similar fractions are obtained from raw 

 clays and soils poor in organic matter. 



The use of ammonium persulphate proved inconvenient in practice 

 and hydrogen j)en)xide was therefore tried as an oxidising agent. After 

 .some prehniiiiary trials it was found that the most convenient method 

 of oxidising was as follows. Ten grams of soil are weighed into a beaker 

 of 600-700 c.c. capacity. Fifty c.c. of hydrogen peroxide (20 vols.) are 

 added and the beaker placed on a boiling water bath. A vigorous 

 reaction soon takes place with considerable frothing owing to the evolu- 

 tion of oxygen. The contents of the beaker are stirred from time to 

 time. After about 30 minutes the reaction dies down and a further 

 25 c.c. of peroxide are added, the froth adhering to the sides of the 

 beaker being washed down with a small volume of water. After a further 

 15-20 minutes' heating the reaction is generally complete and frothing 



