NOTE ON THE MECHANICAL ANALYSIS 

 OF HUMUS SOILS. 



By gilbert WOODING ROBINSON, M.A. 



Adviser in AgricuUuml Chemistri/, University College 

 of North Wales, B<t.ngor. 



It is generally recognised that the mechanical analysis of soils containing 

 large quantities of organic matter presents considerable difficulties and 

 that in the case of peaty soils mechanical analysis can have Httle 

 significance. Apart from the masking effect of organic matter on soil 

 properties which will naturally vitiate any correlations with mechanical 

 composition, the actual dispersion of humus soils is difficult owing to the 

 cementing action of humified organic matter, whereby soil particles are 

 aggregated together into compound structures which resist ordinary 

 methods of dispersion. Various methods have been suggested for the 

 destruction of organic matter as a preliminary to mechanical analysis. 

 Atterberg^ recommends the use of alkaline sodium hypobromite solution. 

 In the case of diatomaceous soils, however, oxidation of the organic 

 matter with hot nitric acid {d. 1-14) is recommended. For soils free 

 from calcium carbonate the use of hydrochloric acid (d. 1-12) is suggested. 



In view of the reactive character of some of the finer soil constituents 

 these and similar methods would appear to be open to objection. Any 

 acid treatment is certain to result in the partial solution of clay and 

 finely divided minerals, whilst alkaline treatment results in the attack 

 of silica or colloidal silicic acid. 



The soils of North Wales are generally higii in organic matter and 

 the writer has long suspected that the figures for clay obtained in the 

 ordinary mechanical analysis might be too low and that a certain 

 amount of clay was reckoned wath the other fractions. Two circum- 

 stances favoured this view. In the first place, the majority of the soils 

 of this area are derived from rock material w^hich might be expected 

 to furnish rather heavy soils. Further the analyses of the hydrochloric 

 acid extracts for such soils show high figures for silica, iron oxide and 

 aluminium oxide such as might be expected from clay soils. 



■ Int. Mitt. Bodeiikunde, 1912, 2, 312-342. 

 Joum. of Agric. Soi. xii 20 



