THE CONSTITUTION OF THE SOIL 



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TABLE XXXI. ANALYSES OF THE ORGANIC MATERIAL EXTRACTED BY ALKALIS FROM 

 SOIL (OFTEN CALLED HUMUS, SOLUBLE HUMUS, ACTIVE HUMUS, MATIBRE NOIRE, 

 ETC.). 



These results are in accordance with the general fact brought out 

 by the field observations, that under similar conditions the humus 

 mixture is tolerably constant, but it is quite clear that each set of soils 

 gives up a different lot of substances to alkalis. Indeed simple varia- 

 tions in the time or the method of extraction cause differences in the 

 results even from the same soil. 1 



The fact that humus is not a definite compound but a complex in- 

 definite colloid was established by van Bemmelen in a remarkable papei 

 in 1888 (19). Baumann's researches (9, 10) have carried the subject a 

 good deal farther and it is now known that " humus " freshly precipi- 

 tated by acids from an alkaline extract of soil, compost, etc., possesses 

 the following colloidal properties : 



(1) Very high capacity for retaining water. 



(2) Extraordinary shrinkage on drying. 



(3) Reversibility, i.e. the freshly precipitated material redissolves 

 when the precipitant is washed away. 



(4) Is coagulated by acids and salts, the electric current and frost. 



(5) Decomposes salts calcium carbonate, calcium phosphate, etc. 



(6) Forms difficultly soluble and easily decomposable colloidal 

 mixtures with other colloids. 



(7) Masks certain ion reactions (e.g. Fe cannot be detected by 

 potassium ferricyanide, etc.). 



(8) Forms absorption compounds. 2 



1 This is shown by the analyses of Miklauz (Zeit.f. Moorkultur u. Torfverwertnng, 

 1908, 285) and Mayer. Sostegni (268) in 1886 had shown that humus is readily fractionated. 



2 The reddening of litmus paper is attributed to the absorption of alkali from the paper 

 and consequently Iiberati6n of the red compound. But there is no reason why acids should 

 not occur in the humus mixture. 



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