72 



SOIL CONDITIONS AND PLANT GROWTH 



in 1888 (19). Baumanti'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 : — 



(i) 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.^ 



Schreiner and Shorey (250) have attempted a resolution of the 

 " humic acid " and of the " crenic acid " (the part not precipitated by 

 HCl) and have obtained the following substances from the alkaline 

 extract : — 



Methods for estimating the amount of " humic acid " or " humates " 

 in the soil have been devised and numerous analyses have been made, 

 but no conclusions of any consequence have been drawn from the 

 results. On the Rothamsted plots about one-half of the total nitrogen 

 is contained in compounds soluble in alkalis, the proportion varying 

 but little with the scheme of manuring. It has been maintained by 

 Grandeau (i 1 1) and Hilgard (133) that these compounds are by far the 



1 The reddening of Utmus paper is attributed to the absorption of alkali from the paper 

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

 not occur in the humus mixture. 



