II.] 



THE NATURE OF HUMUS 



47 



Locality. 



1. Canada 



2. Canada 



3. Russia . 



4. Rhodesia 



5. Monte Video 



6. New Zealand 



Indian Head 



Wide Awake 



Ploty 



Salisbury 



{ 



{ 



Tararua 

 Mountains 



Description of 

 Soil. 



Black Prairie 



Black Steppe 



Black Vlei 



Black 



44 Camp " Soil 



Black Sandy 



Pasture 



) 



Against this, Berthelot and Andr6 have investigated 

 the ratio of carbon to nitrogen in the different portions 

 of the humus which can be dissolved by alkalis or acids, 

 and they find that the most soluble portions contain the 

 highest proportion of nitrogen. It does not, however, 

 follow that the substances most soluble in acids or 

 alkalis are necessarily those which will most readily be 

 converted by bacteria into a form available for plants, 

 and, on the whole, the evidence seems to show that a 

 humus rich in nitrogen will yield it very slowly to crops. 



Humus acts as a weak cement and holds together 

 the particles of soil, thus it serves both to bind a coarse- 

 grained sandy soil, and, by forming aggregates of the 

 finest particles, to render the texture of a clay soil more 

 open. In determining the sizes of the constituent 

 particles of a soil, the " mechanical analysis," it is desir- 

 able to remove the humus as far as possible, and so 

 break up these temporary aggregates. 



. Sampling of Soils. 



The first step in the analysis of any soil, mechanical 

 or chemical, consists in obtaining a sample that shall 

 adequately represent the land in question. 



