202 THE LURE OF THE LAND 



gard is of importance because the humus of the soil is 

 the chief repository of one of the most costly of the 

 plant foods, viz., nitrogen. 



It must not be inferred however that such soils are 

 very deficient in nitrogenous food since it appears that 

 the humus of the arid soil has a higher percentage of 

 nitrogen than that found in soils with abundant water 

 supply. 



The real composition of humus is a matter which has 

 never been definitely determined. Composed as it is 

 of many different but closely related substances it has 

 been difficult to isolate and determine them. 



At the present time we can only regard the various 

 forms of humus bodies as mixtures of many substances 

 mostly of an acid nature, and resulting from a gradual 

 decomposition of organic matter under conditions which 

 partially exclude free access of oxygen. 



Whether humus takes any direct part in the nutri- 

 tion of green plants is not definitely known. De Saus- 

 sure held that soluble humus was assimilated directly 

 by vegetables and this is probably the case with those 

 plants, such as mushrooms, which are devoid of chloro- 

 phyl. 



Liebig held that humic acid is not absorbed by plants 

 and that as such it does not form a part of their food. 

 Humic acid being of a colloidal nature is not well suited 

 to engage in the translations which take place so readily 

 with crystalline bodies in solution and which readily 

 change their location under the stress of osmotic pres- 

 sure. 



Petermann found however that such bodies could be 

 dialyzed in certain circumstances and he succeeded in 

 passing through membranes highly nitrogenized organic 

 matter of an amber color. 



