/(cati 



The Soil. 45 



beans belong, is able, through the agency of nodule forming bac- 

 teria growing on the roots, to derive nitrogen from the inex- 

 haustible stores of the atmosphere. This peculiar property of 

 leguminous plants is quite distinct from the requirements of all 

 other farm crops, which acquire their nitrogen from the nitrogen 

 compounds already in the soil. This fact is of the greatest im- 

 portance to agriculture, for it is " Nature's principal method'' 

 of increasing the nitrogenous food in the soil. The nitrogenous 

 compounds stored in such plants eventually become a part of the 

 soil through their decay, thus furnishing food for other plants 



increasing the fertility of the soil. 



The constituents of soil. A popular and convenient classifi- 

 ation of soil constituents is the following : 



(1) Sand mainly silica, but containing small fragments of 

 feldspar, mica, and other minerals. 



(2) Clay mainly kaolin, but containing small fragments of 

 silica, feldspar, etc. 



(3) Limestone finely divided calcium carbonate. 



(4) Humus the somewhat indefinite nitrogenous and carbon- 

 aceous material, brown or black in color and resulting from the 

 decay of plants. A brief description of these materials will now 

 be given. 



Sand is of low specific heat and has the lowest water retaining 

 power of all soil constituents. It is practically valueless as a 

 plant food, except for the small amounts of potassium, calcium 

 and iron contained in the mineral fragments mixed with the true 

 sand. Its physical properties often have valuable effects upon 

 the character of a soil, particularly with regard to friability, and 

 its relation towards water and heat. 



Clay in its pure form is free from plant food, but is usually 

 well supplied with potash, because of the feldspar present. Com- 

 mon clay contains quartz and calcium carbonate (as in marls) 

 in addition to feldspar. The true clay (kaolin) acts as a cement 

 to the other mineral grains. 



It is thought that even in the purest clay there is a small quan- 



