SOILS AND FERTILIZERS. 367 



that is helped along by the roots themselves, which by their acid secre. 

 tions attack and dissolve mineral matter previously insoluble. This 

 enters the plant, subserving the physiological processes of its develop- 

 ment, and returns at its death to the soil from which it was originally 

 taken, but in altered forms, in combinations not previously existing, 

 more soluble, and therefore more readily accessible to subsequent 

 plants that may need it. 



How considerable this factor is even for a single season in modify- 

 ing the character of a soil will appear further on ; and be it remembered,, 

 it is not only the carbonaceous matter of the plant, not only its altered 

 mineral combinations that produce the change, but also the nitrogen, 

 originally absent from the purely mineral soil, and collected success- 

 ively from the atmosphere, that works a modification in a most import- 

 ant direction. Heat and cold, dryness and moisture, chemical action 

 of the roots of the plants upon the minerals of the soil, and the incor- 

 poration into it of their bodies, with their multiform proximate compo- 

 nents, including their nitrogenous constituents, and air with its oxygen 

 and carbonic acid, bring about finally a result as beneficial to us as it is 

 natural, and one, withal, that is lasting if knowledge and prudence 

 recognize and deal with it in the proper manner. 



Composition. — The soil, then, as it results by the processes indi- 

 cated, is a mixture of fine and coarse material mixed with a greater or 

 smaller amount of organic humus-like substance, holding a variable 

 but large quantity of moisture in its composition. The percentage of 

 this organic matter is usually low, amounting in our soils rarely to more 

 than 10 in 100, but being more frequently only 4 to 5. The water 

 reaches a higher figure, and though 15 per cent in a soil will support 

 a ripening crop, double the amount is preferable in the early season, 

 and more for a time advantageous rather than otherwise. The nitro- 

 gen, though the accumulation of ages, is but a fraction of one per cent, 

 and the average composition of our soils during the summer season 



is as follows : 



4.0 p. c. organic matter, 



O.l p. c. nitrogen. 

 20.0 p. c. water. 

 75.9 p. c. mineral matter. 



The mineral matter is the matter derived from the rocks that 

 originated the soil, modified, as suggested, by the action of weather and 

 the growth of plants. It consists of panicles of quartz sand, feldspar, 

 mica, and a variety of mineral species, easily recognized under the 

 microscope, and, in addition, of clay, soluble silica, simple and double 

 hydrous silicates, the result of the alteration of soil and the addition 



