COMPOSITION AKD FERTILITY OF THE SOIL. 21 



one per cent, of its substance is, at any moment, in a fit 

 condition for nourishiug our crops. The great bulk of 

 it is unayailable to the plant at any one time, and is only 

 slowly liberated by the action of air, of moisture, of heat, 

 and of manure. It is on the rate at which this libera- 

 tion of plant food takes place that the natural fertility of 

 the soil may be said, in a great measure, to depend. 



A soil may contain abundance of phosphoric acid, 

 potash, magnesia, etc., and yet be infertile, if these exist 

 in the soil only as apatite, felspar, and serpentine, be- 

 cause these minerals do not }ield their elements to the 

 solvent agencies of the soil or plant rapidly enough to 

 furnish the required amount of jDlant food. Nitrates and 

 ammonia salts, which are the natural sources of nitrogen 

 to crops, never need be present in the soil in more than 

 the minutest proportion. It is only requisite that they 

 be gathered or generated there as rapidly as crops require 

 them. The process of nitrification, whereby inert or un- 

 assimilable nitrogen existing in the soil is converted into 

 nitric acid, thus becomes of tho utmost agricultural 

 importance. 



On the other hand, the nutritive substances which are 

 yielded naturally by the soil may be in a state so soluble 

 as to be very liable to waste before they can be taken up 

 by the roots of the growing plants. 



Everyday experience proves that soils differ greatly in 

 these respects. Nearly all the materials which go to 

 make up the structure of the earth's crust are such as to 

 aSord, by their decomposition, a soil fit for the support 

 of vegetable life ; but all rock-formations do not furnish 

 equal amounts of these materials ; and, while all soils 

 have considerable power of retaining in their pores even 

 the most soluble substances, some part with them too 

 readily, and others retain them too firmly, or only part 

 with them when exposed to various preparatory processes. 



