24 THE PRINCIPLES OF 



is better spoken of as an " available " condition of plant food. 

 This requires time. Let us then fully appreciate the fact 

 that we have in a soil, as we see it to-day, a certain pro- 

 portion of soluble matter ; that is, of plant food, or mineral 

 constituents in a state in which they can be dissolved by 

 water, and take their place as constituents of the sap of plants. 



Next in order, we have an exceedingly pulverulent and 

 impalpable material in all soils, which cannot be produced 

 by the pestle and mortar. We have in the second place 

 for we have done at present with the soluble materials of 

 soils a large mass of impalpable smooth material in such a 

 finely-divided condition that it feels " greasy " to the touch 

 not " gritty," but greasy. This, in a word, is " clay." It has 

 been produced during the lapse of vast periods of time, 

 assisted by the continued operation of forces already named. 

 It is largely composed of silicate of alumina. It gives a 

 peculiar adhesiveness to soil, and it has certain properties 

 which will shortly occupy our attention. 



In the third place, we find distributed through the finer 

 portions of the soil an abundant stock of coarser material, 

 from sand to large stones, composing the bulk of the soil 

 whose potential value has already been insisted upon. The 

 general structure of the soil may be briefly summed up as 

 follows first, a sufficient, but very small amount of soluble 

 plant food ; secondly, a quantity of finely-divided, pulverulent 

 material which is useful in holding the soluble matter, and 

 produces a moist condition of the mass suitable for the 

 healthy development of the root, and also all grades of 

 compact material fitted for the recouping of the fertility of 

 the soil. These different states of matter sufficiently indicate 

 that the soil is a product of many complex forces, aided by 

 the lapse of time. 



In the next place, before a soil can be fertile, it must 

 contain vegetable matter. Originally, soils must have been 



