PHYSICAL CONDITION OP SOILS 367 



remembered, is in the majority of instances accompanied by a 

 leaching process, whereby original soluble compounds, or new 

 soluble compounds formed during the process of decomposition, 

 are gradually removed. The final result is therefore, as already 

 many times noted, a residue consisting of the least soluble con- 

 stituents, and which forms the ordinary surface soil. Even in 

 cases where the actual amount of soluble matter is greatest in 

 a soil, the apparent excess may be due to water of hydration 

 and to the large amount of sesquioxide of iron, the latter being 

 practically insoluble in meteoric waters so long as there is a 

 free supply of oxygen, though readily soluble in hydrochloric 

 acid. These conclusions are based upon the table on p. 366 in 

 which the total percentage loss on ignition, minus the ignition 

 in the insoluble residue, is tabulated with the soluble matter. 



(3) Physical Condition of the Soil. — Chemically, as previ- 

 ously noted, a soil differs from the parent rock in the amount 

 of leaching it has undergone, and in the finely comminuted and 

 more or less decomposed condition of its particles. There are 

 other distinctions, not the least important of which are its state 

 of loose coherency and porous condition due to interstitial air 

 spaces. It has been estimated by Whitney^ that the approxi- 

 mate number of grains in one gramme of soil varies between 

 2,000,000 and 15,000,000, the lowest estimate being that for a 

 sandy soil containing only some 4.77% of material in such an 

 extremely fine state of comminution as properly to be classed 

 as clay, while the highest number is that in a sub-soil contain- 

 ing some 32.45%. Our interest in these remarkable figures is 

 still further heightened when we are called upon to remember 

 that these grains are not in actual contact, but each separated 

 from the other by thin films of moisture, or, in a dry soil, by 

 actual air spaces. That such spaces exist is easily proven by the 

 fact that any soil may be greatly diminished in bulk by pressure. 

 The amount of this empty space is naturally quite variable, but 

 it is estimated to constitute on an average some 50%, by volume, 

 of the soil That is to say, a cubic foot of soil, in its natural 

 condition, contains an amount of space between its grains, filled 

 by air or water, equal to one-half the entire mass. 



These figures are given, not merely to illustrate the won- 

 derful degree of comminution reached in rock-weathering, but, 

 also, and what is of more importance from the standpoint of 



^Bull. No. 4, IT. S. Dept. of Agriculture, Weather Bureau. 



