(S) = 10 Sacramento. Sacramento Co. (T) = 506 Gila. San Diego Co. (Z) = Hygroscopic Moisture (+7 = to +21°C) ===========+================== | CALIFORNIA. +------------------ Designation | River Deposit. of +---------+-------- Materials. | | | (S) | (T) | | -----------+---------+-------- Grit | | | | { | | { | | .13 Sand { | | .15 { | | .11 { | .32 | .75 | | { | 3.16 | 2.51 { | 10.27 | 8.32 Silt { | 13.67 | 12.64 { | 13.11 | 11.28 { | } 43.61 | } 31.79 { | } | } | | Clay | 12.06 | 23.97 +---------+-------- | 96.20 | 91.71 | | (Z) | 9.18 | 9.26 Ferric Oxid | | -----------+---------+-------- _Number of soil grains per gram._—It is of some interest to consider the number of grains of different sizes that may be contained in, _e. g._, a gram of soil. If for this purpose we assume all the soil grains to be spherical, we shall obtain the minimum figures, for most other shapes will pack more closely. King (Physics of Agriculture, p. 117) calculates such figures for different grain-sizes, assuming the density to be that of quartz (2.65), with the result that while with a diameter of one millimeter (1-25 inch) the number of grains would be 720, and with one-tenth of a mm. 720,000; if made of the finest particles only, viz., one thousandth of a mm., the number would be 720,000 billions. Probably few of the clayey soils we ordinarily deal with are of this order; it is doubtless approached in certain fine plastic clays. _Surface afforded by various grain-sizes._—The amount of _surface_ afforded by a similar amount of soil must naturally be considered in this connection, since upon it depends not only the amount of moisture which the soil may hold in the form of superficial films, but also the extent of surface upon which the weathering agencies as well as the root hairs of plants may act. Quoting again from King’s work, we find on the same premises given above for the _number_ of grains, that their _surface_ would in the case of grains of one mm. diameter be eleven square feet per pound (about half a pint) of material; while in the case of the finest grade we should have 110,538 square feet, or more than two and a half acres.