634 
Journal of Agricultural Research 
Vol. XXVII, No. 9 
might include most of the factors that influence percolation rate. The 
relationship of soil texture to water percolation is too obvious to require 
much elaboration. When the soil is composed chiefly of large particles 
the interstitial spaces are correspondingly large and free movement of 
water is possible. When the soil is composed largely of small particles, 
the interstitial spaces are much smaller and the movement of water is 
consequently retarded. The water relations of the soil may be more 
easily explained and understood by recognizing three main classes of 
soil material: (i) Sand, (2) clay, and (3) soluble salts. 
The class designated as sand may be taken to include all material of 
whatever size that is crystalline and nearly or quite insoluble in water. 
The class designated as clay includes the noncrystalline material, also 
nearly or quite insoluble in water but differing from the sand in that it 
is very much more finely divided. The soluble salts are mostly crystal¬ 
lized when dry and dissolved or dissociated when wet. In this classifi¬ 
cation of the soil material the matter of the size of the particles is not 
OV/L. /VC/. ——» OU/t 
MEPAS LEPCH/A/G PPTE-O.S5SC.C. MEP/V LEPCH/AAS PPTE-0.049C.C. 
PE/? M/A/UTE OP 254 C.C. /A//2 HOOPS. PEP M/A/OTE OP J5.6C. C. /A//2HOOPS". 
Fig. 5.—Quantity of water leached through different soils in 12 hours. 
emphasized. The crystals, particles, or aggregates may be larger or 
smaller. The distinctions are made on the basis of the reactions with 
water. 
With respect to these reactions with water, the material classed as 
sand is regarded as nearly or quite inert. The clay is definitely reactive 
in that though not soluble in water certain of its constituents may enter 
into reactions with substances dissolved in the water. The soluble salts 
are more completely reactive than the clay. They pass into solution and 
dissociate. They move freely with the movement of water in the soil 
and even move somewhat by diffusion independently of the movement 
of the water. 
Of the three classes of soil material just described, the clay exercises 
the most important influence on the movement of water through the soil. 
The extent and character of this influence varies greatly, depending upon 
the physical properties of the clay rather than upon the quantity in pro¬ 
portion to the total soil mass. The physical properties of the clay are 
