EXTENSION COURSE IN SOILS. 19 
of dividing a block of wood 1 foot on each edge by sawing it through 
in the middle in the three directions. This will produce eight cubes 
6 inches on each edge. The large cube will contain 6 square feet of 
surface; each of the smaller cubes will measure 6 inches, or one-half 
of a foot, on each edge and will contain one-fourth of a square foot 
on each surface. This multiplied by 6, the number of surfaces on a 
cube, then by 8, the number of small cubes, gives 12 square feet of 
surface. The area is therefore doubled by the division. The same 
division of each of these smaller cubes would again double the area, 
and so on. In the same way the division of a grain of sand into 
eight smaller particles having one-half the original diameter would 
multiply the entire surface exposed by 2. A cubic foot of coarse, 
sandy soil has about 40,000 square feet of surface, or nearly 1 acre. 
A cubic foot of sandy loam has about 65,000 square feet of surface, 
a cubic foot of clay loam nearly 105,000 square feet, and a heavy 
clay about 200,000 square feet, or nearly 5 acres. It should be noted, 
however, that under certain conditions the particles in soils of fine 
texture tend to flocculate or collect in small aggregates (see p. 20), 
thus reducing the effective area of exposed surface. 
Relation of effective soil surface to fertility. — That the qualities of 
soils are largely influenced by the size of the soil grains is due to the 
fact that many of these qualities actually depend on the area of the 
total effective surface of all the soil grains in the mass of soil that 
the roots of plants occupy. The water held by the soil after draining 
is in the form of fine films surrounding the soil grains, and therefore 
the quantity depends on the extent of surface of the soil grains. 
Chemical and microbiological processes forming available plant food 
also take place on the surface of the soil grains. The finer the par- 
ticles of any soil the greater is the relative quantity of available 
plant-food materials carried in the soil solutions. The total feeding 
area of plant roots is therefore increased as the size of the particles 
composing the soil is decreased. 
STRUCTURE. 
(Ref. Nos. 3, pp. 115-116; 184-197; 10, pp. 99-101.) 
Plasticity and granulation. — The particles of a soil when wet have 
a tendency to stick together and to adhere to other objects with 
which they come in contact. This property of stickiness or ability 
to be molded is called plasticity. Coarse-textured soils show this 
property only to a very small degree. In soil -management studies, 
therefore, plasticity need be considered only in connection with the 
fine-textured soils, especially the clays. The plasticity of soil is due 
principally to the size and arrangement of the soil particles, the 
water present in the soil, and the materials contained in the soil solu- 
