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 multiphed 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-haH 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 quahties 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 abihty 

 to be molded is oaUed 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 

 fiine-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- 



