Studies in Soil Physics. 31 



But the mechanical composition of a soil does not entirely 

 determine its physical condition. Acting with (and on) it is the 

 structure or the mutual arrangement of the soil particles in 

 space. As this is closer or looser all the physical properties of 

 the soil will vary. By these two quantities, then, the physical 

 condition of the soil is determined; the mechanical composition 

 representing the real or ultimate physical nature of the soil, the 

 structure the physical state in which it exists. * 



But physical condition is not in itself a measurable 

 quantity or even a quantity at all. It is a complex of manv in- 

 dividual properties of the most diverse nature and it is onlv by 

 observation of these properties that an idea of physical condition 

 is reached. These properties may be roughly classified into 

 two main groups: first, those denoted by texturej which 

 include those related to the amount and size of the interspaces 

 ("porosity," apparent specific gravity, "permeability," etc.); 

 and second, those denoted by mechanical resistance, 

 including cohesion and adhesion, the draft of plows, resistance 

 to penetration and friction, and similar forces. According to 

 this nomenclature physical condition is determined by 

 mechanical composition and by structure, and is evidenced 

 by the properties classed under texture and mechanical resist- 

 ance. The logical procedure in the study of physical condition 

 is, therefore, the examination of these measurable properties 

 included under texture and mechanical resistance; and the 

 determination, in so far as may be possible, of the ways in which 

 each of them is influenced by mechanical composition and 

 structure and by the external agencies (material and energetic) 

 which control the latter quantity. 



Texture. In a dry soil composed of spherical particles of 

 uniform size (the simplest possible mechanical composition) an 

 infinite number of structures are possible, ranging between the 

 closest possible packing, where each sphere touches twelve others, 

 and the loosest possible, where each sphere is in contact w^ith 



*By a perhaps somewhat remote analogy the mechanical composition may be called a 

 "quantity" factor (analogous to mass), the structure, a "potential". Or, again by 

 analogy, the mechanical composition is a scalar, the structure, a vector. 



JI am aware that texture is frequently used as synonomous with mechanical com- 

 position. However, such usage is by no means universal and is quite imnecessary 

 since another satisfactory term (mechanical composition) already exists. The use of 

 texture here suggested, namely, as referring to the interspaces, is much more closely 

 analogous to its meaning as applied to textiles, from which source it got into soil 

 literature, and supplies a much needed term for this group of properties. As so used 

 it may be defined as "the expression of the size, arrangement and relative total amottnt 

 of the interspaces between the soil particles." 



