TEXTURE AND STRUCTURE OF SOILS 37 



are the larger ones, the only exception being the lime in the 

 limestone residual. The arid soils do not show as great 

 differences as do the others, because they have not been 

 subjected to the same amount of solvent action and tritura- 

 tion. 



36. Soil structure. — By soil structure is meant the ar- 

 rangement of the particles of which the soil consists. These 

 particles may be separated so that each is free to move 

 independently of any other, which is usually true of a dry 

 coarse sand. Such an arrangement is known as the separate 

 grain structure. On the other hand the particles may be 

 arranged in small groups or granules, these being so firmly 

 combined that the granule acts like a separate particle. 

 The latter condition is termed the granular or crumbly 

 structure. When applied to loams and clay soils, these 

 arrangements of the particles have a relation to the condi- 

 tion popularly known as tilth. Good tilth in clays and loams 

 implies a granular structure, poor tilth a separate grain 

 structure. 



The granular structure is not to be confused with a cloddy 

 condition of the soil. In fact clods have the separate grain 

 structure, because the soil has been worked when wet until 

 the granules are broken down and the particles move easily 

 over each other owing to the lubrication of the moisture. 



37. Relation of structure to pore space. — The arrange- 

 ment of the soil particles determines to a considerable degree 

 the amount of free or pore space within the soil, especially in 

 loams and clays. Merely for the purpose of illustrating this 

 let us suppose that the soil particles are perfect spheres of 

 equal size, which, of course, they are not. There would be two 

 arrangements possible, if each sphere were independent of 

 every other: (1) in columnar order, in which each particle 

 is touched on four places by its neighbors ; (2) oblique 

 order, in which each particle is in contact with six of its 



