88 THE PRINCIPLES OF SOIL MANAGEMENT 



of soil is involved, but under field conditions modifica- 

 tions of texture artificially are not practicable, because 

 of the expense involved. The farmer must generally 

 accept the texture of the soil as he finds it, and make 

 the best of his conditions by suitable selection of crops 

 adapted to his soil, and by such modifications of the 

 structure of the soil as its texture will permit. 



33. Structure. Soil structure deals with the arrange- 

 ment of the soil particles independently of their size. 



34. Some aspects of soil structure. The arrangement 

 of the soil particles may be viewed in many different 

 ways. Upon this arrangement depend several very 

 important physical properties, which in turn have a 

 fundamental bearing on chemical and biological prop- 

 erties. 



35. Ideal arrangements. Taking the simplest case 

 first, that of spherical particles of one size, these may 

 be arranged in general forms: (1) In columnar order, 

 with each particle touching its neighbors at only four 

 points. (2) In oblique order, with each particle touch- 

 ing its neighbors at six points. (3) These spheres may 

 be gathered into larger spheres which rest together 

 in the second order. In the first the unoccupied or pore 

 space is 47.64 per cent of the total volume occupied 

 by the spheres. In the second it is 25.95 per cent. In 

 the third case, however, where there are spheres within 

 spheres, the pore space is greatly increased to 74.05' 

 per cent. (4) On the other hand, if there are spheres of 

 several sizes so that the small ones may rest in the 

 spaces between the large ones, the total pore space will 

 be reduced below 25.95 per cent, and the spaces may 



