THE PHYSICAL PROPKKTIES OF THE SOIL. 705 



volume.' Tlie volume at'tuiilly occupied by the solid particles is inde- 

 peiulent of the size of the spheres, in both the above cases, so loug as 

 all the spheres in the mass are the same size.^ 



In ordinary arable soils the conditions are somewhat different, due to 

 the fact that the particles are not spherical and are of different sizes, 

 and that external conditions exercise a considerable influence on the 

 structure. The arrangement may be more compact than in the first case 

 described above on account of the smaller grains depositing themselves 

 in the spaces between the larger ones (Fig. 0), or may be less compact 

 than in the second when the parti(;les arrange themselves as in Fig. 7. 

 Shaking and packing increase the density of the soil, but this is princi- 

 pally caused, and to the greatest extent, by the percolation of water, 

 which acts mechanically on the soil (puddling it). 



Fig. 6. Fig. 7. 



The volume actually occupied by soil particles is smallest, relatively, in 

 soils containing porous fragments, especially those rich in humus (peat). 



From what has been said it is not surprising that the volume of the 

 pores, which depends on the arrangement of the i^articles, varies within 

 wide limits in natural soils. According to the experiments made on 

 this subject by C. Fliigge^ the volume varies in mineral soils between 

 20 and 57.8 per cent, while in peat and moor soils it may rise to 85.2 

 per cent, according to E. Ramann and A. K. von Schwarz.'' 



Crumbly structure. — In many soils aggregates or floccules are formed 

 under certain circumstances. This kind of arrangement differs from 

 that already described principally in the occurrence of a considerable 

 number of larger spaces between the aggregates, owing to the virtually 

 larger size of the particles. These larger spaces bring about very 

 important differences in the physical properties of the soil, especially in 

 its behavior toward water and air. This kind of arrangement of the 

 soil particles is shown in Fig. 8. 



There are various reasons for the formation of aggregates. The 

 tendency is approximately inversely proportional to the size of the 



' F. Soyka, Forscli. Geb. agr. Pbys., 8, p. 1. 



^C. Lang, Ibid., 1, p. 109. 



^Beitriige zur Hygiene, Leipsic, 187Q. E. Wolff, Anleitung zur chemischen Uuter- 

 suchung lauclwirthscbaftlich wichtiger Stoffe, 1875. F. Renk, Ztschr. Biol., 15. E. 

 Kaniann, 1. c. Veitiueyer, Vorarbeiten zur Wasserversorguug tier Statlt Berlin, 1871 

 and later. 



^ Forscb. deb. agr. Pbys., 2, p. 164. 



