SOME PHYSICAL PROPERTIES OF THE SOIL 111 



sand. In such soil would naturally be found also a 

 medium to low pore space, just as has been exemplified 

 in the ideal condition described above. In granular 

 soils, the granules, being made up of small particles, 

 present much internal pore space. This condition occurs 

 only in fine soils, such as loams, silts, and clays, since 

 large particles will not cohere firmly enough to produce 

 a crumb structure. A fine-textured soil, which will 

 puddle more readily than a coarser one, is thus saved 

 from a semi-impervious condition by this tendency toward 

 granulation. 



The ideal soil condition might be considered to be most 

 likely to occur in a loam (Fig. 18). In loamy soil some 

 of the particles are large and 

 function separately; others are 

 medium in size and tend to form 

 the nuclei around which smaller 

 particles may cluster to form 

 granules, or crumbs. There are 

 thus a few large pore spaces 

 which facilitate drainage, and 

 numberless small openings in 

 which water is retained. Air 

 therefore finds easy movement 

 and sanitation is promoted. In 

 such a condition the organic mat- 

 ter plays an important part. 

 This exists usually as dark, par- 

 tially decayed material. It pushes 



apart the grains and lightens the soil, and contributes 

 much in bringing about the loamy condition so favorable 

 to plant development. Because of its water-holding 

 capacity also it proves a valuable addition. Thus with 



FIG. 18. The arrange- 

 ment of particles in loamy 

 soil of good structural 

 condition. (a). Large 

 granule ; (6), small sand 

 particle ; (c) , large pore 

 space ; (d) small granules 

 with small interstitial 

 spaces; (e), large sand 

 grain. 



