CHAPTER IV. 

 PHYSICAL NATURE OF SOILS. 



128. Texture of Soils. The size of soil grains and the 

 way they are grouped in composite clusters forming ker- 

 nels or crumbs has a very great influence in determining 

 'the physical properties of soils and their agricultural value, 

 and as soils vary quite as widely in the size and arrange- 

 ment of their grains as they do in their chemical composi- 

 tion it is clear that this phase of soil problems must take at 

 least equal rank with those considered in the last chapter. 



In all agricultural soils except the very coarse and sandy 

 ones there is a composite granular structure which renders 

 them much more open and porous than they could otherwise 

 be, and when a soil is puddled this structure or texture is 

 destroyed in a large measure and the separate grains are 

 then brought into the closest possible arrangement, and 

 they become nearly or quite impervious to both water and 

 air, approaching the condition of brick and potter's clays. 



129. Size of Soil Grains. When the fragments of rock are 

 so coarse that very few are smaller than .01 of an inch in 

 diameter we have a sand rather than a soil. Most plas- 

 tering sands are made up of grains ranging from .01 up to 

 .08 of an inch in diameter. 



In the table which follows is given the mechanical anal- 

 yses of three types of soil: 



It will be seen from this table that only .8 -per cent, of 

 either soil is made up of grains having diameters so great 

 that only 23 are required to span a linear inch, while the 

 heavy clay soil has nearly one-half of its weight made up 



