108. 



underlying factor* which determine differences in pore space, and, alsg 

 differences in other physical properties. 3oil pnrticles are irregular 

 in Bhai>e and uneven in size, llien brought very close together, as occurs 

 in mixing in a wet consition, th&ir molecular attraction is brought intp 

 operation and, especially when dry, they are held together very secure- 

 ly. In this wry the normal molecular attraction of the soil pnrticles is 

 increased "by the deposition around them of the material in solution. 



Applying these principles to the soil, it is observed tht there 

 / be two general rrrn.mi;er<ents of the particles. 



1} :jla.cl} particle mny be free and separate frorc itn neighbors. 

 This is a separate-.:jr.-?.i.i structure. That is, each particle of soil fun- 

 ctions separniely. lien by proper uanipulaticn the particles are so pack 

 ed together th.t the small particles quite completely fill in the spaces 

 between tlu? l?.rge ones, so that ? very dense raass is formed, the struc- 

 ture i? ten ed "puddled*. The terrr. puddled, in this connection, is rela- 

 ted to the fact that such an arrangement aan be obtained only in fine- 

 textured soils 'vhen they arc mixed (puddled) in a very wert condition, 

 so Dint the fine particles will move into the large spaces. 



2) On the other hand, the small particles may adhere to the 

 large oMc-n, or a number of small particles may adhere together as a 

 group or granule. Mien a number of united particles function together 

 as a single larger particle or granule, the structure is termed "granu- 

 lar". This arrangement is also termed the crumb structure. According as 



:^o grout-3 are prominent or inconspicuous, the soil is said to be well 

 or poorly gr?.nul". t'"d. 



i the granules reach large size, so that they interfere 

 with the b..st functioning of the soil, they are termed clods. That ia, 

 a clod is an un sizable granule. 



It is well known that n box of baseballs, or a pile of boulders, 

 or over, a box of sand, doer? not adhere together to any appreciable ex- 

 tent. I , ill the co arse -textured classes, Certainly down to 

 the size of very fine sand, there is very little tendency to granulate. 

 But in the- silt, to e. small extent, and in the clay, to a very great ex- 

 tent, granulation is strong. 



:. a crumb arm;, t, then, la the most desirable; the parti- 

 cles are glued together by clay and other agents into lumps or crumbs. 

 This lumps may take very irregular shapes. This structure is desirable 

 for all soils. Ml of our arid soils are rich in lime, much of this be- 



.jlucin t r by lime carbonate. The crumb structure nay be changed by 

 in torf ererice vrith the character of the clay. It is advisable not to 

 plow wet soil: the clay puddles and formo baked lumps. Inundation also 

 produces baked clay. 



In Mature the roots give the soil good tillage; the roots conti- 

 this tillage, and protection of the soil by mulch is maintained. The 

 mont important function of mulch in the foreut i to protect the soil, i 

 It prevents packing by water, etc. 



A certain amount of moisture and cold is often desirable, for 

 till .- r^f. Freezing helps the crumb condition and destroys the puddled- 

 condition of clay. The soil expands in frost, and cracks in dry weather. 

 Clay will not crack when there ia less than a minimum of l^'/V clay in the 

 soil7in good land; the minimum is less on poor land. 



