252 Bulletin 254. 



(2) Drainage is directly operative to change an unfavorable physical 

 condition into a desirable one, as well as to reduce the tendency to a bad 

 physical condition of the soil. It may bring about the change from a 

 puddled to a granular soil. Such physical changes are most pronounced in 

 fine textured soil. The change is produced primarily by the alternate wet- 

 ting and drying to which well drained soil is subject. Poorly drained soil 

 is usually in a bad physical condition. It is compact and impervious. This 

 shows that a permanent or long continued wetness prevents the formation 

 of the loose granular condition which is desired. On the other hand, con- 

 tinued dryness produces no important change in tlie physical condition 

 of the soil. It is the alternation from the wet to the dry condition which 

 produces the readjustment of the soil particles. In nature such an alter- 

 nation of wetness and dryness is produced on soil where adequate pro- 

 vision is made for drainage. The rain comes periodically producing the 

 wetness which is followed by drying days during which the soil loses more 

 or less of its moisture. In a saturated soil, the particles are partially 

 floated. As the water is removed the film first breaks across the large 

 spaces. It breaks along any natural line of weakness resulting from a 

 different texture or structure, or a root cavity or animal burrow. A film 

 of moisture then surrounds a large number of particles. It may be a 

 mass a foot in diameter or it may be one so small as to be almost invisible 

 and including only a few hundred particles. As the moisture continues 

 to be removed by evaporation or other means, the film is continually drawn 

 tighter around the group of particles and tends with considerable force to 

 move them nearer together in the same way that — to use a common ex- 

 ample — the hairs of a brush are held together when it is dipped in water 

 and then removed. The inequalities in the mass of soil permit breakage of 

 the film into smaller and smaller areas with the result that new centers 

 of contraction are produced. This contraction is clearly shown by the 

 checking of a clay soil upon drying. The difiference between a cloddy 

 soil and a soil in good tilth is in the size of the granules. In the puddled 

 soil the lines of great weakness are few in number with the result that a 

 few centers of contraction produce a few large clods instead of a very 

 great number of fine granules. The numerous lines of great weakness 

 are gradually produced by this process of alternate wetting and drying 

 and their production is facilitated by the presence of plant roots, by frost, 

 by tillage and by organic matter. It is well known that the tilth of a soil 

 rapidly improves as a result of drainage and it is the result of the opera- 

 tion of all these forces and conditions at the foundation of which is 

 drainage. 



(3) Contrary to a frequent belief, drainage increases the amount of 

 moisture available to crops. This is the result of two factors. First, it 

 has been shown under two, above, that the granular condition is increased 

 and therefore the total amount of pore space in the soil is increased. When 

 the soil is granulated to the condition of good tilth, the total capillary 



