240 THE PRINCIPLES OF SOIL MANAGEMENT 



the available moisture capacity. (4) Improves the 

 aeration of the soil. (5) Raises the average temperature. 



(6) Promotes the growth of desirable organisms. 



(7) Increases the available food supply. (8) Enlarges 

 the root zone of the soil. (9) Reduces "heaving." 



(10) Removes injurious salts from ''alkali soils." 



(11) Reduces erosion. (12) Increases crop yields, and 

 improves sanitary conditions of the region. 



110. Firms the soil. In a saturated soil the particles 

 are held apart and are partially floated by the water, 

 with the result that they afford a poor support for plants, 

 and are largely unable to bear the weight of travel 

 incident to cultural operations. Heavy objects sink 

 into the surface, and become mired as a result of the easy 

 movement of the soil particles from beneath their 

 weight. This movement is greatly facilitated by the 

 lubrication afforded by the water between the particles. 

 It is because of this freedom of movement that a wet 

 soil may readily be "puddled," that is, the small par- 

 ticles moved into the spaces between the large ones, 

 producing a more dense mass, a change not possible 

 in dry or even moderately moist soils. 



111. Improves the structure. Drainage improves 

 the granular structure of fine-textured soil. One of the 

 most important factors in soil granulation is alternate 

 wetting and drying. (See page 105.) In a wet soil, this 

 drying and drawing together does not take place. On 

 the other hand, if a granular soil be kept saturated, the 

 crumb structure will be broken down and a bad physical 

 condition results. This is well illustrated by the fact 

 that nearly all swamp soils are in a puddled, or otherwise 



