150 SOIL PHYSICS AND MANAGEMENT 



The movement of capillary moisture is principally along the 

 surfaces of mineral soil particles that are in contact, and the more 

 points of contact the larger the amount and the greater the rapidity 

 of movement. Organic matter introduces many very irregular par- 

 ticles which diminish the number in contact. As a result capillary 

 movement is slow in soils rich in organic matter. 



3. Puddling. The particles of soils low in organic matter are 

 not cemented together into crumbs and hence are free to move. 

 When these dry soils become wet there is a rearrangement of the 

 particles, due to the drawing force of the surface film, by which they 

 are brought closer together, and the pore space is so diminished that 

 water cannot penetrate the wet stratum very rapidly. This is spoken 

 of as "running together," but is really one form of puddling. The 

 change is produced by the tension of the film of water drawing the 

 particles together. This action may be seen where drops of water 

 fall in dust during a shower. 



Soils low in organic matter are easily puddled if worked when 

 wet, and a longer time is required for the natural agencies to correct 

 this condition than if the soil is well provided with organic matter. 

 Since granules are destroyed by puddling a correction of this con- 

 dition is produced when by any means granulation is restored. 



4. Prevents Loss by Erosion. Erosion causes very serious 

 loss on many soils. A vast amount of the richest soil material is 

 removed annually from the rolling land by the excess of rainfall 

 that runs off as surface drainage. The more the run-off the greater 

 the amount of washing. It is practically impossible to prevent this 

 entirely. The loss may be diminished by methods given in 

 Chapter xxvii. 



5. Increases Temperature. Organic matter imparts a darker 

 color to the soil, thus increasing the absorption of heat, and raising 

 the temperature, and, as a general rule for well-drained soils, the 

 darker the soil the higher the temperature. Light-colored soils are 

 cold, while dark ones are warm. This difference in color may 

 increase the temperature from four to ten degrees F. at a depth of 

 four inches during a clear day and give the crop on the dark soil a 

 distinct advantage. 



0. Biological Effects. Biological and consequently chemical 

 action is increased by organic matter, not only because it provides 

 a food supply for the organisms, but also because it brings about 

 physical conditions favorable to the action of bacteria which produce 

 chemical action. 



