274 SOILS: PROPERTIES AND MANAGEMENT 



stones may not be more than offset by their beneficial 

 effects. 



The materials for mulching mentioned above are all 

 strictly artificial, and their application is greatly limited, 

 due to the lack of material and the expense involved. 

 They are therefore used only under special conditions. 

 The second type of mulch is almost universal in its prac- 

 tical availability. 



By proper cultivation almost any soil surface may be 

 brought into such a condition that evaporation of mois- 

 ture is more rapid than the upward capillary movement. 

 This is because surface tillage produces a loose, open 

 structure, which, while increasing the rate of thermal 

 movement of the water, at the same time obstructs 

 capillary action. The surface layer, therefore, quickly 

 becomes air-dry and is in a condition designated as a 

 soil mulch. As it differs from the soil below only in 

 structure, it has numerous advantages over artificial 

 mulches, at the same time performing successfully all 

 the functions of the latter. Since not only the water in 

 the mulch is sacrificed but also a small quantity pumped 

 upward by capillarity during the operation, speed in 

 formation is of importance. The tillage implements 

 that give 1 the maximum looseness and granulation will 

 prove the most successful. A spike-tooth harrow or a 

 weeder is the instrument ordinarily employed. 



192. The functions of a mulch. — A soil mulch depends 

 for its effectiveness on two functions — (1) the shutting- 

 off of evaporation, and (2) the checking of capillary move- 

 ment upward. It has already been shown that thermal 

 movement of water through dry soil layers is practically 

 nil; 1 therefore, as long as the soil is dry, evaporation is 

 1 This text, paragraph 166. 



