200 THE PRINCIPLES OF SOIL MANAGEMENT 



applied to the surface of the soil. (2) Those composed 

 of the natural soil modified by appropriate tillage. 



The action of both sorts of material depends on the 

 facts shown on pages 180 and 1S9; namely, that capillary 

 action may be changed or broken by sufficient change 

 in the texture or structural properties of the material, 

 and, second, that the diffusion of water vapor, even after 

 evaporation has taken place, is exceedingly slow through 

 small irregular pore spaces, such as exist in all materials 

 effective as mulch. Any material is effective as a mulch 

 in proportion as it fulfils these conditions; and their 

 practical application, therefore, becomes chiefly a matter 

 of selecting that material which meets these require- 

 ments, and may be readily applied. 



Many kinds of material are used as a mulch. Straw, 

 chaff, dead weeds, stubble, leaves, sawdust, manure, 

 boards, canvas, stone, coarse sand all of these are used, 

 and many other waste materials which may be available. 

 They act as a cover to the moist soil, so that water 

 which is held in the surface of the soil, or is brought up 

 by capillarity, must evaporate into this stagnant and 

 therefore soon-saturated atmosphere; under which con- 

 ditions the loss must be much less than where the vapor 

 is freely removed, and dry air brought in contact with 

 the moist soil. All of these materials are very efficient 

 as a mulch, their efficiency depending upon their thick- 

 ness and porosity. Straw and leaves, when fresh and 

 dry, will reduce evaporation below 10 per cent of the 

 normal, when in a layer three or four inches thick. 

 As they decay and become water-soaked from succes- 

 sive rains, their efficiency decreases; but they retain 



