ARTIFICIAL MULCHES 



201 



an efficiency of at least 50 per cent for a long period, 

 or until they are so decayed that they acquire decided 

 capillary capacity. A practice based upon this effect 

 is that of growing potatoes under straw. The potatoes 

 are laid upon the surface of the ground, and covered 

 deeply with straw, which keeps the surface soil so moist 

 that the potatoes sprout and will grow a reasonable 

 crop to maturity, when the straw has simply to be raked 

 back and the tubers, clean and smooth, are found on or 

 very near the surface. Leaves, including pine needles, 



Fig. 63. A very stony soil. Boulders and gravel serve as a mulch, promote 

 drainage, and increase the warmth of the soil. 



