468 WATER SUPPLY AND SEWAGE DISPOSAL 



Scarcely any other kind of soil has equal capacity for holding 

 moisture. 



3. The capacity for holding moisture is greatly increased in 

 most soils by providing good UNDER DRAINAGE. Well-drained 

 soils remain porous while soils not well drained become hard 

 and solid. The rainfall readily enters porous soils and passes 

 down through such soils to the water table (Art. 532) below. 



551. How Moisture may be Conserved. As we have seen 

 (Art. 546), at best, considerable soil moisture is evaporated 

 and passes into the atmosphere on account of soil aeration. 

 Moreover, many soils, when dry, tend to become hard and solid, 

 and shrink. The result is, frequently, that cracks open in the 

 surface soil. Sometimes these cracks are numerous, wide, and 

 deep. Such cracks permit freer circulation of air through the 

 soil and consequently more evaporation takes place; worst of 

 all, such evaporation takes place at considerable depth. This 

 evaporation greatly lessens the amount of film water within the 

 reach of the plant roots. 



Loss of soil moisture through evaporation may be lessened 

 by covering the soil with mulch. This mulch may be provided 

 by either of two methods: 



1. By the application of a coat of manure, straw, dead grass, 

 or any similar material. Mulch of this. nature is difficult to 

 obtain in sufficient quantities for use on large cultivated fields. 



2. By the preparation of SOIL MULCH. A soil mulch consists 

 simply of a layer of finely pulverized soil. This is produced by 

 thorough tillage after every rain. If the top 1 or 2 in. of soil 

 is kept in a finely pulverized condition, loose and open, during 

 the growing season, the loss through evaporation from the 

 surface is greatly reduced. It does this by breaking the capil- 

 lary action at the lower surface of the loose soil. The soil 

 moisture rises readily to th,at point but does not rise farther. 

 It is much the same condition we should have in our kerosene 

 lamp were the wick cut in two a short distance down in the 

 wick tube of the burner. Keeping the surface of the soil 

 covered with this soil mulch is the only practical method of 



