THE SOIL AND SOIL FORMATION 



75 



pSds re of* foii from 

 be h en h forced ^t te bybal- 



keep these zones of soil supplied with the proper amount 

 of moisture and vegetable matter and to provide for the 

 mechanical conditions which best promote the growth 

 and yield of crops. Where there is a lack of mineral 

 plant food this also must be supplied. 



Film water and free water. In order that the relation 

 of water to the roots of plants may be better understood, 

 the following explanation is here pre- 

 sented : Place a pot of soil in a hot oven 

 and leave it until nearly all the moisture 

 has been dried out. If the oven is kept 

 at the boiling point of water a day or 

 l n g QT > practically all the soil moisture 

 will have been changed into steam and 

 driven off into the air as vapor, and the 

 soil will weigh much less than before. Even at the boil- 

 ing point of water the soil will so tenaciously hold to the 

 last particles of water that a very small amount will re- 

 main. We will assume that we have one hundred 

 pounds of water-free soil in the pot. (Figure 20.) Now 

 place this pot of dried soil in a room between 

 windows where the air can freely pass 

 over it, but where no rain can strike 

 it. Upon weighing the pot of soil some 

 days later we find that it weighs a few 

 pounds more than when it was removed 

 from the oven. (Figure 21.) This in- 

 crease in weight is due to moisture 

 which the soil has absorbed from the 

 air, just as ordinary salt will absorb ture ' 

 moisture from the air. The air always contains 

 water in the form of gas, often called vapor. If 

 we now close the room up tightly and place several 

 large kettles of water on a stove and cause them 

 to boil vigorously, the water will " boil away " and the 

 vapor will become an invisible part of the air of the 



open 



