290 



GENERAL SCIENCE 



dissolved for plant use, the growth of soil organisms, and 

 the temperature of the soil. 



Experiment 75. Close the bottom of a tall, straight lamp 

 chimney and fill it with soil, then pour it full of water. If a hole 

 is now made in the stopper (Figure 251), a considerable portion of 

 the water will drain out. This is called gravitational water. Why ? 

 If the hole at the bottom is again closed and the apparatus is 



allowed to stand for some time, the 

 soil will become quite dry, since the 

 water in the bottom will rise by the 

 action of capillarity to replace that 

 lost at the surface by evaporation. 

 The water lost in this way is called 

 capillary water. If some of this dry 

 soil is weighed and then heated to a 

 temperature of 100 C., it will lose still 

 more water. This is called hygroscopic 

 water and is so strongly held in the soil 

 that it is not available for plant use. 



Size of Soil Particles in Rela- 

 tion to Temperature and Crops. 

 - We have learned that evapora- 

 tion of water lowers the tempera- 

 ture of the surrounding objects ; 

 therefore that soil will be coldest 

 from which the most water 

 evaporates. Wet soils are always cold soils. Clay soils 

 are colder than sandy soils for this reason. They have 

 a larger water capacity and hence furnish more water for 

 evaporation. An additional reason for the coldness of 

 wet soils is that the heat capacity of dry soil is but two 

 tenths that of water ; that is, it takes five times as much 

 heat to warm a gram of water through one degree as it 

 does to warm a gram of dry soil through one degree. 

 Hence the sun's heat will warm the drier soil more rapidly. 



FIG. 251. 



