206 ELEMENTS OF GENERAL SCIENCE 



inch, and their aggregate surface area would be 3141.59 

 square inches." In the coarse silt the surface area of the par- 

 ticles reaches an astonishing figure, since in 1 cubic foot of 

 such soil the particles would have a surface area of 37,700 

 square feet. So large a surface area makes possible a large 

 water content in such a soil. In some kinds of clay 100 

 pounds of the wet soil may contain 40 pounds of water, or 40 

 per cent of the whole weight. Since 1 cubic foot of clay ordi- 

 narily weighs about 80 pounds, 40 per cent of it, or 32 pounds, 

 might be water. These facts make clear the tremendous dif- 

 ferences in the water-holding power of soils. 



227. Soil water and soil temperature. Anyone who has 

 walked back and forth along the lake or ocean shore while in 

 bathing has noticed differences in the temperature of the soil, 

 and any well-trained gardener who desires to grow early spring 

 vegetables selects what he calls a " warm soil." What do these 

 things signify ? Soil which is very wet changes temperature 

 much more slowly than dry soil (sect. 157). The water of the 

 soil, being at lower temperature than the air above the soil on 

 a sunny spring day, makes the soil feel cold. If in the hot 

 summer weather the soil may be cultivated in such a manner 

 that the water will be held in it, the soil will not become so 

 hot and will therefore be a better living place for plants. 



228. Amount of water used by crops. From the calculations 

 given it is evident that some soils may contain much water, 

 but it is also true that large amounts of water pass off from 

 the soil by evaporation and that plants take large quantities 

 of water from the soil. Growing plants are constantly carry- 

 ing water through their roots, stems, and leaves, using some 

 of it in food manufacture but allowing most of it to pass off 

 into the air. The following data show the average quantity 

 of water used by three common farm crops : 



Corn : 50 bushels per acre require 1,500,000 pounds of water. 

 Potatoes : 200 bushels per acre require 1,268,000 pounds of water. 

 Oats : 29 bushels per acre require 1,192,000 pounds of water. 



