310 GENERAL SCIENCE 



but a soil made of a mixture of sand and clay in low ground 

 or in river valleys can easily be kept productive. 



The disintegrated rock particles in most soils make 

 from 60 to 95 per cent of the soil's weight. The 

 size of the particles usually determines the nature and 

 productiveness of a soil. A coarse-grained soil cannot 

 retain much water, and the spaces between the particles 

 are so large that the air can move about in it so freely 

 that much of the soil water is carried out by evaporation. 

 This is very noticeable when broken soil is not pulverized 

 before a dry season. Finely divided soil will retain the 

 water better. But soils which hold a large quantity of 

 water will not become warm enough for the growth of 

 plants early in the spring. The best soil for general 

 purposes is, therefore, one composed of a mixture of coarse 

 and fine particles. 



215. Soil Water. Soil water is not all the water that 

 is in the ground, it is only the water which adheres to 

 the soil particles, like the film of moisture which adheres 

 to an object when it is dipped into water. When soil is 

 moist and will not pack if you squeeze it in your hand, 

 each particle of it has a film of water around it. The film 

 of water is what the roots of plants absorb for food. 

 This soil water has the plant food dissolved in it. 



The productiveness of a soil is usually determined by 

 the amount of soil water which the soil can hold and 

 by the ease with which the roots of plants can get to this 

 soil water to remove it. For this reason it is very impor- 

 tant that the excess of water should drain away during a 

 wet season so that the roots of plants can have more soil 

 particles from which to get food; and in dry seasons the 

 soil water should not be permitted to escape from the 

 surface by evaporation. Excess water during a wet sea- 



