The Soil and Soil Water 29 



depending on the relative proportions of sand and clay. How lex- 



The texture of some soils can be improved by adding 



clay, of others by adding sand, and of still others by 

 adding silt. Humus also greatly modifies the texture; 

 it improves both sandy and clayey soils. The texture 

 of all soils is greatly improved by tillage, if the culti- 

 vation is done when the ground is neither too wet nor 

 too dry. 



A " heavy " soil is one containing so much clay that Heavy and 

 it is sticky and hard to work. Sandy soils are called Ii0ht S0lls 

 11 light " because they are easy to work. By weight, 

 sand is heavier than clay, a cubic foot of sandy soil 

 weighing about a hundred pounds, while the same 

 amount of clayey soil weighs about seventy-five pounds. 

 Clay weighs less than sand, not because of being made 

 of any lighter material, but because the total air space 

 in dry clay is greater than the air space in sand. About 

 half of the volume of clay soil is made up of pores filled 

 with air. 



Heavy soils are usually richer and more moist than Their rela- 

 light soils, for they have more water-holding capacity ' 

 and do not allow the humus to escape so readily as do 

 light, sandy soils. 



Subsoil. The surface of the ground to the depth of The 

 six or eight inches (about as deep as the plow goes) has ^weento 



a darker color and contains more humus and other plant S M ar } d 

 food than is found below this depth. If the plow is 

 made to go deeper, it brings up sticky material that 

 has not been exposed to the weather and is therefore 

 not so well suited to growing plants. (Exp. 7.) This 

 under layer of poorer soil is called " subsoil." Deep 



