474 GEOLOGY OF NORTHWESTERN TEXAS. 



tion and will act upon it to adapt their crops to the soil and plant only such 

 crops as can get most readily their food supply from it. 



By repeated chemical analyses of plants it has been found that the follow- 

 ing elements are essential to plant life, and if a soil be lacking in any one of 

 them, or the plant so situated that it can not absorb the missing ingredient 

 from the atmosphere, it is impossible for plant life to exist. These elements 

 are: 1, carbon; 2, hydrogen; 3, oxygen; 4, nitrogen; 5, phosphorus; 6, sul- 

 phur; 7, chlorine; 8, iron; 9, magnesium; 10, calcium; 11, potassium. 



The sources of these elements as appropriated by the vegetables are as fol- 

 lows: 



Carbon. — At least one-half in weight of dried plants is composed of car- 

 bon. The plant gets this material from the atmosphere. The plant is cap- 

 able of absorbing some carbon from the soil from its roots, but not enough 

 to sustain itself. The open leaves take up the carbonic acid from the atmos- 

 phere, and if exposed to the light assimilates the carbon, while the oxygen 

 goes back to the air. 



Hydrogen. — This material exists abundantly in nature. About one-tenth 

 of dried vegetable matter consists of this element. The plant receives this 

 material in the form of water. The water is taken up by both the roots and 

 leaves, but principally by the roots. This element all comes from the atmos- 

 phere, but large amounts of it fall upon the earth in the form of rain and is 

 absorbed by the soil and stored up for the future use of the plant. 



Oxygen. — As has already been noticed, this element is in combination 

 with carbonic acid and water, and the plant receives it in combination with 

 them through both roots and leaves. 



Nitrogen. — This material exists abundantly in the atmosphere, the atmos- 

 phere being composed of oxygen 23.04 parts and nitrogen 76.96 parts, omit- 

 ting carbonic acid and water. But it is doubtful if a plant has the power of 

 assimilating nitrogen from the atmosphere unless in combination with other 

 elements. Large amounts of nitrogen are evolved by decaying vegetable 

 and animal substances, which is taken up by the atmosphere and returned to 

 the earth in rains. 



Phosphorus. — This element is entirely taken from the soils, where it oc- 

 curs as phosphate of lime, or alkalies. It comes from the decomposition of 

 phosphatic rocks, and is taken up by the roots of the plants. 



Sulphur. — This element is taken up by the plants in combination with 

 sulphuric acid and lime, potash, soda, and other sulphates. Nearly all the 

 soils of Northwestern Texas are supplied with this material in the form of sul- 

 phate of lime or gypsum. 



Chlorine. — This element is also taken from the soils, where it occurs in the 



