CROP COMPOSITION 217 



measure the quality of the crops produced. Irrigation, or 

 the artificial application of water to crops, requires added 

 labor. It is, therefore, only upon the basis of certain 

 and larger yields, and better quality of the crops produced, 

 that irrigated areas may compete in the open markets 

 with other sections of the world. 



The relation between irrigation and crop composi- 

 tion has been studied by several investigators. In general, 

 it has been shown that many irrigated crops may be 

 made to possess a composition superior to that of crops 

 grown under the natural rainfall. Yet, it must be admit- 

 ted that we know the merest outlines of the subject. 

 There is here a great and important field open for investi- 

 gation by those who are interested in developing a science 

 of irrigation 



127. Groups of plant constituents. Every plant con- 

 tains five great groups of substances and each has a defi- 

 nite food value and bears important relationships to the 

 soil on which the plant has been grown. These are: (1) 

 water, (2) ash or mineral matter, (3) nitrogenous sub- 

 stances, (4) fats, and (5) carbohydrates. 



128. Water. During the life of the plant, large quan- 

 tities of water are passed rapidly from the soil into the 

 plant, and from the plant leaves into the air. As has been 

 shown, hundreds of pounds of water are thus passed 

 through the plant for the production of one pound of dry 

 matter. That the vital processes of the plant may pro- 

 ceed unhindered, the cells of the green plant must be fully 

 filled with water. The more water is in the soil, the more 

 completely are the plant cells filled with water. That is, 

 on a moist soil, under conditions of abundant irrigation, 

 the green plant probably contains a larger proportion of 

 water than on dry soils, where the quantity of irrigation 



