220 IRRIGATION PRACTICE 



as the supply of -water increases; and potassium, phos- 

 phorus, and other important plant-foods are likewise 

 taken up in larger proportion to the dry matter as the 

 supply of water is increased. 



This law, that the percentage of ash in plants increases 

 as the irrigation water is increased, means, apparently, 

 that more plant-food is used to produce a unit of dry 

 matter as more water is used in irrigation. This is one of 

 the strongest arguments yet found against the excessive 

 use of water. The farmer who uses a small quantity of 

 water in crop-production not only obtains a larger amount 

 of dry matter for each unit of water used, but also uses a 

 smaller quantity of plant-food for each unit of dry matter. 

 The waste due to over-irrigation is, therefore, at least 

 twofold : it diminishes the yield of dry matter to the unit 

 of water used, and it increases the soil-fertility cost per unit 

 of dry matter. This must be a fundamental considera- 

 tion in the establishment of a permanent system of agri- 

 culture under irrigation. 



130. Protein. Protein is the term commonly applied 

 to all organic plant substances containing nitrogen. These 

 nitrogenous plant constituents are of greatest impor- 

 tance in the maintenance of animal life. When organized 

 into proteid forms, they form the basis of blood, muscles 

 and all other primary tissues of the animal body. In fact, 

 as a food for animals, the value of a crop may be well 

 measured by its percentage of nitrogenous substances. 

 The compounds containing nitrogen are not, however, 

 all of equal value. Some furnish merely body heat, while 

 others enter into the fundamental structures of the body. 

 In the investigations of the effect of irrigation on plant 

 composition, attention has been given mainly to the group 

 of substances under the name "protein," and little knowl- 



