SOIL WATER, DRAINAGE, AND IRRIGATION 299 



302. Artificial drainage of soil water. Much of the sur- 

 plus water of the soil will naturally drain downward to the 

 water table, but agricultural plants grow better when by 

 artificial drainage as much -water is removed as will readily 

 flow into underground drains. The advantages are probably 

 at least fourfold : surplus water is removed, the soil is better 

 aerated, the soil becomes warm earlier in the season, and 

 injurious soil substances excreted from plant roots may be 

 removed as the surplus water passes downward. If you will 

 plant two jars of seedlings, one in a well-drained glass jar 

 and the other in a jar without drainage, and give them the 

 same amount of water, you will see some of the advantages 

 of soil drainage. 



303. Irrigation. In many parts of the country the annual 

 rainfall is so small, or is confined to so short a period of the 

 year, that there is not sufficient water in the soil to enable 

 plants to grow. There are many hundred thousand acres of 

 soil the composition of which is favorable to the growth of 

 plants but which are deficient in water. Various devices are 

 used for irrigating some of these lands, with different degrees 

 of success. There are in the United States (fig. 144) over 

 10,000,000 acres of artificially watered lands, and projects 

 under construction will add much to this figure. There are 

 several methods of irrigation, but that along the banks of the 

 Rio Grande River may serve as a type. In the lower stretches 

 of this river the annual rainfall is quite low, but a large vol- 

 ume of water comes from the mountainous country hundreds 

 of miles away in Mexico and the United States. In the lower 

 Rio Grande Valley there is much rich alluvial soil, needing 

 only water to make it fertile. Hundreds of miles of ditches 

 have been constructed to direct the water from the river to 

 the land. A main canal, looking like a river, carries water 

 in one case for more than 100 miles. From this main canal 

 smaller ones branch off, and finally the smallest ditches run to 

 the individual farms. Floodgates are used in turning water 



